linux-kernel - Re: [RFC PATCH 5/5] sched/fair: Add push task callback for EAS

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Message-ID: <73756bc3-5de0-4559-99a3-704db78433f5@arm.com>
Date: Mon, 9 Sep 2024 10:59:49 +0100
From: Christian Loehle <christian.loehle@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, lukasz.luba@....com, rafael.j.wysocki@...el.com,
 linux-kernel@...r.kernel.org
Cc: qyousef@...alina.io, hongyan.xia2@....com
Subject: Re: [RFC PATCH 5/5] sched/fair: Add push task callback for EAS

On 8/30/24 14:03, Vincent Guittot wrote:
> EAS is based on wakeup events to efficiently place tasks on the system, but
> there are cases where a task will not have wakeup events anymore or at a
> far too low pace. For such situation, we can take advantage of the task
> being put back in the enqueued list to check if it should be migrated on
> another CPU. When the task is the only one running on the CPU, the tick
> will check it the task is stuck on this CPU and should migrate on another
> one.
> 
> Wake up events remain the main way to migrate tasks but we now detect
> situation where a task is stuck on a CPU by checking that its utilization
> is larger than the max available compute capacity (max cpu capacity or
> uclamp max setting)

Let me think out loud about this and feel free to object:
If there's other tasks on the rq we don't have that problem, if it is the
only one running it's utilization should be 1024, misfit should take care
of the upmigration on the way up.
If the task utilization is 1024 it needs to be alone on the rq, why would
another CPU be more efficient in that case (which presumably is an idle
CPU of the same PD)?
Or is this patch just for UCLAMP_MAX < 1024 cases altogether?