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

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Date:	Mon, 20 Jul 2015 10:55:45 +0200
From:	Lucas Stach <l.stach@...gutronix.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
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

Am Samstag, den 04.07.2015, 02:08 +0200 schrieb Rafael J. Wysocki:
> 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()?
> 
I'm no RCU expert, but I'm reasonably sure that this is a safe thing to
do, as stop/start_critical_timings() does not use any RCU facilities.

I've ran this patch quite some time with PROVE_RCU enabled and it didn't
indicate any suspicious RCU usage.

> >  	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?
> 
You are right on this one. I'll spin a V2 with this fixed.

Thanks,
Lucas

-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |

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