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_idle field is added to sched_domain so both status and sched_domain will share the same RCU lifecycle and will be always synchronized. In addition, there is no more need to protect nohz_idle against concurrent access as it is only modified by 2 exclusive functions called by local cpu.

This solution has been prefered to the creation of a new struct with an extra pointer indirection for sched_domain.

The synchronization is done at the cost of : - An additional indirection and a rcu_dereference for accessing nohz_idle. - We use only the nohz_idle field of the top sched_domain.

Change since v7: - remove atomic access which is useless now. - refactor the sequence that update nohz_idle status and nr_busy_cpus.

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 vincent.guittot@linaro.org --- include/linux/sched.h | 3 +++ kernel/sched/fair.c | 26 ++++++++++++++++---------- kernel/sched/sched.h | 1 - 3 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h index d35d2b6..22bcbe8 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -899,6 +899,9 @@ struct sched_domain { unsigned int wake_idx; unsigned int forkexec_idx; unsigned int smt_gain; +#ifdef CONFIG_NO_HZ + int nohz_idle; /* NOHZ IDLE status */ +#endif int flags; /* See SD_* */ int level;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7a33e59..5db1817 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5395,13 +5395,16 @@ 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)); - rcu_read_lock(); - for_each_domain(cpu, sd) + sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); + + if (!sd || !sd->nohz_idle) + goto unlock; + sd->nohz_idle = 0; + + for (; sd; sd = sd->parent) atomic_inc(&sd->groups->sgp->nr_busy_cpus); +unlock: rcu_read_unlock(); }

@@ -5410,13 +5413,16 @@ 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)); - rcu_read_lock(); - for_each_domain(cpu, sd) + sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); + + if (!sd || sd->nohz_idle) + goto unlock; + sd->nohz_idle = 1; + + for (; sd; sd = sd->parent) atomic_dec(&sd->groups->sgp->nr_busy_cpus); +unlock: rcu_read_unlock(); }

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index cc03cfd..03b13c8 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1187,7 +1187,6 @@ extern void account_cfs_bandwidth_used(int enabled, int was_enabled); enum rq_nohz_flag_bits { NOHZ_TICK_STOPPED, NOHZ_BALANCE_KICK, - NOHZ_IDLE, };

#define nohz_flags(cpu) (&cpu_rq(cpu)->nohz_flags)