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

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Message-ID: <aQAvnjVvJOUx78Nk@fedora>
Date: Tue, 28 Oct 2025 10:51:10 +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 Mon, Oct 27, 2025 at 06:06:32PM +0100, Thomas Gleixner wrote:
> On Wed, Oct 22 2025 at 20:13, Pingfan Liu wrote:
> > The previous patch lifted the deadline bandwidth check during the kexec
> 
> Once this is applied 'The previous patch' is meaningless.
> 

I will rephrase it.
> > process, which raises a potential issue: as the number of online CPUs
> > decreases, DL tasks may be crowded onto a few CPUs, which may starve the
> > CPU hotplug kthread. As a result, the hot-removal cannot proceed in
> > practice.  On the other hand, as CPUs are offlined one by one, all tasks
> > will eventually be migrated to the kexec CPU.
> >
> > Therefore, this patch marks all other CPUs as inactive to signal the
> 
> git grep "This patch" Documentation/process/
> 

I will rephrase it.
> > scheduler to migrate tasks to the kexec CPU during hot-removal.
> 
> I'm not seeing what this solves. It just changes the timing of moving
> tasks off to the boot CPU where they compete for the CPU for nothing.
> 
> 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.

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.


Thanks,

Pingfan