linux-kernel - Re: [PATCH] sched: cpufreq_schedutil: maintain raw cache when next

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Date:   Fri, 16 Oct 2020 19:36:17 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Wei Wang <wvw@...gle.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Wei Wang <wei.vince.wang@...il.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Quentin Perret <qperret@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: cpufreq_schedutil: maintain raw cache when next_f
 is not changed

On Fri, Oct 16, 2020 at 7:18 PM Wei Wang <wvw@...gle.com> wrote:
>
> On Fri, Oct 16, 2020 at 10:01 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Fri, Oct 16, 2020 at 6:36 PM Wei Wang <wvw@...gle.com> wrote:
> > >
> > > Currently, raw cache will be reset when next_f is changed after
> > > get_next_freq for correctness. However, it may introduce more
> > > cycles. This patch changes it to maintain the cached value instead of
> > > dropping it.
> >
> > IMV you need to be more specific about why this helps.
> >
>
> I think the idea of cached_raw_freq is to reduce the chance of calling
> cpufreq drivers (in some arch those may be costly) but sometimes the
> cache will be wiped for correctness. The purpose of this patch is to
> still keep the cached value instead of wiping them.

Well, I see what the problem is and how the patch is attempting to
address it (which is not the best way to do that because of the extra
struct member that doesn't need to be added if I'm not mistaken), but
IMO the changelog is way too vague from the problem statement
perspective.