linux-kernel - Re: [PATCH] idle: move latency tracing stop/start calls deeper inside the idle loop

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Date:	Sat, 04 Jul 2015 02:08:54 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Lucas Stach <l.stach@...gutronix.de>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de, patchwork-lst@...gutronix.de
Subject: Re: [PATCH] idle: move latency tracing stop/start calls deeper inside the idle loop

On Friday, July 03, 2015 04:19:40 PM Lucas Stach wrote:
> Make sure to stop tracing only once we are past a point where all
> latency tracing events have been processed (irqs are not enabled
> again). This has the slight advantage of capturing more latency
> related events in the idle path, but most importantly it makes sure
> that latency tracing doesn't get re-enabled inadvertently when
> new events are coming in.
> 
> This makes the irqsoff latency tracer useful again, as we stop
> capturing CPU sleep time as IRQ latency.
> 
> Signed-off-by: Lucas Stach <l.stach@...gutronix.de>
> ---
>  drivers/cpuidle/cpuidle.c |  2 ++
>  kernel/sched/idle.c       | 14 +++++---------
>  2 files changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 61c417b9e53f..d78514ac3f5d 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -173,7 +173,9 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>  	trace_cpu_idle_rcuidle(index, dev->cpu);
>  	time_start = ktime_get();
>  
> +	stop_critical_timings();
>  	entered_state = target_state->enter(dev, drv, index);
> +	start_critical_timings();
>  
>  	time_end = ktime_get();
>  	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index fefcb1fa5160..30b799bfc3c6 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -93,12 +93,6 @@ static void cpuidle_idle_call(void)
>  	}
>  
>  	/*
> -	 * During the idle period, stop measuring the disabled irqs
> -	 * critical sections latencies
> -	 */
> -	stop_critical_timings();
> -
> -	/*
>  	 * 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
> @@ -181,7 +175,6 @@ exit_idle:
>  		local_irq_enable();
>  
>  	rcu_idle_exit();
> -	start_critical_timings();

Are you sure we can safely reorder stop/start_critical_timings() with respect
to rcu_idle_enter/exit()?

>  	return;
>  
>  use_default:
> @@ -189,10 +182,13 @@ use_default:
>  	 * We can't use the cpuidle framework, let's use the default
>  	 * idle routine.
>  	 */
> -	if (current_clr_polling_and_test())
> +	if (current_clr_polling_and_test()) {
>  		local_irq_enable();
> -	else
> +	} else {
> +		stop_critical_timings();
>  		arch_cpu_idle();
> +		start_critical_timings();
> +	}
>  
>  	goto exit_idle;
>  }

And what about enter_freeze_proper()?  Don't we need those in there too?


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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