linux-kernel - Re: [PATCH v11 7/8] cpuidle: Pre-store next timer/tick before selecting an idle state

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Date:   Wed, 27 Feb 2019 01:07:39 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Raju P . L . S . S . S . N" <rplsssn@...eaurora.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Tony Lindgren <tony@...mide.com>,
        Kevin Hilman <khilman@...nel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 7/8] cpuidle: Pre-store next timer/tick before
 selecting an idle state

On Wed, 27 Feb 2019 at 00:41, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Feb 27, 2019 at 12:16 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Tue, 26 Feb 2019 at 23:08, Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Tue, Feb 26, 2019 at 3:54 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > >
> > > > A common piece of data used by cpuidle governors, is the information about
> > > > when the next timer/tick is going to fire. Rather than having each governor
> > > > calling tick_nohz_get_next_timer|hrtimer() separately, let's consolidate
> > > > the code by calling these functions before invoking the ->select() callback
> > > > of the governor - and store the output data in the struct cpuidle_device.
> > >
> > > That misses the point IMO.
> > >
> > > You don't need to store two values in struct cpuidle_device, but just
> > > one, and not before running ->select(), but before invoking the
> > > driver's ->enter() callback.
> >
> > Okay! Thanks for letting me know!
> >
> > >
> > > At that point, the decision on whether or not to stop the scheduler
> > > tick has been made already and it should be sufficient to store the
> > > return value of tick_nohz_get_next_hrtimer() introduced by patch
> > > [3/8], because that value represents the next timer regardless of what
> > > has been decided with respect to the tick.
> >
> > Just to make sure I get this correctly, because it seems like I have
> > missed a few points here....
> >
> > If we decided to keep the tick running, then
> > tick_nohz_get_next_hrtimer() gives the next tick or the next hrtimer,
> > whatever that comes first. There are no other timer that can expire
> > earlier than this, right!?
> >
> > If we decided to stop the tick, then tick_nohz_get_next_hrtimer() will
> > give us the next hrtimer. Again, then there are no other timer that
> > can't expire earlier than this, right!?
>
> Right in both cases.
>
> IOW, that is the event that will wake up the CPU unless any other
> (non-timer) interrupts (or equivalent events) occur in the meantime.
>
> > >
> > > And you won't need the tick_nohz_get_next_timer() any more then.
> >
> > Alright, this kind of brings this hole thing back closer to v10 - and
> > then we should stick to use tick_nohz_get_sleep_length() as is for the
> > cpuidle governors. That is what you are saying?
>
> For the menu and teo governors - yes.  IMO
> tick_nohz_get_sleep_length() is as good as it gets in there.

Okay, got it! Thanks for your helpful answers!

I will post a v12 as soon as I can, so we can look into the other
parts of the series.

Kind regards
Uffe