linux-kernel - Re: [PATCH Resend 1/3] sched: fix nr_busy

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Date:	Thu, 24 Jan 2013 17:44:05 +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, ccross@...roid.com
Subject: Re: [PATCH Resend 1/3] sched: fix nr_busy_cpus with coupled cpuidle

2012/12/3 Vincent Guittot <vincent.guittot@...aro.org>:
> 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.

I'm a bit confused with this.

set_cpu_sd_state_busy() is only called from nohz_kick_needed(). And it
checks idle_cpu() before doing anything. So if no task is going to be
scheduled, idle_cpu() prevents from calling set_cpu_sd_state_busy().

I'm probably missing something.

Thanks.

>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.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 955d35b..b8d74ea 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -570,6 +570,8 @@ void tick_nohz_irq_exit(void)
>         if (!ts->inidle)
>                 return;
>
> +       set_cpu_sd_state_idle();
> +
>         /* Cancel the timer because CPU already waken up from the C-states*/
>         menu_hrtimer_cancel();
>         __tick_nohz_idle_enter(ts);
> --
> 1.7.9.5
>
> --
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