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

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Message-ID: <001701d59412$5804f620$080ee260$@net>
Date:   Tue, 5 Nov 2019 11:50:56 -0800
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>
Cc:     "'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 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>