linux-kernel - Re: [PATCH] sched: idle: move need_resched check after function rcu_idle

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Message-ID: <20151123175926.GM17308@twins.programming.kicks-ass.net>
Date:	Mon, 23 Nov 2015 18:59:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lianwei Wang <lianwei.wang@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	rjw@...ysocki.net, mingo@...hat.com
Subject: Re: [PATCH] sched: idle: move need_resched check after function
 rcu_idle_enter

On Sat, Nov 21, 2015 at 01:02:02AM -0800, Lianwei Wang wrote:
> The rcu_idle_endter may call wakeup_softirqd to set the need resched
> flag on idle process. But if we don't check it after that, then the
> cpu will enter idle state with RESCHED flag set and can not be woken
> up by wakeup/resched call anymore.
> 
> Check need_resched after rcu_idle_enter to make sure the cpu is able
> to be out of idle immediatley to run other tasks.
> 
> Signed-off-by: Lianwei Wang <lianwei.wang@...il.com>
> ---
>  kernel/sched/idle.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 4a2ef5a02fd3..6e96a1f41041 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -137,21 +137,21 @@ static void cpuidle_idle_call(void)
>         int next_state, entered_state;
> 
>         /*
> +        * Tell the RCU framework we are entering an idle section,
> +        * so no more rcu read side critical sections and one more
> +        * step to the grace period
> +        */
> +       rcu_idle_enter();
> +
> +       /*
>          * Check if the idle task must be rescheduled. If it is the
>          * case, exit the function after re-enabling the local irq.
>          */
>         if (need_resched()) {
>                 local_irq_enable();
> -               return;
> +               goto exit_idle;
>         }
> 
> -       /*
> -        * Tell the RCU framework we are entering an idle section,
> -        * so no more rcu read side critical sections and one more
> -        * step to the grace period
> -        */
> -       rcu_idle_enter();
> -
>         if (cpuidle_not_available(drv, dev)) {
>                 default_idle_call();
>                 goto exit_idle;

FWIW your patch is whitespace mangled; I could not apply if I wanted to.
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