linux-kernel - Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr

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Message-ID: <7e9545b069f19f0e5c57c53d4633c508009e796c.camel@gmx.de>
Date:   Tue, 21 Sep 2021 07:50:07 +0200
From:   Mike Galbraith <efault@....de>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running

On Tue, 2021-09-21 at 05:52 +0200, Mike Galbraith wrote:
> On Mon, 2021-09-20 at 15:26 +0100, Mel Gorman wrote:
>
>
> The scaled up tweakables on my little quad desktop box:
> sched_nr_latency = 8
> sched_wakeup_granularity = 4ms
> sched_latency = 24ms
>
> Due to the FAIR_SLEEPERS feature, a task can only receive a max of
> sched_latency/2 sleep credit, ie the delta between waking sleeper and
> current is clipped to a max of 12 virtual ms, so the instant our
> preempt threshold reaches 12.000ms, by human booboo or now 3 runnable
> tasks with this change, wakeup preemption is completely disabled, or?

I just dug up a now ancient artifact testcase allegedly distilled from
a real application's control thread sometime back in the dark ages, and
granularity >= latency/2 still does turn off wakeup preemption.

	-Mike