This patchset consolidates several changes in the capacity and the usage tracking of the CPU. It provides a frequency invariant metric of the usage of CPUs and generally improves the accuracy of load/usage tracking in the scheduler. The frequency invariant metric is the foundation required for the consolidation of cpufreq and implementation of a fully invariant load tracking. These are currently WIP and require several changes to the load balancer (including how it will use and interprets load and capacity metrics) and extensive validation. The frequency invariance is done with arch_scale_freq_capacity and this patchset doesn't provide the backends of the function which are architecture dependent.
As discussed at LPC14, Morten and I have consolidated our changes into a single patchset to make it easier to review and merge.
During load balance, the scheduler evaluates the number of tasks that a group of CPUs can handle. The current method assumes that tasks have a fix load of SCHED_LOAD_SCALE and CPUs have a default capacity of SCHED_CAPACITY_SCALE. This assumption generates wrong decision by creating ghost cores or by removing real ones when the original capacity of CPUs is different from the default SCHED_CAPACITY_SCALE. With this patch set, we don't try anymore to evaluate the number of available cores based on the group_capacity but instead we evaluate the usage of a group and compare it with its capacity.
This patchset mainly replaces the old capacity_factor method by a new one and keeps the general policy almost unchanged. These new metrics will be also used in later patches.
The CPU usage is based on a running time tracking version of the current implementation of the load average tracking. I also have a version that is based on the new implementation proposal [1] but I haven't provide the patches and results as [1] is still under review. I can provide change above [1] to change how CPU usage is computed and to adapt to new mecanism.
Change since V8 - reorder patches
Change since V7 - add freq invariance for usage tracking - add freq invariance for scale_rt - update comments and commits' message - fix init of utilization_avg_contrib - fix prefer_sibling
Change since V6 - add group usage tracking - fix some commits' messages - minor fix like comments and argument order
Change since V5 - remove patches that have been merged since v5 : patches 01, 02, 03, 04, 05, 07 - update commit log and add more details on the purpose of the patches - fix/remove useless code with the rebase on patchset [2] - remove capacity_orig in sched_group_capacity as it is not used - move code in the right patch - add some helper function to factorize code
Change since V4 - rebase to manage conflicts with changes in selection of busiest group
Change since V3: - add usage_avg_contrib statistic which sums the running time of tasks on a rq - use usage_avg_contrib instead of runnable_avg_sum for cpu_utilization - fix replacement power by capacity - update some comments
Change since V2: - rebase on top of capacity renaming - fix wake_affine statistic update - rework nohz_kick_needed - optimize the active migration of a task from CPU with reduced capacity - rename group_activity by group_utilization and remove unused total_utilization - repair SD_PREFER_SIBLING and use it for SMT level - reorder patchset to gather patches with same topics
Change since V1: - add 3 fixes - correct some commit messages - replace capacity computation by activity - take into account current cpu capacity
[1] https://lkml.org/lkml/2014/10/10/131 [2] https://lkml.org/lkml/2014/7/25/589
Morten Rasmussen (2): sched: Track group sched_entity usage contributions sched: Make sched entity usage tracking scale-invariant
Vincent Guittot (8): sched: add utilization_avg_contrib sched: remove frequency scaling from cpu_capacity sched: make scale_rt invariant with frequency sched: add per rq cpu_capacity_orig sched: get CPU's usage statistic sched: replace capacity_factor by usage sched: add SD_PREFER_SIBLING for SMT level sched: move cfs task on a CPU with higher capacity
include/linux/sched.h | 21 ++- kernel/sched/core.c | 15 +- kernel/sched/debug.c | 12 +- kernel/sched/fair.c | 369 ++++++++++++++++++++++++++++++++------------------ kernel/sched/sched.h | 15 +- 5 files changed, 276 insertions(+), 156 deletions(-)
Add new statistics which reflect the average time a task is running on the CPU and the sum of these running time of the tasks on a runqueue. The latter is named utilization_load_avg.
This patch is based on the usage metric that was proposed in the 1st versions of the per-entity load tracking patchset by Paul Turner pjt@google.com but that has be removed afterwards. This version differs from the original one in the sense that it's not linked to task_group.
The rq's utilization_load_avg will be used to check if a rq is overloaded or not instead of trying to compute how many tasks a group of CPUs can handle.
Rename runnable_avg_period into avg_period as it is now used with both runnable_avg_sum and running_avg_sum
Add some descriptions of the variables to explain their differences
cc: Paul Turner pjt@google.com cc: Ben Segall bsegall@google.com
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- include/linux/sched.h | 21 ++++++++++++--- kernel/sched/debug.c | 10 ++++--- kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++++----------- kernel/sched/sched.h | 8 +++++- 4 files changed, 89 insertions(+), 24 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index 18f5262..b576b29 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1071,15 +1071,28 @@ struct load_weight { };
struct sched_avg { + u64 last_runnable_update; + s64 decay_count; + /* + * utilization_avg_contrib describes the amount of time that a + * sched_entity is running on a CPU. It is based on running_avg_sum + * and is scaled in the range [0..SCHED_LOAD_SCALE]. + * load_avg_contrib described the amount of time that a sched_entity + * is runnable on a rq. It is based on both runnable_avg_sum and the + * weight of the task. + */ + unsigned long load_avg_contrib, utilization_avg_contrib; /* * These sums represent an infinite geometric series and so are bound * above by 1024/(1-y). Thus we only need a u32 to store them for all * choices of y < 1-2^(-32)*1024. + * running_avg_sum reflects the time that the sched_entity is + * effectively running on the CPU. + * runnable_avg_sum represents the amount of time a sched_entity is on + * a runqueue which includes the running time that is monitored by + * running_avg_sum. */ - u32 runnable_avg_sum, runnable_avg_period; - u64 last_runnable_update; - s64 decay_count; - unsigned long load_avg_contrib; + u32 runnable_avg_sum, avg_period, running_avg_sum; };
#ifdef CONFIG_SCHEDSTATS diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index ce33780..f384452 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -71,7 +71,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group if (!se) { struct sched_avg *avg = &cpu_rq(cpu)->avg; P(avg->runnable_avg_sum); - P(avg->runnable_avg_period); + P(avg->avg_period); return; }
@@ -94,7 +94,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group P(se->load.weight); #ifdef CONFIG_SMP P(se->avg.runnable_avg_sum); - P(se->avg.runnable_avg_period); + P(se->avg.avg_period); P(se->avg.load_avg_contrib); P(se->avg.decay_count); #endif @@ -214,6 +214,8 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq) cfs_rq->runnable_load_avg); SEQ_printf(m, " .%-30s: %ld\n", "blocked_load_avg", cfs_rq->blocked_load_avg); + SEQ_printf(m, " .%-30s: %ld\n", "utilization_load_avg", + cfs_rq->utilization_load_avg); #ifdef CONFIG_FAIR_GROUP_SCHED SEQ_printf(m, " .%-30s: %ld\n", "tg_load_contrib", cfs_rq->tg_load_contrib); @@ -628,8 +630,10 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m) P(se.load.weight); #ifdef CONFIG_SMP P(se.avg.runnable_avg_sum); - P(se.avg.runnable_avg_period); + P(se.avg.running_avg_sum); + P(se.avg.avg_period); P(se.avg.load_avg_contrib); + P(se.avg.utilization_avg_contrib); P(se.avg.decay_count); #endif P(policy); diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index bd61cff..3a91ae6 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -670,6 +670,7 @@ static int select_idle_sibling(struct task_struct *p, int cpu); static unsigned long task_h_load(struct task_struct *p);
static inline void __update_task_entity_contrib(struct sched_entity *se); +static inline void __update_task_entity_utilization(struct sched_entity *se);
/* Give new task start runnable values to heavy its load in infant time */ void init_task_runnable_average(struct task_struct *p) @@ -678,9 +679,10 @@ void init_task_runnable_average(struct task_struct *p)
p->se.avg.decay_count = 0; slice = sched_slice(task_cfs_rq(p), &p->se) >> 10; - p->se.avg.runnable_avg_sum = slice; - p->se.avg.runnable_avg_period = slice; + p->se.avg.runnable_avg_sum = p->se.avg.running_avg_sum = slice; + p->se.avg.avg_period = slice; __update_task_entity_contrib(&p->se); + __update_task_entity_utilization(&p->se); } #else void init_task_runnable_average(struct task_struct *p) @@ -1548,7 +1550,7 @@ static u64 numa_get_avg_runtime(struct task_struct *p, u64 *period) *period = now - p->last_task_numa_placement; } else { delta = p->se.avg.runnable_avg_sum; - *period = p->se.avg.runnable_avg_period; + *period = p->se.avg.avg_period; }
p->last_sum_exec_runtime = runtime; @@ -2294,7 +2296,8 @@ static u32 __compute_runnable_contrib(u64 n) */ static __always_inline int __update_entity_runnable_avg(u64 now, struct sched_avg *sa, - int runnable) + int runnable, + int running) { u64 delta, periods; u32 runnable_contrib; @@ -2320,7 +2323,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now, sa->last_runnable_update = now;
/* delta_w is the amount already accumulated against our next period */ - delta_w = sa->runnable_avg_period % 1024; + delta_w = sa->avg_period % 1024; if (delta + delta_w >= 1024) { /* period roll-over */ decayed = 1; @@ -2333,7 +2336,9 @@ static __always_inline int __update_entity_runnable_avg(u64 now, delta_w = 1024 - delta_w; if (runnable) sa->runnable_avg_sum += delta_w; - sa->runnable_avg_period += delta_w; + if (running) + sa->running_avg_sum += delta_w; + sa->avg_period += delta_w;
delta -= delta_w;
@@ -2343,20 +2348,26 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
sa->runnable_avg_sum = decay_load(sa->runnable_avg_sum, periods + 1); - sa->runnable_avg_period = decay_load(sa->runnable_avg_period, + sa->running_avg_sum = decay_load(sa->running_avg_sum, + periods + 1); + sa->avg_period = decay_load(sa->avg_period, periods + 1);
/* Efficiently calculate \sum (1..n_period) 1024*y^i */ runnable_contrib = __compute_runnable_contrib(periods); if (runnable) sa->runnable_avg_sum += runnable_contrib; - sa->runnable_avg_period += runnable_contrib; + if (running) + sa->running_avg_sum += runnable_contrib; + sa->avg_period += runnable_contrib; }
/* Remainder of delta accrued against u_0` */ if (runnable) sa->runnable_avg_sum += delta; - sa->runnable_avg_period += delta; + if (running) + sa->running_avg_sum += delta; + sa->avg_period += delta;
return decayed; } @@ -2372,6 +2383,8 @@ static inline u64 __synchronize_entity_decay(struct sched_entity *se) return 0;
se->avg.load_avg_contrib = decay_load(se->avg.load_avg_contrib, decays); + se->avg.utilization_avg_contrib = + decay_load(se->avg.utilization_avg_contrib, decays); se->avg.decay_count = 0;
return decays; @@ -2408,7 +2421,7 @@ static inline void __update_tg_runnable_avg(struct sched_avg *sa,
/* The fraction of a cpu used by this cfs_rq */ contrib = div_u64((u64)sa->runnable_avg_sum << NICE_0_SHIFT, - sa->runnable_avg_period + 1); + sa->avg_period + 1); contrib -= cfs_rq->tg_runnable_contrib;
if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) { @@ -2461,7 +2474,8 @@ static inline void __update_group_entity_contrib(struct sched_entity *se)
static inline void update_rq_runnable_avg(struct rq *rq, int runnable) { - __update_entity_runnable_avg(rq_clock_task(rq), &rq->avg, runnable); + __update_entity_runnable_avg(rq_clock_task(rq), &rq->avg, runnable, + runnable); __update_tg_runnable_avg(&rq->avg, &rq->cfs); } #else /* CONFIG_FAIR_GROUP_SCHED */ @@ -2479,7 +2493,7 @@ static inline void __update_task_entity_contrib(struct sched_entity *se)
/* avoid overflowing a 32-bit type w/ SCHED_LOAD_SCALE */ contrib = se->avg.runnable_avg_sum * scale_load_down(se->load.weight); - contrib /= (se->avg.runnable_avg_period + 1); + contrib /= (se->avg.avg_period + 1); se->avg.load_avg_contrib = scale_load(contrib); }
@@ -2498,6 +2512,27 @@ static long __update_entity_load_avg_contrib(struct sched_entity *se) return se->avg.load_avg_contrib - old_contrib; }
+ +static inline void __update_task_entity_utilization(struct sched_entity *se) +{ + u32 contrib; + + /* avoid overflowing a 32-bit type w/ SCHED_LOAD_SCALE */ + contrib = se->avg.running_avg_sum * scale_load_down(SCHED_LOAD_SCALE); + contrib /= (se->avg.avg_period + 1); + se->avg.utilization_avg_contrib = scale_load(contrib); +} + +static long __update_entity_utilization_avg_contrib(struct sched_entity *se) +{ + long old_contrib = se->avg.utilization_avg_contrib; + + if (entity_is_task(se)) + __update_task_entity_utilization(se); + + return se->avg.utilization_avg_contrib - old_contrib; +} + static inline void subtract_blocked_load_contrib(struct cfs_rq *cfs_rq, long load_contrib) { @@ -2514,7 +2549,7 @@ static inline void update_entity_load_avg(struct sched_entity *se, int update_cfs_rq) { struct cfs_rq *cfs_rq = cfs_rq_of(se); - long contrib_delta; + long contrib_delta, utilization_delta; u64 now;
/* @@ -2526,18 +2561,22 @@ static inline void update_entity_load_avg(struct sched_entity *se, else now = cfs_rq_clock_task(group_cfs_rq(se));
- if (!__update_entity_runnable_avg(now, &se->avg, se->on_rq)) + if (!__update_entity_runnable_avg(now, &se->avg, se->on_rq, + cfs_rq->curr == se)) return;
contrib_delta = __update_entity_load_avg_contrib(se); + utilization_delta = __update_entity_utilization_avg_contrib(se);
if (!update_cfs_rq) return;
- if (se->on_rq) + if (se->on_rq) { cfs_rq->runnable_load_avg += contrib_delta; - else + cfs_rq->utilization_load_avg += utilization_delta; + } else { subtract_blocked_load_contrib(cfs_rq, -contrib_delta); + } }
/* @@ -2612,6 +2651,7 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq, }
cfs_rq->runnable_load_avg += se->avg.load_avg_contrib; + cfs_rq->utilization_load_avg += se->avg.utilization_avg_contrib; /* we force update consideration on load-balancer moves */ update_cfs_rq_blocked_load(cfs_rq, !wakeup); } @@ -2630,6 +2670,7 @@ static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq, update_cfs_rq_blocked_load(cfs_rq, !sleep);
cfs_rq->runnable_load_avg -= se->avg.load_avg_contrib; + cfs_rq->utilization_load_avg -= se->avg.utilization_avg_contrib; if (sleep) { cfs_rq->blocked_load_avg += se->avg.load_avg_contrib; se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter); @@ -2967,6 +3008,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se) */ update_stats_wait_end(cfs_rq, se); __dequeue_entity(cfs_rq, se); + update_entity_load_avg(se, 1); }
update_stats_curr_start(cfs_rq, se); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 6130251..c34bd11 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -343,8 +343,14 @@ struct cfs_rq { * Under CFS, load is tracked on a per-entity basis and aggregated up. * This allows for the description of both thread and group usage (in * the FAIR_GROUP_SCHED case). + * runnable_load_avg is the sum of the load_avg_contrib of the + * sched_entities on the rq. + * blocked_load_avg is similar to runnable_load_avg except that its + * the blocked sched_entities on the rq. + * utilization_load_avg is the sum of the average running time of the + * sched_entities on the rq. */ - unsigned long runnable_load_avg, blocked_load_avg; + unsigned long runnable_load_avg, blocked_load_avg, utilization_load_avg; atomic64_t decay_counter; u64 last_decay; atomic_long_t removed_load;
From: Morten Rasmussen morten.rasmussen@arm.com
Adds usage contribution tracking for group entities. Unlike se->avg.load_avg_contrib, se->avg.utilization_avg_contrib for group entities is the sum of se->avg.utilization_avg_contrib for all entities on the group runqueue. It is _not_ influenced in any way by the task group h_load. Hence it is representing the actual cpu usage of the group, not its intended load contribution which may differ significantly from the utilization on lightly utilized systems.
cc: Paul Turner pjt@google.com cc: Ben Segall bsegall@google.com
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/debug.c | 2 ++ kernel/sched/fair.c | 3 +++ 2 files changed, 5 insertions(+)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index f384452..efb47ed 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -94,8 +94,10 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group P(se->load.weight); #ifdef CONFIG_SMP P(se->avg.runnable_avg_sum); + P(se->avg.running_avg_sum); P(se->avg.avg_period); P(se->avg.load_avg_contrib); + P(se->avg.utilization_avg_contrib); P(se->avg.decay_count); #endif #undef PN diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3a91ae6..a171e1b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2529,6 +2529,9 @@ static long __update_entity_utilization_avg_contrib(struct sched_entity *se)
if (entity_is_task(se)) __update_task_entity_utilization(se); + else + se->avg.utilization_avg_contrib = + group_cfs_rq(se)->utilization_load_avg;
return se->avg.utilization_avg_contrib - old_contrib; }
Now that arch_scale_cpu_capacity has been introduced to scale the original capacity, the arch_scale_freq_capacity is no longer used (it was previously used by ARM arch). Remove arch_scale_freq_capacity from the computation of cpu_capacity. The frequency invariance will be handled in the load tracking and not in the CPU capacity. arch_scale_freq_capacity will be revisited for scaling load with the current frequency of the CPUs in a later patch.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 7 ------- 1 file changed, 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a171e1b..a96affd 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5823,13 +5823,6 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
sdg->sgc->capacity_orig = capacity;
- if (sched_feat(ARCH_CAPACITY)) - capacity *= arch_scale_freq_capacity(sd, cpu); - else - capacity *= default_scale_capacity(sd, cpu); - - capacity >>= SCHED_CAPACITY_SHIFT; - capacity *= scale_rt_capacity(cpu); capacity >>= SCHED_CAPACITY_SHIFT;
From: Morten Rasmussen morten.rasmussen@arm.com
Apply frequency scale-invariance correction factor to usage tracking. Each segment of the running_load_avg geometric series is now scaled by the current frequency so the utilization_avg_contrib of each entity will be invariant with frequency scaling. As a result, utilization_load_avg which is the sum of utilization_avg_contrib, becomes invariant too. So the usage level that is returned by get_cpu_usage, stays relative to the max frequency as the cpu_capacity which is is compared against. Then, we want the keep the load tracking values in a 32bits type, which implies that the max value of {runnable|running}_avg_sum must be lower than 2^32/88761=48388 (88761 is the max weigth of a task). As LOAD_AVG_MAX = 47742, arch_scale_freq_capacity must return a value less than (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024). So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.
cc: Paul Turner pjt@google.com cc: Ben Segall bsegall@google.com
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a96affd..a5039da 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2266,6 +2266,8 @@ static u32 __compute_runnable_contrib(u64 n) return contrib + runnable_avg_yN_sum[n]; }
+unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu); + /* * We can represent the historical contribution to runnable average as the * coefficients of a geometric series. To do this we sub-divide our runnable @@ -2294,7 +2296,7 @@ static u32 __compute_runnable_contrib(u64 n) * load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... ) * = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}] */ -static __always_inline int __update_entity_runnable_avg(u64 now, +static __always_inline int __update_entity_runnable_avg(u64 now, int cpu, struct sched_avg *sa, int runnable, int running) @@ -2302,6 +2304,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now, u64 delta, periods; u32 runnable_contrib; int delta_w, decayed = 0; + unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
delta = now - sa->last_runnable_update; /* @@ -2337,7 +2340,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now, if (runnable) sa->runnable_avg_sum += delta_w; if (running) - sa->running_avg_sum += delta_w; + sa->running_avg_sum += delta_w * scale_freq + >> SCHED_CAPACITY_SHIFT; sa->avg_period += delta_w;
delta -= delta_w; @@ -2358,7 +2362,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now, if (runnable) sa->runnable_avg_sum += runnable_contrib; if (running) - sa->running_avg_sum += runnable_contrib; + sa->running_avg_sum += runnable_contrib * scale_freq + >> SCHED_CAPACITY_SHIFT; sa->avg_period += runnable_contrib; }
@@ -2366,7 +2371,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now, if (runnable) sa->runnable_avg_sum += delta; if (running) - sa->running_avg_sum += delta; + sa->running_avg_sum += delta * scale_freq + >> SCHED_CAPACITY_SHIFT; sa->avg_period += delta;
return decayed; @@ -2474,8 +2480,8 @@ static inline void __update_group_entity_contrib(struct sched_entity *se)
static inline void update_rq_runnable_avg(struct rq *rq, int runnable) { - __update_entity_runnable_avg(rq_clock_task(rq), &rq->avg, runnable, - runnable); + __update_entity_runnable_avg(rq_clock_task(rq), cpu_of(rq), &rq->avg, + runnable, runnable); __update_tg_runnable_avg(&rq->avg, &rq->cfs); } #else /* CONFIG_FAIR_GROUP_SCHED */ @@ -2553,6 +2559,7 @@ static inline void update_entity_load_avg(struct sched_entity *se, { struct cfs_rq *cfs_rq = cfs_rq_of(se); long contrib_delta, utilization_delta; + int cpu = cpu_of(rq_of(cfs_rq)); u64 now;
/* @@ -2564,7 +2571,7 @@ static inline void update_entity_load_avg(struct sched_entity *se, else now = cfs_rq_clock_task(group_cfs_rq(se));
- if (!__update_entity_runnable_avg(now, &se->avg, se->on_rq, + if (!__update_entity_runnable_avg(now, cpu, &se->avg, se->on_rq, cfs_rq->curr == se)) return;
On Thu, Jan 15, 2015 at 11:09:24AM +0100, Vincent Guittot wrote:
From: Morten Rasmussen morten.rasmussen@arm.com
Apply frequency scale-invariance correction factor to usage tracking. Each segment of the running_load_avg geometric series is now scaled by the current frequency so the utilization_avg_contrib of each entity will be invariant with frequency scaling.
As a result, utilization_load_avg which is the sum of utilization_avg_contrib, becomes invariant too. So the usage level that is returned by get_cpu_usage, stays relative to the max frequency as the cpu_capacity which is is compared against.
Then, we want the keep the load tracking values in a 32bits type, which implies that the max value of {runnable|running}_avg_sum must be lower than 2^32/88761=48388 (88761 is the max weigth of a task).
As LOAD_AVG_MAX = 47742, arch_scale_freq_capacity must return a value less than (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024). So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.
cc: Paul Turner pjt@google.com cc: Ben Segall bsegall@google.com
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
kernel/sched/fair.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a96affd..a5039da 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2266,6 +2266,8 @@ static u32 __compute_runnable_contrib(u64 n) return contrib + runnable_avg_yN_sum[n]; } +unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
There is no actual function definition in this patch; this would hinder linking, no?
if (running)
sa->running_avg_sum += delta_w * scale_freq>> SCHED_CAPACITY_SHIFT;
if (running)
sa->running_avg_sum += runnable_contrib * scale_freq>> SCHED_CAPACITY_SHIFT;
if (running)
sa->running_avg_sum += delta * scale_freq>> SCHED_CAPACITY_SHIFT;
If you're going to respin; please but {} around that.
On Thu, Feb 19, 2015 at 04:34:42PM +0000, Peter Zijlstra wrote:
On Thu, Jan 15, 2015 at 11:09:24AM +0100, Vincent Guittot wrote:
From: Morten Rasmussen morten.rasmussen@arm.com
Apply frequency scale-invariance correction factor to usage tracking. Each segment of the running_load_avg geometric series is now scaled by the current frequency so the utilization_avg_contrib of each entity will be invariant with frequency scaling.
As a result, utilization_load_avg which is the sum of utilization_avg_contrib, becomes invariant too. So the usage level that is returned by get_cpu_usage, stays relative to the max frequency as the cpu_capacity which is is compared against.
Then, we want the keep the load tracking values in a 32bits type, which implies that the max value of {runnable|running}_avg_sum must be lower than 2^32/88761=48388 (88761 is the max weigth of a task).
As LOAD_AVG_MAX = 47742, arch_scale_freq_capacity must return a value less than (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024). So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.
cc: Paul Turner pjt@google.com cc: Ben Segall bsegall@google.com
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
kernel/sched/fair.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a96affd..a5039da 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2266,6 +2266,8 @@ static u32 __compute_runnable_contrib(u64 n) return contrib + runnable_avg_yN_sum[n]; } +unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
There is no actual function definition in this patch; this would hinder linking, no?
The function definition already lives further down in fair.c. I can move it up here if you prefer?
if (running)
sa->running_avg_sum += delta_w * scale_freq>> SCHED_CAPACITY_SHIFT;if (running)
sa->running_avg_sum += runnable_contrib * scale_freq>> SCHED_CAPACITY_SHIFT;if (running)
sa->running_avg_sum += delta * scale_freq>> SCHED_CAPACITY_SHIFT;If you're going to respin; please but {} around that.
Sure.
On Thu, Feb 19, 2015 at 05:02:36PM +0000, Morten Rasmussen wrote:
There is no actual function definition in this patch; this would hinder linking, no?
The function definition already lives further down in fair.c. I can move it up here if you prefer?
argh.. ok.
With previous patches removing all that I had missed you ;eft this one.
I thought everything got ripped and we went with new stuff.
On Thu, Feb 19, 2015 at 05:05:28PM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 05:02:36PM +0000, Morten Rasmussen wrote:
There is no actual function definition in this patch; this would hinder linking, no?
The function definition already lives further down in fair.c. I can move it up here if you prefer?
argh.. ok.
With previous patches removing all that I had missed you ;eft this one.
I thought everything got ripped and we went with new stuff.
I see. Vincent's patch only removes the old calls to the function, not the function itself. The history might get more clear if we wipe it completely before reintroducing again with a slightly different meaning.
The average running time of RT tasks is used to estimate the remaining compute capacity for CFS tasks. This remaining capacity is the original capacity scaled down by a factor (aka scale_rt_capacity). This estimation of available capacity must also be invariant with frequency scaling.
A frequency scaling factor is applied on the running time of the RT tasks for computing scale_rt_capacity.
In sched_rt_avg_update, we scale the RT execution time like below: rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
Then, scale_rt_capacity can be summarized by: scale_rt_capacity = SCHED_CAPACITY_SCALE - ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
We can optimize by removing right and left shift in the computation of rq->rt_avg and scale_rt_capacity
The call to arch_scale_frequency_capacity in the rt scheduling path might be a concern for RT folks because I'm not sure whether we can rely on arch_scale_freq_capacity to be short and efficient ?
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 17 +++++------------ kernel/sched/sched.h | 4 +++- 2 files changed, 8 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a5039da..b37c27b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct sched_domain *sd, int cpu) static unsigned long scale_rt_capacity(int cpu) { struct rq *rq = cpu_rq(cpu); - u64 total, available, age_stamp, avg; + u64 total, used, age_stamp, avg; s64 delta;
/* @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu)
total = sched_avg_period() + delta;
- if (unlikely(total < avg)) { - /* Ensures that capacity won't end up being negative */ - available = 0; - } else { - available = total - avg; - } + used = div_u64(avg, total);
- if (unlikely((s64)total < SCHED_CAPACITY_SCALE)) - total = SCHED_CAPACITY_SCALE; + if (likely(used < SCHED_CAPACITY_SCALE)) + return SCHED_CAPACITY_SCALE - used;
- total >>= SCHED_CAPACITY_SHIFT; - - return div_u64(available, total); + return 1; }
static void update_cpu_capacity(struct sched_domain *sd, int cpu) diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index c34bd11..fc5b152 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq)
#ifdef CONFIG_SMP extern void sched_avg_update(struct rq *rq); +extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu); + static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta) { - rq->rt_avg += rt_delta; + rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL, cpu_of(rq)); sched_avg_update(rq); } #else
On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:
The average running time of RT tasks is used to estimate the remaining compute capacity for CFS tasks. This remaining capacity is the original capacity scaled down by a factor (aka scale_rt_capacity). This estimation of available capacity must also be invariant with frequency scaling.
A frequency scaling factor is applied on the running time of the RT tasks for computing scale_rt_capacity.
In sched_rt_avg_update, we scale the RT execution time like below: rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
Then, scale_rt_capacity can be summarized by: scale_rt_capacity = SCHED_CAPACITY_SCALE - ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
We can optimize by removing right and left shift in the computation of rq->rt_avg and scale_rt_capacity
So far so good..
The call to arch_scale_frequency_capacity in the rt scheduling path might be a concern for RT folks because I'm not sure whether we can rely on arch_scale_freq_capacity to be short and efficient ?
No, that is, arch_scale_frequency_capacity() _must_ be short and efficient, event for the fair class, its called in very hot paths.
I think we've talked about this before; this function should basically only return a cached value, which is periodically updated through some means.
But lets see, I've yet to see an actual implementation of it; and its got that sd argument, curious what you're going to do with that.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
kernel/sched/fair.c | 17 +++++------------ kernel/sched/sched.h | 4 +++- 2 files changed, 8 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a5039da..b37c27b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c
@@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu) total = sched_avg_period() + delta;
- if (unlikely(total < avg)) {
/* Ensures that capacity won't end up being negative */available = 0;- } else {
available = total - avg;- }
- used = div_u64(avg, total);
- if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
total = SCHED_CAPACITY_SCALE;
- if (likely(used < SCHED_CAPACITY_SCALE))
return SCHED_CAPACITY_SCALE - used;
- total >>= SCHED_CAPACITY_SHIFT;
- return div_u64(available, total);
- return 1;
}
This makes a lot of changes not commented on in the Changelog.
I think it makes sense, but this has always been horrible.
On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote:
On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:
The average running time of RT tasks is used to estimate the remaining compute capacity for CFS tasks. This remaining capacity is the original capacity scaled down by a factor (aka scale_rt_capacity). This estimation of available capacity must also be invariant with frequency scaling.
A frequency scaling factor is applied on the running time of the RT tasks for computing scale_rt_capacity.
In sched_rt_avg_update, we scale the RT execution time like below: rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
Then, scale_rt_capacity can be summarized by: scale_rt_capacity = SCHED_CAPACITY_SCALE - ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
We can optimize by removing right and left shift in the computation of rq->rt_avg and scale_rt_capacity
So far so good..
The call to arch_scale_frequency_capacity in the rt scheduling path might be a concern for RT folks because I'm not sure whether we can rely on arch_scale_freq_capacity to be short and efficient ?
No, that is, arch_scale_frequency_capacity() _must_ be short and efficient, event for the fair class, its called in very hot paths.
... and very frequently too.
I think we've talked about this before; this function should basically only return a cached value, which is periodically updated through some means.
Agreed. I think it is reasonable to assume that the arch code implementing arch_scale_freq_capacity() does it's best to make it fast for the particular architecture. Since the scaling factor to be returned by the function may be obtained in different ways for different architectures the caching should be done on the arch side.
But lets see, I've yet to see an actual implementation of it; and its got that sd argument, curious what you're going to do with that.
So we do have an RFC implementation for ARM already which I posted in December and is also included in the rather large RFC posting I did some weeks ago. That one basically reads two atomic variables and returns the ratio between the two. I have yet to benchmark how horribly expensive it is though. The sd argument is ignored. We might actually not need it at all?
On 19 February 2015 at 18:18, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote:
On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:
The average running time of RT tasks is used to estimate the remaining compute capacity for CFS tasks. This remaining capacity is the original capacity scaled down by a factor (aka scale_rt_capacity). This estimation of available capacity must also be invariant with frequency scaling.
A frequency scaling factor is applied on the running time of the RT tasks for computing scale_rt_capacity.
In sched_rt_avg_update, we scale the RT execution time like below: rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
Then, scale_rt_capacity can be summarized by: scale_rt_capacity = SCHED_CAPACITY_SCALE - ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
We can optimize by removing right and left shift in the computation of rq->rt_avg and scale_rt_capacity
So far so good..
The call to arch_scale_frequency_capacity in the rt scheduling path might be a concern for RT folks because I'm not sure whether we can rely on arch_scale_freq_capacity to be short and efficient ?
No, that is, arch_scale_frequency_capacity() _must_ be short and efficient, event for the fair class, its called in very hot paths.
... and very frequently too.
I think we've talked about this before; this function should basically only return a cached value, which is periodically updated through some means.
Agreed. I think it is reasonable to assume that the arch code implementing arch_scale_freq_capacity() does it's best to make it fast for the particular architecture. Since the scaling factor to be returned by the function may be obtained in different ways for different architectures the caching should be done on the arch side.
But lets see, I've yet to see an actual implementation of it; and its got that sd argument, curious what you're going to do with that.
So we do have an RFC implementation for ARM already which I posted in December and is also included in the rather large RFC posting I did some weeks ago. That one basically reads two atomic variables and returns the ratio between the two. I have yet to benchmark how horribly expensive it is though. The sd argument is ignored. We might actually not need it at all?
For consistency across all arch_scale_xx_capacity, i would prefer to keep the same prototype interface (struct sched_domain *sd, int cpu) even if it's not used ofr now
-- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On 24/02/15 10:21, Vincent Guittot wrote:
On 19 February 2015 at 18:18, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote:
On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:
[...]
Agreed. I think it is reasonable to assume that the arch code implementing arch_scale_freq_capacity() does it's best to make it fast for the particular architecture. Since the scaling factor to be returned by the function may be obtained in different ways for different architectures the caching should be done on the arch side.
But lets see, I've yet to see an actual implementation of it; and its got that sd argument, curious what you're going to do with that.
So we do have an RFC implementation for ARM already which I posted in December and is also included in the rather large RFC posting I did some weeks ago. That one basically reads two atomic variables and returns the ratio between the two. I have yet to benchmark how horribly expensive it is though. The sd argument is ignored. We might actually not need it at all?
For consistency across all arch_scale_xx_capacity, i would prefer to keep the same prototype interface (struct sched_domain *sd, int cpu) even if it's not used ofr now
Agreed.
Once we call arch_scale_xx_capacity [xx = freq, cpu] in the PELT code (i.e. w/ sd = NULL) we have to make sure that default_scale_cpu_capacity() can be called w/ sd = NULL too for platforms which are not providing their own arch_scale_cpu_capacity() function.
It's already part of '[RFCv3 PATCH 00/48] sched: Energy cost model for energy-aware scheduling'
https://lkml.org/lkml/2015/2/4/573
[...]
This new field cpu_capacity_orig reflects the original capacity of a CPU before being altered by rt tasks and/or IRQ
The cpu_capacity_orig will be used: - to detect when the capacity of a CPU has been noticeably reduced so we can trig load balance to look for a CPU with better capacity. As an example, we can detect when a CPU handles a significant amount of irq (with CONFIG_IRQ_TIME_ACCOUNTING) but this CPU is seen as an idle CPU by scheduler whereas CPUs, which are really idle, are available. - evaluate the available capacity for CFS tasks
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org Reviewed-by: Kamalesh Babulal kamalesh@linux.vnet.ibm.com --- kernel/sched/core.c | 2 +- kernel/sched/fair.c | 8 +++++++- kernel/sched/sched.h | 1 + 3 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c84bdc0..45ae52d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7087,7 +7087,7 @@ void __init sched_init(void) #ifdef CONFIG_SMP rq->sd = NULL; rq->rd = NULL; - rq->cpu_capacity = SCHED_CAPACITY_SCALE; + rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE; rq->post_schedule = 0; rq->active_balance = 0; rq->next_balance = jiffies; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index b37c27b..4782733 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4141,6 +4141,11 @@ static unsigned long capacity_of(int cpu) return cpu_rq(cpu)->cpu_capacity; }
+static unsigned long capacity_orig_of(int cpu) +{ + return cpu_rq(cpu)->cpu_capacity_orig; +} + static unsigned long cpu_avg_load_per_task(int cpu) { struct rq *rq = cpu_rq(cpu); @@ -5821,6 +5826,7 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
capacity >>= SCHED_CAPACITY_SHIFT;
+ cpu_rq(cpu)->cpu_capacity_orig = capacity; sdg->sgc->capacity_orig = capacity;
capacity *= scale_rt_capacity(cpu); @@ -5875,7 +5881,7 @@ void update_group_capacity(struct sched_domain *sd, int cpu) * Runtime updates will correct capacity_orig. */ if (unlikely(!rq->sd)) { - capacity_orig += capacity_of(cpu); + capacity_orig += capacity_orig_of(cpu); capacity += capacity_of(cpu); continue; } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index fc5b152..aaaa3e4 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -585,6 +585,7 @@ struct rq { struct sched_domain *sd;
unsigned long cpu_capacity; + unsigned long cpu_capacity_orig;
unsigned char idle_balance; /* For active balancing */
Monitor the usage level of each group of each sched_domain level. The usage is the portion of cpu_capacity_orig that is currently used on a CPU or group of CPUs. We use the utilization_load_avg to evaluate the usage level of each group.
The utilization_load_avg only takes into account the running time of the CFS tasks on a CPU with a maximum value of SCHED_LOAD_SCALE when the CPU is fully utilized. Nevertheless, we must cap utilization_load_avg which can be temporaly greater than SCHED_LOAD_SCALE after the migration of a task on this CPU and until the metrics are stabilized.
The utilization_load_avg is in the range [0..SCHED_LOAD_SCALE] to reflect the running load on the CPU whereas the available capacity for the CFS task is in the range [0..cpu_capacity_orig]. In order to test if a CPU is fully utilized by CFS tasks, we have to scale the utilization in the cpu_capacity_orig range of the CPU to get the usage of the latter. The usage can then be compared with the available capacity (ie cpu_capacity) to deduct the usage level of a CPU.
The frequency scaling invariance of the usage is not taken into account in this patch, it will be solved in another patch which will deal with frequency scaling invariance on the running_load_avg.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4782733..884578e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4559,6 +4559,33 @@ static int select_idle_sibling(struct task_struct *p, int target) done: return target; } +/* + * get_cpu_usage returns the amount of capacity of a CPU that is used by CFS + * tasks. The unit of the return value must capacity so we can compare the + * usage with the capacity of the CPU that is available for CFS task (ie + * cpu_capacity). + * cfs.utilization_load_avg is the sum of running time of runnable tasks on a + * CPU. It represents the amount of utilization of a CPU in the range + * [0..SCHED_LOAD_SCALE]. The usage of a CPU can't be higher than the full + * capacity of the CPU because it's about the running time on this CPU. + * Nevertheless, cfs.utilization_load_avg can be higher than SCHED_LOAD_SCALE + * because of unfortunate rounding in avg_period and running_load_avg or just + * after migrating tasks until the average stabilizes with the new running + * time. So we need to check that the usage stays into the range + * [0..cpu_capacity_orig] and cap if necessary. + * Without capping the usage, a group could be seen as overloaded (CPU0 usage + * at 121% + CPU1 usage at 80%) whereas CPU1 has 20% of available capacity/ + */ +static int get_cpu_usage(int cpu) +{ + unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg; + unsigned long capacity = capacity_orig_of(cpu); + + if (usage >= SCHED_LOAD_SCALE) + return capacity; + + return (usage * capacity) >> SCHED_LOAD_SHIFT; +}
/* * select_task_rq_fair: Select target runqueue for the waking task in domains @@ -5688,6 +5715,7 @@ struct sg_lb_stats { unsigned long sum_weighted_load; /* Weighted load of group's tasks */ unsigned long load_per_task; unsigned long group_capacity; + unsigned long group_usage; /* Total usage of the group */ unsigned int sum_nr_running; /* Nr tasks running in the group */ unsigned int group_capacity_factor; unsigned int idle_cpus; @@ -6036,6 +6064,7 @@ static inline void update_sg_lb_stats(struct lb_env *env, load = source_load(i, load_idx);
sgs->group_load += load; + sgs->group_usage += get_cpu_usage(i); sgs->sum_nr_running += rq->cfs.h_nr_running;
if (rq->nr_running > 1)
The scheduler tries to compute how many tasks a group of CPUs can handle by assuming that a task's load is SCHED_LOAD_SCALE and a CPU's capacity is SCHED_CAPACITY_SCALE. group_capacity_factor divides the capacity of the group by SCHED_LOAD_SCALE to estimate how many task can run in the group. Then, it compares this value with the sum of nr_running to decide if the group is overloaded or not. But the group_capacity_factor is hardly working for SMT system, it sometimes works for big cores but fails to do the right thing for little cores.
Below are two examples to illustrate the problem that this patch solves:
1- If the original capacity of a CPU is less than SCHED_CAPACITY_SCALE (640 as an example), a group of 3 CPUS will have a max capacity_factor of 2 (div_round_closest(3x640/1024) = 2) which means that it will be seen as overloaded even if we have only one task per CPU.
2 - If the original capacity of a CPU is greater than SCHED_CAPACITY_SCALE (1512 as an example), a group of 4 CPUs will have a capacity_factor of 4 (at max and thanks to the fix [0] for SMT system that prevent the apparition of ghost CPUs) but if one CPU is fully used by rt tasks (and its capacity is reduced to nearly nothing), the capacity factor of the group will still be 4 (div_round_closest(3*1512/1024) = 5 which is cap to 4 with [0]).
So, this patch tries to solve this issue by removing capacity_factor and replacing it with the 2 following metrics : -The available CPU's capacity for CFS tasks which is already used by load_balance. -The usage of the CPU by the CFS tasks. For the latter, utilization_avg_contrib has been re-introduced to compute the usage of a CPU by CFS tasks.
group_capacity_factor and group_has_free_capacity has been removed and replaced by group_no_capacity. We compare the number of task with the number of CPUs and we evaluate the level of utilization of the CPUs to define if a group is overloaded or if a group has capacity to handle more tasks.
For SD_PREFER_SIBLING, a group is tagged overloaded if it has more than 1 task so it will be selected in priority (among the overloaded groups). Since [1], SD_PREFER_SIBLING is no more concerned by the computation of load_above_capacity because local is not overloaded.
Finally, the sched_group->sched_group_capacity->capacity_orig has been removed because it's no more used during load balance.
[1] https://lkml.org/lkml/2014/8/12/295
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/core.c | 12 ----- kernel/sched/fair.c | 150 +++++++++++++++++++++++++-------------------------- kernel/sched/sched.h | 2 +- 3 files changed, 75 insertions(+), 89 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 45ae52d..37fb92c 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5373,17 +5373,6 @@ static int sched_domain_debug_one(struct sched_domain *sd, int cpu, int level, break; }
- /* - * Even though we initialize ->capacity to something semi-sane, - * we leave capacity_orig unset. This allows us to detect if - * domain iteration is still funny without causing /0 traps. - */ - if (!group->sgc->capacity_orig) { - printk(KERN_CONT "\n"); - printk(KERN_ERR "ERROR: domain->cpu_capacity not set\n"); - break; - } - if (!cpumask_weight(sched_group_cpus(group))) { printk(KERN_CONT "\n"); printk(KERN_ERR "ERROR: empty group\n"); @@ -5868,7 +5857,6 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu) * die on a /0 trap. */ sg->sgc->capacity = SCHED_CAPACITY_SCALE * cpumask_weight(sg_span); - sg->sgc->capacity_orig = sg->sgc->capacity;
/* * Make sure the first group of this domain contains the diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 884578e..db392a6 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5717,11 +5717,10 @@ struct sg_lb_stats { unsigned long group_capacity; unsigned long group_usage; /* Total usage of the group */ unsigned int sum_nr_running; /* Nr tasks running in the group */ - unsigned int group_capacity_factor; unsigned int idle_cpus; unsigned int group_weight; enum group_type group_type; - int group_has_free_capacity; + int group_no_capacity; #ifdef CONFIG_NUMA_BALANCING unsigned int nr_numa_running; unsigned int nr_preferred_running; @@ -5855,7 +5854,6 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu) capacity >>= SCHED_CAPACITY_SHIFT;
cpu_rq(cpu)->cpu_capacity_orig = capacity; - sdg->sgc->capacity_orig = capacity;
capacity *= scale_rt_capacity(cpu); capacity >>= SCHED_CAPACITY_SHIFT; @@ -5871,7 +5869,7 @@ void update_group_capacity(struct sched_domain *sd, int cpu) { struct sched_domain *child = sd->child; struct sched_group *group, *sdg = sd->groups; - unsigned long capacity, capacity_orig; + unsigned long capacity; unsigned long interval;
interval = msecs_to_jiffies(sd->balance_interval); @@ -5883,7 +5881,7 @@ void update_group_capacity(struct sched_domain *sd, int cpu) return; }
- capacity_orig = capacity = 0; + capacity = 0;
if (child->flags & SD_OVERLAP) { /* @@ -5903,19 +5901,15 @@ void update_group_capacity(struct sched_domain *sd, int cpu) * Use capacity_of(), which is set irrespective of domains * in update_cpu_capacity(). * - * This avoids capacity/capacity_orig from being 0 and + * This avoids capacity from being 0 and * causing divide-by-zero issues on boot. - * - * Runtime updates will correct capacity_orig. */ if (unlikely(!rq->sd)) { - capacity_orig += capacity_orig_of(cpu); capacity += capacity_of(cpu); continue; }
sgc = rq->sd->groups->sgc; - capacity_orig += sgc->capacity_orig; capacity += sgc->capacity; } } else { @@ -5926,39 +5920,24 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
group = child->groups; do { - capacity_orig += group->sgc->capacity_orig; capacity += group->sgc->capacity; group = group->next; } while (group != child->groups); }
- sdg->sgc->capacity_orig = capacity_orig; sdg->sgc->capacity = capacity; }
/* - * Try and fix up capacity for tiny siblings, this is needed when - * things like SD_ASYM_PACKING need f_b_g to select another sibling - * which on its own isn't powerful enough. - * - * See update_sd_pick_busiest() and check_asym_packing(). + * Check whether the capacity of the rq has been noticeably reduced by side + * activity. The imbalance_pct is used for the threshold. + * Return true is the capacity is reduced */ static inline int -fix_small_capacity(struct sched_domain *sd, struct sched_group *group) +check_cpu_capacity(struct rq *rq, struct sched_domain *sd) { - /* - * Only siblings can have significantly less than SCHED_CAPACITY_SCALE - */ - if (!(sd->flags & SD_SHARE_CPUCAPACITY)) - return 0; - - /* - * If ~90% of the cpu_capacity is still there, we're good. - */ - if (group->sgc->capacity * 32 > group->sgc->capacity_orig * 29) - return 1; - - return 0; + return ((rq->cpu_capacity * sd->imbalance_pct) < + (rq->cpu_capacity_orig * 100)); }
/* @@ -5996,37 +5975,54 @@ static inline int sg_imbalanced(struct sched_group *group) }
/* - * Compute the group capacity factor. - * - * Avoid the issue where N*frac(smt_capacity) >= 1 creates 'phantom' cores by - * first dividing out the smt factor and computing the actual number of cores - * and limit unit capacity with that. + * group_has_capacity returns true if the group has spare capacity that could + * be used by some tasks. We consider that a group has spare capacity if the + * number of task is smaller than the number of CPUs or if the usage is lower + * than the available capacity for CFS tasks. For the latter, we use a + * threshold to stabilize the state, to take into account the variance of the + * tasks' load and to return true if the available capacity in meaningful for + * the load balancer. As an example, an available capacity of 1% can appear + * but it doesn't make any benefit for the load balance. */ -static inline int sg_capacity_factor(struct lb_env *env, struct sched_group *group) +static inline bool +group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs) { - unsigned int capacity_factor, smt, cpus; - unsigned int capacity, capacity_orig; + if ((sgs->group_capacity * 100) > + (sgs->group_usage * env->sd->imbalance_pct)) + return true;
- capacity = group->sgc->capacity; - capacity_orig = group->sgc->capacity_orig; - cpus = group->group_weight; + if (sgs->sum_nr_running < sgs->group_weight) + return true; + + return false; +}
- /* smt := ceil(cpus / capacity), assumes: 1 < smt_capacity < 2 */ - smt = DIV_ROUND_UP(SCHED_CAPACITY_SCALE * cpus, capacity_orig); - capacity_factor = cpus / smt; /* cores */ +/* + * group_is_overloaded returns true if the group has more tasks than it can + * handle. We consider that a group is overloaded if the number of tasks is + * greater than the number of CPUs and the tasks already use all available + * capacity for CFS tasks. For the latter, we use a threshold to stabilize + * the state, to take into account the variance of tasks' load and to return + * true if available capacity is no more meaningful for load balancer + */ +static inline bool +group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs) +{ + if (sgs->sum_nr_running <= sgs->group_weight) + return false;
- capacity_factor = min_t(unsigned, - capacity_factor, DIV_ROUND_CLOSEST(capacity, SCHED_CAPACITY_SCALE)); - if (!capacity_factor) - capacity_factor = fix_small_capacity(env->sd, group); + if ((sgs->group_capacity * 100) < + (sgs->group_usage * env->sd->imbalance_pct)) + return true;
- return capacity_factor; + return false; }
-static enum group_type -group_classify(struct sched_group *group, struct sg_lb_stats *sgs) +static enum group_type group_classify(struct lb_env *env, + struct sched_group *group, + struct sg_lb_stats *sgs) { - if (sgs->sum_nr_running > sgs->group_capacity_factor) + if (sgs->group_no_capacity) return group_overloaded;
if (sg_imbalanced(group)) @@ -6087,11 +6083,9 @@ static inline void update_sg_lb_stats(struct lb_env *env, sgs->load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
sgs->group_weight = group->group_weight; - sgs->group_capacity_factor = sg_capacity_factor(env, group); - sgs->group_type = group_classify(group, sgs);
- if (sgs->group_capacity_factor > sgs->sum_nr_running) - sgs->group_has_free_capacity = 1; + sgs->group_no_capacity = group_is_overloaded(env, sgs); + sgs->group_type = group_classify(env, group, sgs); }
/** @@ -6213,17 +6207,20 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
/* * In case the child domain prefers tasks go to siblings - * first, lower the sg capacity factor to one so that we'll try + * first, lower the sg capacity so that we'll try * and move all the excess tasks away. We lower the capacity * of a group only if the local group has the capacity to fit - * these excess tasks, i.e. nr_running < group_capacity_factor. The - * extra check prevents the case where you always pull from the - * heaviest group when it is already under-utilized (possible - * with a large weight task outweighs the tasks on the system). + * these excess tasks. The extra check prevents the case where + * you always pull from the heaviest group when it is already + * under-utilized (possible with a large weight task outweighs + * the tasks on the system). */ if (prefer_sibling && sds->local && - sds->local_stat.group_has_free_capacity) - sgs->group_capacity_factor = min(sgs->group_capacity_factor, 1U); + group_has_capacity(env, &sds->local_stat) && + (sgs->sum_nr_running > 1)) { + sgs->group_no_capacity = 1; + sgs->group_type = group_overloaded; + }
if (update_sd_pick_busiest(env, sds, sg, sgs)) { sds->busiest = sg; @@ -6402,11 +6399,12 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s */ if (busiest->group_type == group_overloaded && local->group_type == group_overloaded) { - load_above_capacity = - (busiest->sum_nr_running - busiest->group_capacity_factor); - - load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_CAPACITY_SCALE); - load_above_capacity /= busiest->group_capacity; + load_above_capacity = busiest->sum_nr_running * + SCHED_LOAD_SCALE; + if (load_above_capacity > busiest->group_capacity) + load_above_capacity -= busiest->group_capacity; + else + load_above_capacity = ~0UL; }
/* @@ -6469,6 +6467,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env) local = &sds.local_stat; busiest = &sds.busiest_stat;
+ /* ASYM feature bypasses nice load balance check */ if ((env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE) && check_asym_packing(env, &sds)) return sds.busiest; @@ -6489,8 +6488,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env) goto force_balance;
/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */ - if (env->idle == CPU_NEWLY_IDLE && local->group_has_free_capacity && - !busiest->group_has_free_capacity) + if (env->idle == CPU_NEWLY_IDLE && group_has_capacity(env, local) && + busiest->group_no_capacity) goto force_balance;
/* @@ -6549,7 +6548,7 @@ static struct rq *find_busiest_queue(struct lb_env *env, int i;
for_each_cpu_and(i, sched_group_cpus(group), env->cpus) { - unsigned long capacity, capacity_factor, wl; + unsigned long capacity, wl; enum fbq_type rt;
rq = cpu_rq(i); @@ -6578,9 +6577,6 @@ static struct rq *find_busiest_queue(struct lb_env *env, continue;
capacity = capacity_of(i); - capacity_factor = DIV_ROUND_CLOSEST(capacity, SCHED_CAPACITY_SCALE); - if (!capacity_factor) - capacity_factor = fix_small_capacity(env->sd, group);
wl = weighted_cpuload(i);
@@ -6588,7 +6584,9 @@ static struct rq *find_busiest_queue(struct lb_env *env, * When comparing with imbalance, use weighted_cpuload() * which is not scaled with the cpu capacity. */ - if (capacity_factor && rq->nr_running == 1 && wl > env->imbalance) + + if (rq->nr_running == 1 && wl > env->imbalance && + !check_cpu_capacity(rq, env->sd)) continue;
/* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index aaaa3e4..8fd30c1 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -759,7 +759,7 @@ struct sched_group_capacity { * CPU capacity of this group, SCHED_LOAD_SCALE being max capacity * for a single CPU. */ - unsigned int capacity, capacity_orig; + unsigned int capacity; unsigned long next_update; int imbalance; /* XXX unrelated to capacity but shared group state */ /*
On Thu, Jan 15, 2015 at 11:09:28AM +0100, Vincent Guittot wrote:
Finally, the sched_group->sched_group_capacity->capacity_orig has been removed because it's no more used during load balance.
Maybe do that in a separate patch to avoid cluttering this one?
Patch references are like: 9a5d9ba6a363 ("sched/fair: Allow calculate_imbalance() to move idle cpus")
/*
- Check whether the capacity of the rq has been noticeably reduced by side
- activity. The imbalance_pct is used for the threshold.
*/
- Return true is the capacity is reduced
static inline int +check_cpu_capacity(struct rq *rq, struct sched_domain *sd) {
- return ((rq->cpu_capacity * sd->imbalance_pct) <
(rq->cpu_capacity_orig * 100));}
How about cpu_has_capacity() to be consistent with the below function?
This comment could use whitespace:
/*
- group_has_capacity returns true if the group has spare capacity that could
- be used by some tasks.
We consider that a group has spare capacity if the
- number of task is smaller than the number of CPUs or if the usage is lower
- than the available capacity for CFS tasks.
For the latter, we use a
- threshold to stabilize the state, to take into account the variance of the
- tasks' load and to return true if the available capacity in meaningful for
- the load balancer.
As an example, an available capacity of 1% can appear
*/
- but it doesn't make any benefit for the load balance.
+static inline bool +group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs) {
- if ((sgs->group_capacity * 100) >
(sgs->group_usage * env->sd->imbalance_pct))return true;
- if (sgs->sum_nr_running < sgs->group_weight)
return true;- return false;
+}
Would it not make sense to first do the nr_running test, its cheaper than the multiplication thing.
+/*
- group_is_overloaded returns true if the group has more tasks than it can
- handle.
We consider that a group is overloaded if the number of tasks is
- greater than the number of CPUs and the tasks already use all available
- capacity for CFS tasks.
For the latter, we use a threshold to stabilize
- the state, to take into account the variance of tasks' load and to return
- true if available capacity is no more meaningful for load balancer
- */
+static inline bool +group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs) +{
- if (sgs->sum_nr_running <= sgs->group_weight)
return false;
- if ((sgs->group_capacity * 100) <
(sgs->group_usage * env->sd->imbalance_pct))return true;
- return false;
}
Maybe a note on the difference between group_is_overloaded() and !group_has_capacity()?
As to the comment, I think it can be reduced by referring to the comment of group_has_capacity().
/* * In case the child domain prefers tasks go to siblings
* first, lower the sg capacity so that we'll try
- and move all the excess tasks away. We lower the capacity
- of a group only if the local group has the capacity to fit
* these excess tasks.
The extra check prevents the case where
* you always pull from the heaviest group when it is already* under-utilized (possible with a large weight task outweighs* the tasks on the system).group_has_capacity(env, &sds->local_stat) &&(sgs->sum_nr_running > 1)) {sgs->group_no_capacity = 1;sgs->group_type = group_overloaded;}
Looks OK otherwise I suppose.
On 20 February 2015 at 12:14, Peter Zijlstra peterz@infradead.org wrote:
On Thu, Jan 15, 2015 at 11:09:28AM +0100, Vincent Guittot wrote:
Finally, the sched_group->sched_group_capacity->capacity_orig has been removed because it's no more used during load balance.
Maybe do that in a separate patch to avoid cluttering this one?
ok
Patch references are like: 9a5d9ba6a363 ("sched/fair: Allow calculate_imbalance() to move idle cpus")
ok, I'm going to update the reference
/*
- Check whether the capacity of the rq has been noticeably reduced by side
- activity. The imbalance_pct is used for the threshold.
*/
- Return true is the capacity is reduced
static inline int +check_cpu_capacity(struct rq *rq, struct sched_domain *sd) {
return ((rq->cpu_capacity * sd->imbalance_pct) <(rq->cpu_capacity_orig * 100));}
How about cpu_has_capacity() to be consistent with the below function?
I can change the name for consistency but the 2 function are not testing the same thing , so i would not create any confusion. group_has_capacity tests if some capacity is not used whereas check_cpu_capacity/cpu_has_capacity check if the capacity has been reduced by side activity but doesn't give information about spare capacity.
This comment could use whitespace:
/*
- group_has_capacity returns true if the group has spare capacity that could
- be used by some tasks.
We consider that a group has spare capacity if the
- number of task is smaller than the number of CPUs or if the usage is lower
- than the available capacity for CFS tasks.
For the latter, we use a
- threshold to stabilize the state, to take into account the variance of the
- tasks' load and to return true if the available capacity in meaningful for
- the load balancer.
As an example, an available capacity of 1% can appear
*/
- but it doesn't make any benefit for the load balance.
+static inline bool +group_has_capacity(struct lb_env *env, struct sg_lb_stats *sgs) {
if ((sgs->group_capacity * 100) >(sgs->group_usage * env->sd->imbalance_pct))return true;if (sgs->sum_nr_running < sgs->group_weight)return true;return false;+}
Would it not make sense to first do the nr_running test, its cheaper than the multiplication thing.
good point, i will reorder the test
+/*
- group_is_overloaded returns true if the group has more tasks than it can
- handle.
We consider that a group is overloaded if the number of tasks is
- greater than the number of CPUs and the tasks already use all available
- capacity for CFS tasks.
For the latter, we use a threshold to stabilize
- the state, to take into account the variance of tasks' load and to return
- true if available capacity is no more meaningful for load balancer
- */
+static inline bool +group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs) +{
if (sgs->sum_nr_running <= sgs->group_weight)return false;if ((sgs->group_capacity * 100) <(sgs->group_usage * env->sd->imbalance_pct))return true;return false;}
Maybe a note on the difference between group_is_overloaded() and !group_has_capacity()?
ok, i will add a comment
As to the comment, I think it can be reduced by referring to the comment of group_has_capacity().
ok, i 'm going to update it
/* * In case the child domain prefers tasks go to siblings
* first, lower the sg capacity so that we'll try * and move all the excess tasks away. We lower the capacity * of a group only if the local group has the capacity to fit* these excess tasks.The extra check prevents the case where
* you always pull from the heaviest group when it is already* under-utilized (possible with a large weight task outweighs* the tasks on the system). */ if (prefer_sibling && sds->local &&group_has_capacity(env, &sds->local_stat) &&(sgs->sum_nr_running > 1)) {sgs->group_no_capacity = 1;sgs->group_type = group_overloaded;}Looks OK otherwise I suppose.
Add the SD_PREFER_SIBLING flag for SMT level in order to ensure that the scheduler will put at least 1 task per core.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org Reviewed-by: Preeti U. Murthy preeti@linux.vnet.ibm.com --- kernel/sched/core.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 37fb92c..731f2ad 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6165,6 +6165,7 @@ sd_init(struct sched_domain_topology_level *tl, int cpu) */
if (sd->flags & SD_SHARE_CPUCAPACITY) { + sd->flags |= SD_PREFER_SIBLING; sd->imbalance_pct = 110; sd->smt_gain = 1178; /* ~15% */
When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining capacity for CFS tasks can be significantly reduced. Once we detect such situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle load balance to check if it's worth moving its tasks on an idle CPU.
Once the idle load_balance has selected the busiest CPU, it will look for an active load balance for only two cases : - there is only 1 task on the busiest CPU. - we haven't been able to move a task of the busiest rq.
A CPU with a reduced capacity is included in the 1st case, and it's worth to actively migrate its task if the idle CPU has got full capacity. This test has been added in need_active_balance.
As a sidenote, this will note generate more spurious ilb because we already trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that has a task, we will trig the ilb once for migrating the task.
The nohz_kick_needed function has been cleaned up a bit while adding the new test
env.src_cpu and env.src_rq must be set unconditionnally because they are used in need_active_balance which is called even if busiest->nr_running equals 1
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 53 insertions(+), 21 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index db392a6..02e8f7f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6634,6 +6634,28 @@ static int need_active_balance(struct lb_env *env) return 1; }
+ /* + * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task. + * It's worth migrating the task if the src_cpu's capacity is reduced + * because of other sched_class or IRQs whereas capacity stays + * available on dst_cpu. + */ + if ((env->idle != CPU_NOT_IDLE) && + (env->src_rq->cfs.h_nr_running == 1)) { + unsigned long src_eff_capacity, dst_eff_capacity; + + dst_eff_capacity = 100; + dst_eff_capacity *= capacity_of(env->dst_cpu); + dst_eff_capacity *= capacity_orig_of(env->src_cpu); + + src_eff_capacity = sd->imbalance_pct; + src_eff_capacity *= capacity_of(env->src_cpu); + src_eff_capacity *= capacity_orig_of(env->dst_cpu); + + if (src_eff_capacity < dst_eff_capacity) + return 1; + } + return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2); }
@@ -6733,6 +6755,9 @@ static int load_balance(int this_cpu, struct rq *this_rq,
schedstat_add(sd, lb_imbalance[idle], env.imbalance);
+ env.src_cpu = busiest->cpu; + env.src_rq = busiest; + ld_moved = 0; if (busiest->nr_running > 1) { /* @@ -6742,8 +6767,6 @@ static int load_balance(int this_cpu, struct rq *this_rq, * correctly treated as an imbalance. */ env.flags |= LBF_ALL_PINNED; - env.src_cpu = busiest->cpu; - env.src_rq = busiest; env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
more_balance: @@ -7443,22 +7466,25 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
/* * Current heuristic for kicking the idle load balancer in the presence - * of an idle cpu is the system. + * of an idle cpu in the system. * - This rq has more than one task. - * - At any scheduler domain level, this cpu's scheduler group has multiple - * busy cpu's exceeding the group's capacity. + * - This rq has at least one CFS task and the capacity of the CPU is + * significantly reduced because of RT tasks or IRQs. + * - At parent of LLC scheduler domain level, this cpu's scheduler group has + * multiple busy cpu. * - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler * domain span are idle. */ -static inline int nohz_kick_needed(struct rq *rq) +static inline bool nohz_kick_needed(struct rq *rq) { unsigned long now = jiffies; struct sched_domain *sd; struct sched_group_capacity *sgc; int nr_busy, cpu = rq->cpu; + bool kick = false;
if (unlikely(rq->idle_balance)) - return 0; + return false;
/* * We may be recently in ticked or tickless idle mode. At the first @@ -7472,38 +7498,44 @@ static inline int nohz_kick_needed(struct rq *rq) * balancing. */ if (likely(!atomic_read(&nohz.nr_cpus))) - return 0; + return false;
if (time_before(now, nohz.next_balance)) - return 0; + return false;
if (rq->nr_running >= 2) - goto need_kick; + return true;
rcu_read_lock(); sd = rcu_dereference(per_cpu(sd_busy, cpu)); - if (sd) { sgc = sd->groups->sgc; nr_busy = atomic_read(&sgc->nr_busy_cpus);
- if (nr_busy > 1) - goto need_kick_unlock; + if (nr_busy > 1) { + kick = true; + goto unlock; + } + }
- sd = rcu_dereference(per_cpu(sd_asym, cpu)); + sd = rcu_dereference(rq->sd); + if (sd) { + if ((rq->cfs.h_nr_running >= 1) && + check_cpu_capacity(rq, sd)) { + kick = true; + goto unlock; + } + }
+ sd = rcu_dereference(per_cpu(sd_asym, cpu)); if (sd && (cpumask_first_and(nohz.idle_cpus_mask, sched_domain_span(sd)) < cpu)) - goto need_kick_unlock; + kick = true;
+unlock: rcu_read_unlock(); - return 0; - -need_kick_unlock: - rcu_read_unlock(); -need_kick: - return 1; + return kick; } #else static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) { }
On Thu, Jan 15, 2015 at 11:09:30AM +0100, Vincent Guittot wrote:
As a sidenote, this will note generate more spurious ilb because we already
s/note/not/
trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that has a task, we will trig the ilb once for migrating the task.
+static inline bool nohz_kick_needed(struct rq *rq) { unsigned long now = jiffies; struct sched_domain *sd; struct sched_group_capacity *sgc; int nr_busy, cpu = rq->cpu;
- bool kick = false;
if (unlikely(rq->idle_balance))
return false;/*
- We may be recently in ticked or tickless idle mode. At the first
@@ -7472,38 +7498,44 @@ static inline int nohz_kick_needed(struct rq *rq) * balancing. */ if (likely(!atomic_read(&nohz.nr_cpus)))
return false;if (time_before(now, nohz.next_balance))
return false;if (rq->nr_running >= 2)
return true;
So this,
rcu_read_lock(); sd = rcu_dereference(per_cpu(sd_busy, cpu)); if (sd) { sgc = sd->groups->sgc; nr_busy = atomic_read(&sgc->nr_busy_cpus);
if (nr_busy > 1) {kick = true;goto unlock;}- }
- sd = rcu_dereference(rq->sd);
- if (sd) {
if ((rq->cfs.h_nr_running >= 1) &&check_cpu_capacity(rq, sd)) {kick = true;goto unlock;}- }
vs this: how would we ever get here?
If h_nr_running > 1, must then not nr_running > 1 as well?
- sd = rcu_dereference(per_cpu(sd_asym, cpu)); if (sd && (cpumask_first_and(nohz.idle_cpus_mask, sched_domain_span(sd)) < cpu))
kick = true;
For consistencies sake I would've added a goto unlock here as well.
+unlock: rcu_read_unlock();
- return kick;
}
On 20 February 2015 at 12:27, Peter Zijlstra peterz@infradead.org wrote:
On Thu, Jan 15, 2015 at 11:09:30AM +0100, Vincent Guittot wrote:
As a sidenote, this will note generate more spurious ilb because we already
s/note/not/
trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that has a task, we will trig the ilb once for migrating the task.
+static inline bool nohz_kick_needed(struct rq *rq) { unsigned long now = jiffies; struct sched_domain *sd; struct sched_group_capacity *sgc; int nr_busy, cpu = rq->cpu;
bool kick = false; if (unlikely(rq->idle_balance))return false; /* * We may be recently in ticked or tickless idle mode. At the first@@ -7472,38 +7498,44 @@ static inline int nohz_kick_needed(struct rq *rq) * balancing. */ if (likely(!atomic_read(&nohz.nr_cpus)))
return false; if (time_before(now, nohz.next_balance))return false; if (rq->nr_running >= 2)return true;So this,
rcu_read_lock(); sd = rcu_dereference(per_cpu(sd_busy, cpu)); if (sd) { sgc = sd->groups->sgc; nr_busy = atomic_read(&sgc->nr_busy_cpus);
if (nr_busy > 1) {kick = true;goto unlock;}}sd = rcu_dereference(rq->sd);if (sd) {if ((rq->cfs.h_nr_running >= 1) &&check_cpu_capacity(rq, sd)) {kick = true;goto unlock;}}vs this: how would we ever get here?
If h_nr_running > 1, must then not nr_running > 1 as well?
you're right, but the test above can trig a kick with h_nr_running == 1 whereas the other tests may not
sd = rcu_dereference(per_cpu(sd_asym, cpu)); if (sd && (cpumask_first_and(nohz.idle_cpus_mask, sched_domain_span(sd)) < cpu))kick = true;For consistencies sake I would've added a goto unlock here as well.
i can add the goto
+unlock: rcu_read_unlock();
return kick;}
On Fri, Feb 20, 2015 at 02:54:09PM +0100, Vincent Guittot wrote:
if (rq->nr_running >= 2)
return true;So this,
sd = rcu_dereference(rq->sd);if (sd) {if ((rq->cfs.h_nr_running >= 1) &&check_cpu_capacity(rq, sd)) {kick = true;goto unlock;}}vs this: how would we ever get here?
If h_nr_running > 1, must then not nr_running > 1 as well?
you're right, but the test above can trig a kick with h_nr_running == 1 whereas the other tests may not
Duh, clearly I cannot read today.
On Thu, Jan 15, 2015 at 10:09:20AM +0000, Vincent Guittot wrote:
This patchset consolidates several changes in the capacity and the usage tracking of the CPU. It provides a frequency invariant metric of the usage of CPUs and generally improves the accuracy of load/usage tracking in the scheduler. The frequency invariant metric is the foundation required for the consolidation of cpufreq and implementation of a fully invariant load tracking. These are currently WIP and require several changes to the load balancer (including how it will use and interprets load and capacity metrics) and extensive validation. The frequency invariance is done with arch_scale_freq_capacity and this patchset doesn't provide the backends of the function which are architecture dependent.
As discussed at LPC14, Morten and I have consolidated our changes into a single patchset to make it easier to review and merge.
I'm happy with patch 1, 3, 5, 6, and 7. Add my acked-by if you like :)
The last few needs buy-in from somebody running SMT systems I think.
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
Morten
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
On 20 February 2015 at 15:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is
we want to trig the migration before the cpu is fully utilized by rt/irq (which almost never occurs)
currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig) enables us to know if the cpu is significantly used by irq/rt so it's worth to do an active load balance of the task. Then the absolute comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu checks that the dst_cpu is a better choice
something like : if ((check_cpu_capacity(src_rq, sd)) && (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100)) return 1;
On Fri, Feb 20, 2015 at 02:54:09PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 15:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
Also, it still not clear why patch 10 uses relative capacity reduction instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is
we want to trig the migration before the cpu is fully utilized by rt/irq (which almost never occurs)
I meant fully utilized by rt/irq and cfs tasks, sorry. Essentially, get_cpu_usage() ~= capacity_of(). If get_cpu_usage() is signficantly smaller than capacity_of() which is may be reduced by rt/irq utilization, there are still spare cycles and it is not strictly required to migrate tasks away using active LB. But, tasks would be moved away if the tasks are being allowed less cpu time due to rt/irq (get_cpu_usage() >= capacity_of()). Wouldn't that work? Or, do you want to migrate tasks regardless of whether there are still spare cycles available on the cpu doing rt/irq work?
The advantage of comparing get_cpu_usage() with capacity_of() is that it would work for migrating cpu-intensive tasks away from little cpu on big.LITTLE as well. Then we don't need another almost identical check for that purpose :)
currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig) enables us to know if the cpu is significantly used by irq/rt so it's worth to do an active load balance of the task. Then the absolute comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu checks that the dst_cpu is a better choice
something like : if ((check_cpu_capacity(src_rq, sd)) && (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100)) return 1;
It should solve the big.LITTLE issue. Though I would prefer get_cpu_usage() ~= capacity_of() approach as it could even improve performance on big.LITTLE.
On 23 February 2015 at 16:45, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:54:09PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 15:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
> Also, it still not clear why patch 10 uses relative capacity reduction > instead of absolute capacity available to CFS tasks.
As present in your asymmetric big and small systems? Yes it would be unfortunate to migrate a task to an idle small core when the big core is still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is
we want to trig the migration before the cpu is fully utilized by rt/irq (which almost never occurs)
I meant fully utilized by rt/irq and cfs tasks, sorry. Essentially, get_cpu_usage() ~= capacity_of(). If get_cpu_usage() is signficantly smaller than capacity_of() which is may be reduced by rt/irq utilization, there are still spare cycles and it is not strictly required to migrate tasks away using active LB. But, tasks would be moved away if the tasks are being allowed less cpu time due to rt/irq (get_cpu_usage() >= capacity_of()). Wouldn't that work? Or, do you want to migrate tasks regardless of whether there are still spare cycles available on the cpu doing rt/irq work?
In fact, we can see perf improvement even if the cpu is not fully used by thread and interrupts because the task becomes significantly preempted by interruptions.
The advantage of comparing get_cpu_usage() with capacity_of() is that it would work for migrating cpu-intensive tasks away from little cpu on big.LITTLE as well. Then we don't need another almost identical check for that purpose :)
I understand your point but the patch becomes inefficient for part of the issue that it's trying to originally solve if we compare get_cpu_usage with capacity_of. So we will probably need to add few more tests for the issue you point out above
currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig) enables us to know if the cpu is significantly used by irq/rt so it's worth to do an active load balance of the task. Then the absolute comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu checks that the dst_cpu is a better choice
something like : if ((check_cpu_capacity(src_rq, sd)) && (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100)) return 1;
It should solve the big.LITTLE issue. Though I would prefer get_cpu_usage() ~= capacity_of() approach as it could even improve performance on big.LITTLE.
ok. IMHO, it's worth having a dedicated patch for this issue
Vincent
On Tue, Feb 24, 2015 at 10:38:29AM +0000, Vincent Guittot wrote:
On 23 February 2015 at 16:45, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:54:09PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 15:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote: > On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote: > > > Also, it still not clear why patch 10 uses relative capacity reduction > > instead of absolute capacity available to CFS tasks. > > As present in your asymmetric big and small systems? Yes it would be > unfortunate to migrate a task to an idle small core when the big core is > still faster, even if reduced by rt/irq work.
Yes, exactly. I don't think it would cause any harm for symmetric cases to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is
we want to trig the migration before the cpu is fully utilized by rt/irq (which almost never occurs)
I meant fully utilized by rt/irq and cfs tasks, sorry. Essentially, get_cpu_usage() ~= capacity_of(). If get_cpu_usage() is signficantly smaller than capacity_of() which is may be reduced by rt/irq utilization, there are still spare cycles and it is not strictly required to migrate tasks away using active LB. But, tasks would be moved away if the tasks are being allowed less cpu time due to rt/irq (get_cpu_usage() >= capacity_of()). Wouldn't that work? Or, do you want to migrate tasks regardless of whether there are still spare cycles available on the cpu doing rt/irq work?
In fact, we can see perf improvement even if the cpu is not fully used by thread and interrupts because the task becomes significantly preempted by interruptions.
Unless the tasks are the consumers of those interrupts, then it would harm performance to migrate them away :) I get your point though. Could we have a short comment stating the intentions so we don't forget in a couple of months?
The advantage of comparing get_cpu_usage() with capacity_of() is that it would work for migrating cpu-intensive tasks away from little cpu on big.LITTLE as well. Then we don't need another almost identical check for that purpose :)
I understand your point but the patch becomes inefficient for part of the issue that it's trying to originally solve if we compare get_cpu_usage with capacity_of. So we will probably need to add few more tests for the issue you point out above
Right. If your goal is to avoid preemptions and not just make sure that cpus aren't fully utilized then my proposal isn't sufficient. We will have to add another condition to solve the big.LITTLE capacity thing later. In fact we already have that somewhere deep down in the pile of patches I posted some weeks ago.
currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig) enables us to know if the cpu is significantly used by irq/rt so it's worth to do an active load balance of the task. Then the absolute comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu checks that the dst_cpu is a better choice
something like : if ((check_cpu_capacity(src_rq, sd)) && (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100)) return 1;
It should solve the big.LITTLE issue. Though I would prefer get_cpu_usage() ~= capacity_of() approach as it could even improve performance on big.LITTLE.
ok. IMHO, it's worth having a dedicated patch for this issue
Fine by me as long as we get the extra check you proposed above to fix the big.LITTLE issue.
Morten
On 24 February 2015 at 12:29, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Tue, Feb 24, 2015 at 10:38:29AM +0000, Vincent Guittot wrote:
On 23 February 2015 at 16:45, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:54:09PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 15:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
On 20 February 2015 at 12:52, Morten Rasmussen morten.rasmussen@arm.com wrote: > On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote: >> On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote: >> >> > Also, it still not clear why patch 10 uses relative capacity reduction >> > instead of absolute capacity available to CFS tasks. >> >> As present in your asymmetric big and small systems? Yes it would be >> unfortunate to migrate a task to an idle small core when the big core is >> still faster, even if reduced by rt/irq work. > > Yes, exactly. I don't think it would cause any harm for symmetric cases > to use absolute capacity instead. Am I missing something?
If absolute capacity is used, we will trig an active load balance from little to big core each time a little has got 1 task and a big core is idle whereas we only want to trig an active migration is the src_cpu's capacity that is available for the cfs task is significantly reduced by rt tasks.
I can mix absolute and relative tests by 1st testing that the capacity of the src is reduced and then ensure that the dst_cpu has more absolute capacity than src_cpu
If we use absolute capacity and check if the source cpu is fully utilized, wouldn't that work? We want to migrate the task if it is
we want to trig the migration before the cpu is fully utilized by rt/irq (which almost never occurs)
I meant fully utilized by rt/irq and cfs tasks, sorry. Essentially, get_cpu_usage() ~= capacity_of(). If get_cpu_usage() is signficantly smaller than capacity_of() which is may be reduced by rt/irq utilization, there are still spare cycles and it is not strictly required to migrate tasks away using active LB. But, tasks would be moved away if the tasks are being allowed less cpu time due to rt/irq (get_cpu_usage() >= capacity_of()). Wouldn't that work? Or, do you want to migrate tasks regardless of whether there are still spare cycles available on the cpu doing rt/irq work?
In fact, we can see perf improvement even if the cpu is not fully used by thread and interrupts because the task becomes significantly preempted by interruptions.
Unless the tasks are the consumers of those interrupts, then it would harm performance to migrate them away :) I get your point though. Could we have a short comment stating the intentions so we don't forget in a couple of months?
I will add more details in the commit log
The advantage of comparing get_cpu_usage() with capacity_of() is that it would work for migrating cpu-intensive tasks away from little cpu on big.LITTLE as well. Then we don't need another almost identical check for that purpose :)
I understand your point but the patch becomes inefficient for part of the issue that it's trying to originally solve if we compare get_cpu_usage with capacity_of. So we will probably need to add few more tests for the issue you point out above
Right. If your goal is to avoid preemptions and not just make sure that cpus aren't fully utilized then my proposal isn't sufficient. We will have to add another condition to solve the big.LITTLE capacity thing later. In fact we already have that somewhere deep down in the pile of patches I posted some weeks ago.
currently being restricted by the available capacity (due to rt/irq work, being a little cpu, or both) and if there is a destination cpu with more absolute capacity available. No?
yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig) enables us to know if the cpu is significantly used by irq/rt so it's worth to do an active load balance of the task. Then the absolute comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu checks that the dst_cpu is a better choice
something like : if ((check_cpu_capacity(src_rq, sd)) && (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100)) return 1;
It should solve the big.LITTLE issue. Though I would prefer get_cpu_usage() ~= capacity_of() approach as it could even improve performance on big.LITTLE.
ok. IMHO, it's worth having a dedicated patch for this issue
Fine by me as long as we get the extra check you proposed above to fix the big.LITTLE issue.
ok
Morten
linaro-kernel@lists.linaro.org
-
Dietmar Eggemann -
Morten Rasmussen -
Peter Zijlstra -
Vincent Guittot