linux-kernel - Re: [PATCH v2 1/2] sched: fix init NOHZ

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Date:	Fri, 1 Feb 2013 19:03:38 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	peterz@...radead.org, mingo@...nel.org
Subject: Re: [PATCH v2 1/2] sched: fix init NOHZ_IDLE flag

2013/1/29 Vincent Guittot <vincent.guittot@...aro.org>:
> 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@...aro.org>
> ---
>  kernel/sched/core.c |    1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 257002c..fd41924 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5884,6 +5884,7 @@ 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));

So that's a real issue indeed.  nr_busy_cpus was never correct.

Now I'm still a bit worried with this solution. What if an idle task
started in smp_init() has not yet stopped its tick, but is about to do
so? The domains are not yet available to the task but the nohz flags
are. When it later restarts the tick, it's going to erroneously
increase nr_busy_cpus.

It probably won't happen in practice. But then there is more: sched
domains can be concurrently rebuild anytime, right?  So what if we
call set_cpu_sd_state_idle() and decrease nr_busy_cpus while the
domain is switched concurrently. Are we having a new sched group along
the way? If so we have a bug here as well because we can have
NOHZ_IDLE set but nr_busy_cpus accounting the CPU.

May be we need to set the per cpu nohz flags on the child leaf sched
domain? This way it's initialized and stored on the same RCU pointer
and we nohz_flags and nr_busy_cpus become sync.

Also we probably still need the first patch of your previous round.
Because the current patch may introduce situations where we have idle
CPUs with NOHZ_IDLE flags cleared.
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