[PATCH v3 0/2] sched: fix nr_busy_cpus

[PATCH v3 0/2] sched: fix nr_busy_cpus

[Vincent Guittot]

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 some CPUs enter idle state while booting all CPUs
 - when a CPU is unplug and/or replug

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.

Vincent Guittot (2):
  sched: fix init NOHZ_IDLE flag
  sched: fix update NOHZ_IDLE flag

 kernel/sched/core.c |    4 +++-
 kernel/sched/fair.c |    2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

-- 
1.7.9.5

[PATCH v3 1/2] sched: fix init NOHZ_IDLE flag

[Vincent Guittot]

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 set the NOHZ_IDLE flag when nr_busy_cpus is initialized to 0 in order to
have a coherent configuration.

The patch 2/2 protects this init against an update of NOHZ_IDLE flag because
a NULL sched_domain is attached to the CPU during the build of the new
sched_domain so nohz_kick_needed and set_cpu_sd_state_busy are not called and
can't clear the NOHZ_IDLE flag

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/core.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

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)
-- 
1.7.9.5

[PATCH v3 2/2] sched: fix update NOHZ_IDLE flag

[Vincent Guittot]

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 (during the boot or 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 5eea870..dac2edf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5695,7 +5695,7 @@ 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
 }
-- 
1.7.9.5

Re: [PATCH v3 1/2] sched: fix init NOHZ_IDLE flag

[Vincent Guittot]

On 8 February 2013 19:05, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> 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 set the NOHZ_IDLE flag when nr_busy_cpus is initialized to 0 in order to
> have a coherent configuration.
>
> The patch 2/2 protects this init against an update of NOHZ_IDLE flag because
> a NULL sched_domain is attached to the CPU during the build of the new
> sched_domain so nohz_kick_needed and set_cpu_sd_state_busy are not called and
> can't clear the NOHZ_IDLE flag

Hi Frederic,

With this v3, the initialization of  NOHZ_IDLE and nr_busy_cpus is be
protected against any race condition because the initial state is the
idle state so if a cpu enters idle, it keeps the same state and we
can't set a busy state until the initialization is complete thanks to
the NULL domain.

Regards,
Vincent

>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/core.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> 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)
> --
> 1.7.9.5
>