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Message-ID: <7e9545b069f19f0e5c57c53d4633c508009e796c.camel@gmx.de>
Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running
From:   Mike Galbraith <efault@gmx.de>
To:     Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Tue, 21 Sep 2021 07:50:07 +0200
In-Reply-To: <22e7133d674b82853a5ee64d3f5fc6b35a8e18d6.camel@gmx.de>
References: <20210920142614.4891-1-mgorman@techsingularity.net>
         <20210920142614.4891-3-mgorman@techsingularity.net>
         <22e7133d674b82853a5ee64d3f5fc6b35a8e18d6.camel@gmx.de>
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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 =3D 8
> sched_wakeup_granularity =3D 4ms
> sched_latency =3D 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 >=3D latency/2 still does turn off wakeup preemption.

	-Mike