linux-kernel - Re: [PATCH v11] cpuidle: New timer events oriented governor for tickless systems

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Date:   Wed, 16 Jan 2019 13:03:06 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     Giovanni Gherdovich <ggherdovich@...e.cz>,
        Doug Smythies <dsmythies@...us.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Chen, Hu" <hu1.chen@...el.com>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH v11] cpuidle: New timer events oriented governor for tickless systems

On Friday, January 4, 2019 12:30:47 PM CET Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The venerable menu governor does some things that are quite
> questionable in my view.
> 
> First, it includes timer wakeups in the pattern detection data and
> mixes them up with wakeups from other sources which in some cases
> causes it to expect what essentially would be a timer wakeup in a
> time frame in which no timer wakeups are possible (because it knows
> the time until the next timer event and that is later than the
> expected wakeup time).
> 
> Second, it uses the extra exit latency limit based on the predicted
> idle duration and depending on the number of tasks waiting on I/O,
> even though those tasks may run on a different CPU when they are
> woken up.  Moreover, the time ranges used by it for the sleep length
> correction factors depend on whether or not there are tasks waiting
> on I/O, which again doesn't imply anything in particular, and they
> are not correlated to the list of available idle states in any way
> whatever.
> 
> Also, the pattern detection code in menu may end up considering
> values that are too large to matter at all, in which cases running
> it is a waste of time.
> 
> A major rework of the menu governor would be required to address
> these issues and the performance of at least some workloads (tuned
> specifically to the current behavior of the menu governor) is likely
> to suffer from that.  It is thus better to introduce an entirely new
> governor without them and let everybody use the governor that works
> better with their actual workloads.
> 
> The new governor introduced here, the timer events oriented (TEO)
> governor, uses the same basic strategy as menu: it always tries to
> find the deepest idle state that can be used in the given conditions.
> However, it applies a different approach to that problem.
> 
> First, it doesn't use "correction factors" for the time till the
> closest timer, but instead it tries to correlate the measured idle
> duration values with the available idle states and use that
> information to pick up the idle state that is most likely to "match"
> the upcoming CPU idle interval.
> 
> Second, it doesn't take the number of "I/O waiters" into account at
> all and the pattern detection code in it avoids taking timer wakeups
> into account.  It also only uses idle duration values less than the
> current time till the closest timer (with the tick excluded) for that
> purpose.

Given the lack of negative feedback and my confidence in this, I'm
queuing it up for 5.1.

Cheers,
Rafael