The nr_busy_cpus field of the sched_group_power is sometime different from 0 whereas the platform is fully idle. This serie fixes 3 use cases: - when the SCHED softirq is raised on an idle core for idle load balance but the platform doesn't go out of the cpuidle state - when some CPUs enter idle state while booting all CPUs - when a CPU is unplug and/or replug
Vincent Guittot (3): sched: fix nr_busy_cpus with coupled cpuidle sched: fix init NOHZ_IDLE flag sched: fix update NOHZ_IDLE flag
kernel/sched/core.c | 1 + kernel/sched/fair.c | 2 +- kernel/time/tick-sched.c | 2 ++ 3 files changed, 4 insertions(+), 1 deletion(-)
With the coupled cpuidle driver (but probably also with other drivers), a CPU loops in a temporary safe state while waiting for other CPUs of its cluster to be ready to enter the coupled C-state. If an IRQ or a softirq occurs, the CPU will stay in this internal loop if there is no need to resched. The SCHED softirq clears the NOHZ and increases nr_busy_cpus. If there is no need to resched, we will not call set_cpu_sd_state_idle because of this internal loop in a cpuidle state. We have to call set_cpu_sd_state_idle in tick_nohz_irq_exit which is used to handle such situation.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/time/tick-sched.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index a402608..e19bbc9 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -526,6 +526,8 @@ void tick_nohz_irq_exit(void) if (!ts->inidle) return;
+ set_cpu_sd_state_idle(); + __tick_nohz_idle_enter(ts); }
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. We clear the NOHZ_IDLE flag when nr_busy_cpus is initialized in order to have a coherent configuration.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/core.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 5dae0d2..05058e8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5817,6 +5817,7 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)
update_group_power(sd, cpu); atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight); + clear_bit(NOHZ_IDLE, nohz_flags(cpu)); }
int __weak arch_sd_sibling_asym_packing(void)
The function nohz_kick_needed modifies NOHZ_IDLE flag that is used to update the nr_busy_cpus of the sched_group. When the sched_domain are updated (because of the unplug of a CPUs as an example) a null_domain is attached to CPUs. We have to test likely(!on_null_domain(cpu) first in order to detect such intialization step and to not modify the NOHZ_IDLE flag
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3d0686c..1bf7c87 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5490,7 +5490,7 @@ void trigger_load_balance(struct rq *rq, int cpu) likely(!on_null_domain(cpu))) raise_softirq(SCHED_SOFTIRQ); #ifdef CONFIG_NO_HZ - if (nohz_kick_needed(rq, cpu) && likely(!on_null_domain(cpu))) + if (likely(!on_null_domain(cpu)) && nohz_kick_needed(rq, cpu)) nohz_balancer_kick(cpu); #endif }
On 19 November 2012 22:08, Vincent Guittot vincent.guittot@linaro.org wrote:
The nr_busy_cpus field of the sched_group_power is sometime different from 0 whereas the platform is fully idle. This serie fixes 3 use cases:
- when the SCHED softirq is raised on an idle core for idle load balance but the platform doesn't go out of the cpuidle state
- when some CPUs enter idle state while booting all CPUs
- when a CPU is unplug and/or replug
Applied to a independent branch in big LITTLE MP tree: fix-nr-busy-cpus-v1 Isn't merged with any other branch.
-- viresh
-
Vincent Guittot -
Viresh Kumar