linux-kernel - Re: [PATCH 2/3] cpuidle: teo: Avoid expecting unrealistic idle times

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Date:   Fri, 15 Nov 2019 10:07:38 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] cpuidle: teo: Avoid expecting unrealistic idle times

On Fri, Nov 15, 2019 at 2:24 AM Doug Smythies <dsmythies@...us.net> wrote:
>
> On 2019.11.14 15:51 Rafael J. Wysocki wrote:
> > On Wed, Nov 13, 2019 at 1:11 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >>
> >> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>
> >> If an idle state shallower than the one "matching" the time till the
> >> next timer event is considered for selection, expect the idle duration
> >> to fall in the middle of the "bin" corresponding to that state rather
> >> than at the beginning of it which is unrealistic.
> >>
> >> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >> ---
> >>  drivers/cpuidle/governors/teo.c |    9 ++++++++-
> >>  1 file changed, 8 insertions(+), 1 deletion(-)
> >>
> >> Index: linux-pm/drivers/cpuidle/governors/teo.c
> >> ===================================================================
> >> --- linux-pm.orig/drivers/cpuidle/governors/teo.c
> >> +++ linux-pm/drivers/cpuidle/governors/teo.c
> >> @@ -360,7 +360,14 @@ static int teo_select(struct cpuidle_dri
> >>
> >>                 if (max_early_idx >= 0) {
> >>                         idx = max_early_idx;
> >> -                       duration_ns = drv->states[idx].target_residency_ns;
> >> +                       /*
> >> +                        * Expect the idle duration to fall in the middle of the
> >> +                        * "bin" corresponding to idx (note that the maximum
> >> +                        * state index is guaranteed to be greater than idx at
> >> +                        * this point).
> >> +                        */
> >> +                       duration_ns = (drv->states[idx].target_residency_ns +
> >> +                               drv->states[idx+1].target_residency_ns) / 2;
> >>                 }
> >>         }
> >
> > This change turns out to cause the governor to choose idle states that
> > are too deep or too shallow too often, so I'm withdrawing it.
>
> O.K. thanks for letting us know.
> I did see some differences in the testing I did so far, but hadn't drilled down
> into it yet.
> I am somewhat wondering about the above and below stats in general.
>
> By the way, I had a daft mistake in my post processing program, such that the
> "below" graph for idle state 0 was always plotting 0.
>
> Reference for that sweep test that I do (which is as far I got so far):
> http://www.smythies.com/~doug/linux/idle/teo-2019-11/sweep/index.html
>
> Legend:
>
> teo-v2: re-run of previous teo-v2 so that I could get non-zero idle state "below" data
> linux-next 2019.11.07 + cpuidle: Consolidate disabled state checks +
> [PATCH v2] cpuidle: Use nanoseconds as the unit of time
>
> teo-v3: teo-v2 + cpuidle: teo: Exclude cpuidle overhead from computations
>
> teo-v4: linux-pm + linux-next 2019.11.12 +
> cpuidle: teo: Avoid code duplication in conditionals
> cpuidle: teo: Avoid expecting unrealistic idle times
> cpuidle: teo: Avoid using "early hits" incorrectly
>
> teo-v5: teo-v4 + cpuidle: teo: Exclude cpuidle overhead from computations

Thanks for running the tests, appreciated!