linux-kernel - Re: [RFT][PATCH v4 3/7] sched: idle: Do not stop the tick before cpuidle_idle

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Date:   Fri, 16 Mar 2018 15:16:35 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFT][PATCH v4 3/7] sched: idle: Do not stop the tick before
 cpuidle_idle_call()

On Thu, Mar 15, 2018 at 09:41:10PM +0100, Rafael J. Wysocki wrote:
> On Thu, Mar 15, 2018 at 7:19 PM, Frederic Weisbecker
> <frederic@...nel.org> wrote:
> > On Mon, Mar 12, 2018 at 10:53:25AM +0100, Rafael J. Wysocki wrote:
> >> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>
> >> Make cpuidle_idle_call() decide whether or not to stop the tick.
> >>
> >> First, the cpuidle_enter_s2idle() path deals with the tick (and with
> >> the entire timekeeping for that matter) by itself and it doesn't need
> >> the tick to be stopped beforehand.
> >
> > Not sure you meant timekeeping either :)
> 
> Yeah, I meant nohz.
> 
> >>       if (idle_should_enter_s2idle() || dev->use_deepest_state) {
> >>               if (idle_should_enter_s2idle()) {
> >> +                     rcu_idle_enter();
> >> +
> >>                       entered_state = cpuidle_enter_s2idle(drv, dev);
> >>                       if (entered_state > 0) {
> >>                               local_irq_enable();
> >>                               goto exit_idle;
> >>                       }
> >> +
> >> +                     rcu_idle_exit();
> >>               }
> >
> > I'm not sure how the tick is stopped on suspend to idle. Perhaps through
> > hrtimer (tick_cancel_sched_timer()) or clockevents code.
> 
> The latter.
> 
> It does clockevents_shutdown() down the road, eventually.

Ah good. And I see tick_resume_oneshot() takes care of restoring if necessary.

> 
> IOW, it couldn't care less. :-)
> 
> > But we may have a similar problem than with idle_poll() called right after
> > call_cpuidle(). Ie: we arrive in cpuidle_enter_s2idle() with a tick that
> > should be reprogrammed while it is not. No idea if that can hurt somehow.
> >
> > I guess it depends what happens to the tick on s2idle, I'm not clear with that.
> 
> No problem there, AFAICS.

Yep, all good.

Thanks!