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. A new nohz_flags has been added to sched_domain so both flags and sched_domain will share the same RCU lifecycle and will be always synchronized. This solution is prefered to the creation of a new struct with an extra pointer indirection. The synchronization is done at the cost of : - An additional indirection and a rcu_dereference for accessing the NOHZ_IDLE flag. - We use only the nohz_flags field of the top sched_domain. Change since v6: - Add the flags in struct sched_domain instead of creating a sched_domain_rq. Change since v5: - minor variable and function name change. - remove a useless null check before kfree - fix a compilation error when NO_HZ is not set. 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 <[email protected]> --- include/linux/sched.h | 1 + kernel/sched/fair.c | 34 ++++++++++++++++++++++++---------- 2 files changed, 25 insertions(+), 10 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index d35d2b6..cde4f7f 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -899,6 +899,7 @@ struct sched_domain { unsigned int wake_idx; unsigned int forkexec_idx; unsigned int smt_gain; + unsigned long nohz_flags; /* NOHZ_IDLE flag status */ int flags; /* See SD_* */ int level; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7a33e59..09e440f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5394,14 +5394,21 @@ static inline void set_cpu_sd_state_busy(void) { struct sched_domain *sd; int cpu = smp_processor_id(); - - if (!test_bit(NOHZ_IDLE, nohz_flags(cpu))) - return; - clear_bit(NOHZ_IDLE, nohz_flags(cpu)); + int first_nohz_idle = 1; rcu_read_lock(); - for_each_domain(cpu, sd) + for_each_domain(cpu, sd) { + if (first_nohz_idle) { + if (!test_bit(NOHZ_IDLE, &sd->nohz_flags)) + goto unlock; + + clear_bit(NOHZ_IDLE, &sd->nohz_flags); + first_nohz_idle = 0; + } + atomic_inc(&sd->groups->sgp->nr_busy_cpus); + } +unlock: rcu_read_unlock(); } @@ -5409,14 +5416,21 @@ void set_cpu_sd_state_idle(void) { struct sched_domain *sd; int cpu = smp_processor_id(); - - if (test_bit(NOHZ_IDLE, nohz_flags(cpu))) - return; - set_bit(NOHZ_IDLE, nohz_flags(cpu)); + int first_nohz_idle = 1; rcu_read_lock(); - for_each_domain(cpu, sd) + for_each_domain(cpu, sd) { + if (first_nohz_idle) { + if (test_bit(NOHZ_IDLE, &sd->nohz_flags)) + goto unlock; + + set_bit(NOHZ_IDLE, &sd->nohz_flags); + first_nohz_idle = 0; + } + atomic_dec(&sd->groups->sgp->nr_busy_cpus); + } +unlock: rcu_read_unlock(); } -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
