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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 13 Mar 2020 14:21:02 +0100
Message-ID: <CAKfTPtBYDOypA1sMdtNniiTw_C7fNieWXXwPe7oDOzPUW0PW_Q@mail.gmail.com>
Subject: Re: [PATCH V2] sched: fair: Use the earliest break even
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Qais Yousef <qais.yousef@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>
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On Fri, 13 Mar 2020 at 14:17, Daniel Lezcano <daniel.lezcano@linaro.org> wr=
ote:
>
> On 13/03/2020 14:15, Vincent Guittot wrote:
> > On Fri, 13 Mar 2020 at 13:15, Daniel Lezcano <daniel.lezcano@linaro.org=
> wrote:
>
> [ ... ]
>
> >>>>>> +
> >>>>>> +       if (idle_state)
> >>>>>> +               idle_set_break_even(rq, ktime_get_ns() +
> >>>>>
> >>>>> What worries me a bit is that it adds one ktime_get call each time =
a
> >>>>> cpu enters idle
> >>>>
> >>>> Right, we can improve this in the future by folding the local_clock(=
) in
> >>>> cpuidle when entering idle with this ktime_get.
> >>>
> >>> Using local_clock() would be more latency friendly
> >>
> >> Unfortunately we are comparing the deadline across CPUs, so the
> >> local_clock() can not be used here.
> >>
> >> But if we have one ktime_get() instead of a local_clock() + ktime_get(=
),
> >> that should be fine, no?
> >
> > Can't this computation of break_even be done in cpuidle framework
> > while computing other statistics for selecting the idle state instead
> > ? cpuidle already uses ktime_get for next hrtimer as an example.
> > So cpuidle compute break_even and make it available to scheduler like
> > exit_latency. And I can imagine that system wide time value will also
> > be needed when looking at next wakeup event of cluster/group of CPUs
>
> Ok, so you suggest to revisit and consolidate the whole time capture in
> cpuidle? I think that makes sense.

Yes

>
>
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