linux-kernel - Re: [PATCH] sched/fair: Fix overutilized update in enqueue_task

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Message-ID: <20201112123854.GA2222462@google.com>
Date:   Thu, 12 Nov 2020 12:38:54 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     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>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...rret.net>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>,
        kernel-team@...roid.com, Rick Yiu <rickyiu@...gle.com>
Subject: Re: [PATCH] sched/fair: Fix overutilized update in
 enqueue_task_fair()

On Thursday 12 Nov 2020 at 12:29:59 (+0000), Valentin Schneider wrote:
> Alternatively: how much does not updating the overutilized status here help
> us? The next tick will unconditionally update it, which for arm64 is
> anywhere in the next ]0, 4]ms. That "fake" fork-time util_avg should already
> be accounted in the rq util_avg, and even if the new task was running the
> entire time, 4ms doesn't buy us much decay.

Yes, this is arguably a dodgy hack, which will not save us in a number
of cases. The only situation where this helps is for short-lived tasks
that will run only once. And this is a sadly common programming pattern.

So yeah, this is not the prettiest thing in the world, but it doesn't
cost us much and helps some real-world workloads, so ...

Thanks,
Quentin