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

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Message-ID: <CAF+s44SuXgZ4MD5z8m_q-m03j_p0u0RTGhH-VXF25iwQ=Jy-SA@mail.gmail.com>
Date: Wed, 29 Oct 2025 21:39:40 +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 Wed, Oct 29, 2025 at 8:13 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, Oct 29 2025 at 19:36, Pingfan Liu wrote:
> > On Tue, Oct 28, 2025 at 01:59:11PM +0100, Thomas Gleixner wrote:
> >> 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.
>
> No. Hibernation brings the non-boot CPUs down in order to create the
> disk image.
>

Oh, I see. Since there are no DL tasks in the runqueue, no migration
occurs to activate the DL bandwidth. This approach, similar to PM, is
perfect for addressing this issue.


Thanks,

Pingfan