On 4 April 2013 19:30, Vincent Guittot vincent.guittot@linaro.org wrote:
On 4 April 2013 19:07, Frederic Weisbecker fweisbec@gmail.com wrote:
2013/4/3 Vincent Guittot vincent.guittot@linaro.org:
On my smp platform which is made of 5 cores in 2 clusters, I have the nr_busy_cpu field of sched_group_power struct that is not null when the platform is fully idle. The root cause is: During the boot sequence, some CPUs reach the idle loop and set their NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus field is initialized later with the assumption that all CPUs are in the busy state whereas some CPUs have already set their NOHZ_IDLE flag.
More generally, the NOHZ_IDLE flag must be initialized when new sched_domains are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned.
This condition can be ensured by adding a synchronize_rcu between the destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE flag will not be updated with old sched_domain once it has been initialized. But this solution introduces a additionnal latency in the rebuild sequence that is called during cpu hotplug.
As suggested by Frederic Weisbecker, another solution is to have the same rcu lifecycle for both NOHZ_IDLE and sched_domain struct. I have introduce a new sched_domain_rq struct that is the entry point for both sched_domains and objects that must follow the same lifecycle like NOHZ_IDLE flags. They will share the same RCU lifecycle and will be always synchronized.
The synchronization is done at the cost of :
- an additional indirection for accessing the first sched_domain level
- an additional indirection and a rcu_dereference before accessing to the NOHZ_IDLE flag.
Change since v4:
- link both sched_domain and NOHZ_IDLE flag in one RCU object so their states are always synchronized.
Change since V3;
- NOHZ flag is not cleared if a NULL domain is attached to the CPU
- Remove patch 2/2 which becomes useless with latest modifications
Change since V2:
- change the initialization to idle state instead of busy state so a CPU that enters idle during the build of the sched_domain will not corrupt the initialization state
Change since V1:
- remove the patch for SCHED softirq on an idle core use case as it was a side effect of the other use cases.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
include/linux/sched.h | 6 +++ kernel/sched/core.c | 105 ++++++++++++++++++++++++++++++++++++++++++++----- kernel/sched/fair.c | 35 +++++++++++------ kernel/sched/sched.h | 24 +++++++++-- 4 files changed, 145 insertions(+), 25 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index d35d2b6..2a52188 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -959,6 +959,12 @@ struct sched_domain { unsigned long span[0]; };
+struct sched_domain_rq {
struct sched_domain *sd;unsigned long flags;struct rcu_head rcu; /* used during destruction */+};
static inline struct cpumask *sched_domain_span(struct sched_domain *sd) { return to_cpumask(sd->span); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7f12624..69e2313 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5602,6 +5602,15 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu) destroy_sched_domain(sd, cpu); }
+static void destroy_sched_domain_rq(struct sched_domain_rq *sd_rq, int cpu) +{
if (!sd_rq)return;destroy_sched_domains(sd_rq->sd, cpu);kfree_rcu(sd_rq, rcu);+}
/*
- Keep a special pointer to the highest sched_domain that has
- SD_SHARE_PKG_RESOURCE set (Last Level Cache Domain) for this
@@ -5632,10 +5641,23 @@ static void update_top_cache_domain(int cpu)
- hold the hotplug lock.
*/ static void -cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) +cpu_attach_domain(struct sched_domain_rq *sd_rq, struct root_domain *rd,
int cpu){ struct rq *rq = cpu_rq(cpu);
struct sched_domain *tmp;
struct sched_domain_rq *tmp_rq;struct sched_domain *tmp, *sd = NULL;/** If we don't have any sched_domain and associated object, we can* directly jump to the attach sequence otherwise we try to degenerate* the sched_domain*/if (!sd_rq)goto attach;/* Get a pointer to the 1st sched_domain */sd = sd_rq->sd; /* Remove the sched domains which do not contribute to scheduling. */ for (tmp = sd; tmp; ) {@@ -5658,14 +5680,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) destroy_sched_domain(tmp, cpu); if (sd) sd->child = NULL;
/* update sched_domain_rq */sd_rq->sd = sd; }+attach: sched_domain_debug(sd, cpu);
rq_attach_root(rq, rd);
tmp = rq->sd;rcu_assign_pointer(rq->sd, sd);destroy_sched_domains(tmp, cpu);
tmp_rq = rq->sd_rq;rcu_assign_pointer(rq->sd_rq, sd_rq);destroy_sched_domain_rq(tmp_rq, cpu); update_top_cache_domain(cpu);} @@ -5695,12 +5720,14 @@ struct sd_data { };
struct s_data {
struct sched_domain_rq ** __percpu sd_rq; struct sched_domain ** __percpu sd; struct root_domain *rd;};
enum s_alloc { sa_rootdomain,
sa_sd_rq, sa_sd, sa_sd_storage, sa_none,@@ -5935,7 +5962,7 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd) return;
update_group_power(sd, cpu);
atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight);
atomic_set(&sg->sgp->nr_busy_cpus, 0);Is it possible that we can be dealing here with a sched_group/sched_group_power that is used on another CPU (from that CPU's rq->rq_sd->sd) concurrently? When we call build_sched_groups(), we might reuse an exisiting struct sched_group used elsewhere right? If so, is there a race with the above initialization?
No we are not reusing an existing struct, the sched_group/sched_group_power that is initialized here, has just been created by __visit_domain_allocation_hell in build_sched_domains. The sched_group/sched_group_power is not already attached to any CPU
Hi Frederic,
Does it answer your concern and do you have more comments about the patch?
Vincent
Vincent