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Date:   Tue, 17 Jan 2023 09:32:50 +0000
From:   Mel Gorman <mgorman@suse.de>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org, zhangqiao22@huawei.com
Subject: Re: [PATCH v4] sched/fair: limit sched slice duration
Message-ID: <20230117093250.asfytnijs7zytscv@suse.de>
References: <20230113133613.257342-1-vincent.guittot@linaro.org>
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On Fri, Jan 13, 2023 at 02:36:13PM +0100, Vincent Guittot wrote:
> In presence of a lot of small weight tasks like sched_idle tasks, normal
> or high weight tasks can see their ideal runtime (sched_slice) to increase
> to hundreds ms whereas it normally stays below sysctl_sched_latency.
> 
> 2 normal tasks running on a CPU will have a max sched_slice of 12ms
> (half of the sched_period). This means that they will make progress
> every sysctl_sched_latency period.
> 
> If we now add 1000 idle tasks on the CPU, the sched_period becomes
> 3006 ms and the ideal runtime of the normal tasks becomes 609 ms.
> It will even become 1500ms if the idle tasks belongs to an idle cgroup.
> This means that the scheduler will look for picking another waiting task
> after 609ms running time (1500ms respectively). The idle tasks change
> significantly the way the 2 normal tasks interleave their running time
> slot whereas they should have a small impact.
> 
> Such long sched_slice can delay significantly the release of resources
> as the tasks can wait hundreds of ms before the next running slot just
> because of idle tasks queued on the rq.
> 
> Cap the ideal_runtime to sysctl_sched_latency to make sure that tasks will
> regularly make progress and will not be significantly impacted by
> idle/background tasks queued on the rq.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> Tested-By: Dietmar Eggemann <dietmar.eggemann@arm.com>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs