linux-kernel - Re: [RFC 2/3] kernel/cpu: Mark nonboot cpus as inactive when shutting down nonboot cpus

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Message-ID: <87qzunuqqo.ffs@tglx>
Date: Tue, 28 Oct 2025 13:59:11 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Pingfan Liu <piliu@...hat.com>
Cc: kexec@...ts.infradead.org, linux-kernel@...r.kernel.org, Waiman Long
 <longman@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>, Pierre Gondois <pierre.gondois@....com>, Andrew
 Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, Ingo
 Molnar <mingo@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
 <rostedt@...dmis.org>, Valentin Schneider <vschneid@...hat.com>, "Rafael
 J. Wysocki" <rafael.j.wysocki@...el.com>, Joel Granados
 <joel.granados@...nel.org>
Subject: Re: [RFC 2/3] kernel/cpu: Mark nonboot cpus as inactive when
 shutting down nonboot cpus

On Tue, Oct 28 2025 at 10:51, Pingfan Liu wrote:
> On Mon, Oct 27, 2025 at 06:06:32PM +0100, Thomas Gleixner wrote:
>> When kexec() is in progress, then running user space tasks at all is a
>> completely pointless exercise.
>> 
>> So the obvious solution to the problem is to freeze all user space tasks
>
> I agree, but what about a less intrusive approach? Simply stopping the 
> DL tasks should suffice, as everything works correctly without them.

What's intrusive about that? Task freezing exists already.

> I have a draft patch ready. Let's discuss it and go from there.
>
>> when kexec() is invoked. No horrible hacks in the deadline scheduler and
>> elsewhere required to make that work. No?
>
> To clarify, skipping the dl_bw_deactivate() validation is necessary 
> because it prevents CPU hot-removal.

If you freeze stuff there is nothing to do. Hibernation works exactly
that way without any magic hacks in a particular scheduling class, no?

Thanks,

        tglx