linux-kernel - Re: [PATCH] trace: stop critical timings before idle loop on idle=poll

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Message-ID: <20151009123830.03b4cb5c@gandalf.local.home>
Date:	Fri, 9 Oct 2015 12:38:30 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH] trace: stop critical timings before idle loop on
 idle=poll

On Thu,  8 Oct 2015 15:36:06 -0300
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> When using idle=poll, the preemptoff tracer is always showing the idle
> task as the culprit for long latencies. That happens because critical
> timings are not stopped before idle loop. This patch stops critical
> timings before entering the idle loop, starting it again after the
> idle loop.
> 
> This problem does not affect the irqsoff tracer because interruptions
> are enabled before entering the idle loop.
> 
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> Reviewed-by: Luis Claudio R. Goncalves <lgoncalv@...hat.com>

Acked-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve

> ---
>  kernel/sched/idle.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8f177c7..4a2ef5a 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -57,9 +57,11 @@ static inline int cpu_idle_poll(void)
>  	rcu_idle_enter();
>  	trace_cpu_idle_rcuidle(0, smp_processor_id());
>  	local_irq_enable();
> +	stop_critical_timings();
>  	while (!tif_need_resched() &&
>  		(cpu_idle_force_poll || tick_check_broadcast_expired()))
>  		cpu_relax();
> +	start_critical_timings();
>  	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
>  	rcu_idle_exit();
>  	return 1;

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