Should the subject mention that the patch adds utilization tracking? Maybe: 'sched: Add utilization tracking' ?

On Mon, Nov 03, 2014 at 04:54:38PM +0000, Vincent Guittot wrote:

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

Should we do ourselves and anybody else who feels like going through the pain of understanding the load-tracking code a favor and drop the use of the term 'usage' and use 'utilization' everywhere instead? 'usage' isn't clearly defined anywhere.

Referring to 'usage' here in the reference to original patch is fine, but I suggest that we remove it from the code and comment on subsequent patches unless there is a very good reason to keep it.

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.
  

i.e. runnable_avg_sum = running_avg_sum + 'waiting time'

     */

•   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.
  

This is slightly misleading. Blocked entities are not on the rq. Maybe say: "... except it is the blocked sched_entities associated with the rq." ?

s/usage/utilization/ in subject.

On Mon, Nov 03, 2014 at 04:54:39PM +0000, Vincent Guittot wrote:

From: Morten Rasmussen morten.rasmussen@arm.com

Adds usage contribution tracking for group entities. Unlike

s/usage contribution/utilization/

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

s/usage/utilization/

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; }

What happened to the update of se->avg.utilization_avg_contrib in __synchronize_entity_decay()? It seems to have disapperead after v7.

On 21 November 2014 at 13:35, Morten Rasmussen morten.rasmussen@arm.com wrote:

On Mon, Nov 03, 2014 at 04:54:42PM +0000, Vincent Guittot wrote:

[snip]

...

The average running time of RT tasks is used to estimate the remaining compute @@ -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);
  

I haven't looked through all the details of the rt avg tracking, but if 'used' is in the range [0..SCHED_CAPACITY_SCALE], I believe it should work. Is it guaranteed that total > 0 so we don't get division by zero?

static inline u64 sched_avg_period(void) { return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2; }

It does get a slightly more complicated if we want to figure out the available capacity at the current frequency (current < max) later. Say, rt eats 25% of the compute capacity, but the current frequency is only 50%. In that case get:

curr_avail_capacity = (arch_scale_cpu_capacity() * (arch_scale_freq_capacity() - (SCHED_SCALE_CAPACITY - scale_rt_capacity())))

...

...

SCHED_CAPACITY_SHIFT

You don't have to be so complicated but simply need to do: curr_avail_capacity for CFS = (capacity_of(CPU) * arch_scale_freq_capacity()) >> SCHED_CAPACITY_SHIFT

capacity_of(CPU) = 600 is the max available capacity for CFS tasks once we have removed the 25% of capacity that is used by RT tasks arch_scale_freq_capacity = 512 because we currently run at 50% of max freq

so curr_avail_capacity for CFS = 300

Vincent

With numbers assuming arch_scale_cpu_capacity() = 800:

curr_avail_capacity = 800 * (512 - (1024 - 758)) >> 10 = 200

Which isn't actually that bad. Anyway, it isn't needed until we start invovling energy models.

...

• 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;
  

}

On 24 November 2014 at 18:05, Morten Rasmussen morten.rasmussen@arm.com wrote:

On Mon, Nov 24, 2014 at 02:24:00PM +0000, Vincent Guittot wrote:

...

On 21 November 2014 at 13:35, Morten Rasmussen morten.rasmussen@arm.com wrote:

...

On Mon, Nov 03, 2014 at 04:54:42PM +0000, Vincent Guittot wrote:

[snip]

...

...

The average running time of RT tasks is used to estimate the remaining compute @@ -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);
  

I haven't looked through all the details of the rt avg tracking, but if 'used' is in the range [0..SCHED_CAPACITY_SCALE], I believe it should work. Is it guaranteed that total > 0 so we don't get division by zero?

static inline u64 sched_avg_period(void) { return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2; }

I see.

...

...

It does get a slightly more complicated if we want to figure out the available capacity at the current frequency (current < max) later. Say, rt eats 25% of the compute capacity, but the current frequency is only 50%. In that case get:

curr_avail_capacity = (arch_scale_cpu_capacity() * (arch_scale_freq_capacity() - (SCHED_SCALE_CAPACITY - scale_rt_capacity())))

...

...

SCHED_CAPACITY_SHIFT

You don't have to be so complicated but simply need to do: curr_avail_capacity for CFS = (capacity_of(CPU) * arch_scale_freq_capacity()) >> SCHED_CAPACITY_SHIFT

capacity_of(CPU) = 600 is the max available capacity for CFS tasks once we have removed the 25% of capacity that is used by RT tasks arch_scale_freq_capacity = 512 because we currently run at 50% of max freq

so curr_avail_capacity for CFS = 300

I don't think that is correct. It is at least not what I had in mind.

capacity_orig_of(cpu) = 800, we run at 50% frequency which means:

curr_capacity = capacity_orig_of(cpu) * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT = 400

So the total capacity at the current frequency (50%) is 400, without considering RT. scale_rt_capacity() is frequency invariant, so it takes away capacity_orig_of(cpu) - capacity_of(cpu) = 200 worth of capacity for RT. We need to subtract that from the current capacity to get the available capacity at the current frequency.

curr_available_capacity = curr_capacity - (capacity_orig_of(cpu) - capacity_of(cpu)) = 200

you're right, this one looks good to me too

In other words, 800 is the max capacity, we are currently running at 50% frequency, which gives us 400. RT takes away 25% of 800 (frequency-invariant) from the 400, which leaves us with 200 left for CFS tasks at the current frequency.

In your calculations you subtract the RT load before computing the current capacity using arch_scale_freq_capacity(), where I think it should be done after. You find the amount spare capacity you would have at the maximum frequency when RT has been subtracted and then scale the result by frequency which means indirectly scaling the RT load contribution again (the rt avg has already been scaled). So instead of taking away 200 of the 400 (current capacity @ 50% frequency), it only takes away 100 which isn't right.

scale_rt_capacity() is frequency-invariant, so if the RT load is 50% and the frequency is 50%, there are no spare cycles left. curr_avail_capacity should be 0. But using your expression above you would get capacity_of(cpu) = 400 after removing RT, arch_scale_freq_capacity = 512 and you get 200. I don't think that is right.

Morten

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Hi Vincent, On 11/4/14, 12:54 AM, 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)

The 'period' aka 'total' in the scale_rt_capacity(), why it is sched_avg_period() + delta instead of sched_avg_period()?

Regards, Wanpeng Li

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

Hi Vincent, On 11/25/14, 9:52 PM, Vincent Guittot wrote:

On 25 November 2014 at 03:24, Wanpeng Li kernellwp@gmail.com wrote:

...

Hi Vincent, On 11/4/14, 12:54 AM, 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)

The 'period' aka 'total' in the scale_rt_capacity(), why it is sched_avg_period() + delta instead of sched_avg_period()?

The default value of sched_avg_period is 1sec which is "long" so we take into account the time consumed by RT tasks in the ongoing period .

Do you mean 'sched_avg_period() + delta' should be replaced by 'delta' since sched_avg_period() is "long"?

Regards, Wanpeng Li

...

Regards, Wanpeng Li

...

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 Mon, Nov 03, 2014 at 04:54:44PM +0000, Vincent Guittot wrote:

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.

Here 'usage' is defined for the first time.

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.

So 'usage' is more precisely scaled utilization (by cpu_capacity_orig/SCHED_LOAD_SCALE). Do we need to use 'usage' to describe this?

So far we only have introduced frequency invariant load tracking, once we add uarch invariance utilization_load_avg will be in the range [0..cpu_capacity_orig] as the scaling will happen as part of the load tracking (just like the frequency invariance). Then 'usage' becomes equal to utilization_load_avg which means that there is very little reason to keep the term. No?

I haven't pointed out all uses of 'usage' in this and following patches. If 'usage' is kept the previous patches should be revisited to define it.

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

s/must/must be/

• • 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

s/ / /

• • capacity of the CPU because it's about the running time on this CPU.

Maybe add (cpu_capacity_orig) to make it clear what full capacity means.

• • 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 last two hunks do not appear to be used in this patch. Would it be better to have them with the code that uses the statistics? The patch however do what the subject says. Just a thought.

On 4 November 2014 00:54, Vincent Guittot vincent.guittot@linaro.org wrote:

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))
  

Hi Vincent,

In case of when some CPU(s) is used to handle heavy IRQs or RT tasks, get_cpu_usage() will get low usage(capacity), and cpu_capacity gets low as well, so do those of the whole group correspondingly. So in this case, is there any guarantee that this math will return false?

Thanks, Xunlei

•           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 */
    /*
  

-- 1.9.1

---

linaro-kernel mailing list linaro-kernel@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-kernel

On Mon, Nov 03, 2014 at 04:54:45PM +0000, Vincent Guittot wrote:

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.

[...]

@@ -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;
  

•           }
  

I'm still a bit confused about SD_PREFER_SIBLING. What is the flag supposed to do and why?

It looks like a weak load balancing bias attempting to consolidate tasks on domains with spare capacity. It does so by marking non-local groups as overloaded regardless of their actual load if the local group has spare capacity. Correct?

In patch 9 this behaviour is enabled for SMT level domains, which implies that tasks will be consolidated in MC groups, that is we prefer multiple tasks on sibling cpus (hw threads). I must be missing something essential. I was convinced that we wanted to avoid using sibling cpus on SMT systems as much as possible?