linux-kernel - Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record

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Message-ID: <CAJZ5v0guU0GDs06W98boFpdCopHTiF_ojwTPrZFNP0Bk3DiQXQ@mail.gmail.com>
Date:   Fri, 25 Sep 2020 13:09:30 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Lukasz Luba <lukasz.luba@....com>, cristian.marussi@....com,
        Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()

On Fri, Sep 25, 2020 at 12:58 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 25-09-20, 12:04, Rafael J. Wysocki wrote:
> > I'm actually wondering if reset_time is necessary at all.
> >
> > If cpufreq_stats_record_transition() is the only updater of the stats,
> > which will be the case after applying this series IIUC, it may as well
> > simply set the new starting point and discard all of the data
> > collected so far if reset_pending is set.
> >
> > IOW, the time when the reset has been requested isn't particularly
> > relevant IMV (and it is not exact anyway), because the user is
> > basically asking for discarding "history" and that may very well be
> > interpreted to include the current sample.
>
> There are times when this would be visible to userspace and won't look nice.
>
> Like, set governor to performance, reset the stats and after 10 seconds, read
> the stats again, everything will be 0.

Unless I'm missing something, the real reset happens when
cpufreq_stats_record_transition() runs next time, so the old stats
will still be visible at that point, won't they?

> Because cpufreq_stats_record_transition()
> doesn't get called at all here, we would never clear them until the time
> governor is changed and so we need to keep a track of reset-time.

Or trigger a forced update.