linux-kernel - Re: [PATCH 4/4] cpuidle: teo: Fix "early hits" handling for disabled idle states

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Message-ID: <CAJZ5v0h_cBa_q_XjWk-9RuOnf2yS+u3eH7A-tT=tH+Gmjh8H1w@mail.gmail.com>
Date:   Tue, 5 Nov 2019 23:15:35 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 4/4] cpuidle: teo: Fix "early hits" handling for disabled
 idle states

On Tue, Nov 5, 2019 at 8:51 PM Doug Smythies <dsmythies@...us.net> wrote:
>
> On 2019.10.10 14:38 Rafael J. Wysocki wrote:
>
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > The TEO governor uses idle duration "bins" defined in accordance with
> > the CPU idle states table provided by the driver, so that each "bin"
> > covers the idle duration range between the target residency of the
> > idle state corresponding to it and the target residency of the closest
> > deeper idle state.  The governor collects statistics for each bin
> > regardless of whether or not the idle state corresponding to it is
> > currently enabled.
> >
> > In particular, the "early hits" metric measures the likelihood of a
> > situation in which the idle duration measured after wakeup falls into
> > to given bin, but the time till the next timer (sleep length) falls
> > into a bin corresponding to one of the deeper idle states.  It is
> > used when the "hits" and "misses" metrics indicate that the state
> > "matching" the sleep length should not be selected, so that the state
> > with the maximum "early hits" value is selected instead of it.
> >
> > If the idle state corresponding to the given bin is disabled, it
> > cannot be selected and if it turns out to be the one that should be
> > selected, a shallower idle state needs to be used instead of it.
> > Nevertheless, the metrics collected for the bin corresponding to it
> > are still valid and need to be taken into account as though that
> > state had not been disabled.
> >
> > As far as the "early hits" metric is concerned, teo_select() tries to
> > take disabled states into account, but the state index corresponding
> > to the maximum "early hits" value computed by it may be incorrect.
> > Namely, it always uses the index of the previous maximum "early hits"
> > state then, but there may be enabled idle states closer to the
> > disabled one in question.  In particular, if the current candidate
> > state (whose index is the idx value) is closer to the disabled one
> > and the "early hits" value of the disabled state is greater than the
> > current maximum, the index of the current candidate state (idx)
> > should replace the "maximum early hits state" index.
> >
> > Modify the code to handle that case correctly.
> >
> > Fixes: b26bf6ab716f ("cpuidle: New timer events oriented governor for tickless systems")
> > Reported-by: Doug Smythies <dsmythies@...us.net>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> I tested this pretty patch set thoroughly, but can not claim exhaustively.
> I did my best to mess it up via trying weird scenarios.
> Unrelated issues discovered during testing are being handled on
> other e-mail threads.
>
> Tested-by: Doug Smythies <dsmythies@...us.net>

Thanks for the testing and reviews, much appreciated!