linux-kernel - Re: [PATCH v23 6/9] sched: Handle blocked-waiter migration (and return migration)

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Message-ID: <9de6fd90-441c-4ddd-9f79-754bf240a3ab@amd.com>
Date: Thu, 20 Nov 2025 08:25:19 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: John Stultz <jstultz@...gle.com>
CC: LKML <linux-kernel@...r.kernel.org>, Joel Fernandes
	<joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>, 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>, Valentin Schneider
	<vschneid@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall
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	<xuewen.yan94@...il.com>, Thomas Gleixner <tglx@...utronix.de>, "Daniel
 Lezcano" <daniel.lezcano@...aro.org>, Suleiman Souhlal <suleiman@...gle.com>,
	kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>,
	<kernel-team@...roid.com>
Subject: Re: [PATCH v23 6/9] sched: Handle blocked-waiter migration (and
 return migration)

Hello John,

On 11/20/2025 7:30 AM, John Stultz wrote:
>> Ok, so you're suggesting maybe putting the
>>     if (task_on_rq_migrating(owner))
>> case ahead of the
>>     if (owner_cpu != this_cpu)
>> check?
>>
>> Let me give that a whirl and see how it does.
> 
> That said, thinking another second on it, I also realize once we
> decide to proxy_migrate, there is always the chance the owner gets
> migrated somewhere else. So we can check task_on_rq_migrating() but
> then right after we check that it might be migrated, and we can't
> really prevent this.  And in that case, doing the proxy-migration to
> the wrong place will be ok, as that cpu will then bounce the tasks to
> the owner's new cpu.
> 
> Hopefully this would be rare though. :)

Ack! I was just thinking of some extreme scenarios. We can probably
think about it if and when we run into a problem with it :)

That said, once we decide to move the first donors to owner's CPU
should we task some care to retain the owner on the same CPU as much
as possible - take it out of the purview of load balancing and only
move it if the owner is no long runnable on that CPU as a result of
affinity changes?

-- 
Thanks and Regards,
Prateek