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 seems to be: 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. During the initialization of the sched_domain, we set the NOHZ_IDLE flag when nr_busy_cpus is initialized to 0 in order to have a coherent configuration. If a CPU enters idle and call set_cpu_sd_state_idle during the build of the new sched_domain it will not corrupt the initial state set_cpu_sd_state_busy is modified and clears the NOHZ_IDLE only if a non NULL sched_domain is attached to the CPU (which is the case during the rebuild)
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 --- kernel/sched/core.c | 4 +++- kernel/sched/fair.c | 9 +++++++-- 2 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 26058d0..c730a4e 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5884,7 +5884,9 @@ 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); + set_bit(NOHZ_IDLE, nohz_flags(cpu)); + }
int __weak arch_sd_sibling_asym_packing(void) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 81fa536..2701a92 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5403,15 +5403,20 @@ static inline void set_cpu_sd_state_busy(void) { struct sched_domain *sd; int cpu = smp_processor_id(); + int clear = 0;
if (!test_bit(NOHZ_IDLE, nohz_flags(cpu))) return; - clear_bit(NOHZ_IDLE, nohz_flags(cpu));
rcu_read_lock(); - for_each_domain(cpu, sd) + for_each_domain(cpu, sd) { atomic_inc(&sd->groups->sgp->nr_busy_cpus); + clear = 1; + } rcu_read_unlock(); + + if (likely(clear)) + clear_bit(NOHZ_IDLE, nohz_flags(cpu)); }
void set_cpu_sd_state_idle(void)