linux-kernel - Re: [PATCH] sched: fix clear NOHZ_BALANCE

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Date:	Tue, 4 Jun 2013 00:48:38 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	peterz@...radead.org, mingo@...nel.org
Subject: Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

On Thu, May 30, 2013 at 05:23:05PM +0200, Vincent Guittot wrote:
> I have faced a sequence where the Idle Load Balance was sometime not
>  triggered for a while on my platform.
> 
> CPU 0 and CPU 1 are running tasks and CPU 2 is idle
> 
> CPU 1 kicks the Idle Load Balance
> CPU 1 selects CPU 2 as the new Idle Load Balancer
> CPU 1 sets NOHZ_BALANCE_KICK for CPU 2
> CPU 1 sends a reschedule IPI to CPU 2
> While CPU 2 wakes up, CPU 0 or CPU 1 migrates a waking task A on CPU 2
> CPU 2 finally wakes up, runs task A and discards the Idle Load Balance
> Task A quickly goes back to sleep (before a tick occurs on CPU 2)
> CPU 2 goes back to idle with NOHZ_BALANCE_KICK set
> 
> Whenever CPU 2 will be selected for the ILB, reschedule IPI will be not
> sent to CPU2, which is idle, because NOHZ_BALANCE_KICK is already set
> and no Idle Load Balance will be performed.
> 
> We must wait for the sched softirq to be raised on CPU 2 thanks to
> another part of the kernel to clear NOHZ_BALANCE_KICKand come back to
> a normal situation.
> 
> The proposed solution clears NOHZ_BALANCE_KICK in schedule_ipi if
> we can't raise the sched_softirq for the Idle Load Balance.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/core.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 58453b8..51fc715 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1420,7 +1420,8 @@ void scheduler_ipi(void)
>  	if (unlikely(got_nohz_idle_kick() && !need_resched())) {
>  		this_rq()->idle_balance = 1;
>  		raise_softirq_irqoff(SCHED_SOFTIRQ);
> -	}
> +	} else
> +		clear_bit(NOHZ_BALANCE_KICK, nohz_flags(smp_processor_id()));

But then do we reach this if the IPI happens while running the non-idle task in
CPU 2? The first got_nohz_idle_kick() test would drop us out early from scheduler_ipi()
due to the idle_cpu() test. So the flag doesn't get cleared in this case.
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