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

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Message-ID: <aQH8KJ8RsnzbLfN2@fedora>
Date: Wed, 29 Oct 2025 19:36:08 +0800
From: Pingfan Liu <piliu@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
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 01:59:11PM +0100, Thomas Gleixner wrote:
> 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.
> 

Thanks for your guidance. That's a good point -- system suspending is a
good analogy. I will check how PM handles it.

> > 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?
> 

There is a nuance: DL bandwidth represents a commitment, not necessarily
the actual payload. Even a blocked DL task still occupies DL bandwidth.
The system's DL bandwidth remains unchanged as long as the CPUs stay
online, which is the case in hibernation.


Best Regards,

Pingfan