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

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Date:   Tue, 26 Feb 2019 23:17:51 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.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 Tue, Feb 26, 2019 at 11:08 PM 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.
>
> 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],

Or the difference between in and ts->idle_entrytime for that matter,
whichever is more useful.

> because that value represents the next timer regardless of what
> has been decided with respect to the tick.
>
> And you won't need the tick_nohz_get_next_timer() any more then.