linux-kernel - Re: [patch 06/19] sched/mmcid: Prevent pointless work in mm_update_cpus

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Message-ID: <20251017175853.GH3419281@noisy.programming.kicks-ass.net>
Date: Fri, 17 Oct 2025 19:58:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Jeanson <mjeanson@...icios.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>,
	TCMalloc Team <tcmalloc-eng@...gle.com>
Subject: Re: [patch 06/19] sched/mmcid: Prevent pointless work in
 mm_update_cpus_allowed()

On Fri, Oct 17, 2025 at 02:49:29PM +0200, Thomas Gleixner wrote:
> On Fri, Oct 17 2025 at 13:12, Peter Zijlstra wrote:
> > On Wed, Oct 15, 2025 at 07:29:34PM +0200, Thomas Gleixner wrote:
> >
> >> +	if (!mm || READ_ONCE(mm->mm_cid.nr_cpus_allowed) == nr_cpu_ids)
> >>  		return;
> >
> > FWIW this doesn't work on architectures that change their
> > cpu_possible_mask around (eg. Power).
> 
> No. Power does not change it after boot either. Half of the kernel would
> explode if that'd be the case.

Power very much does changes cpu_possible_mask; it doesn't change
nr_cpu_ids. Anyway, the point is that a full mask won't be nr_cpu_ids.

Same is true when you offline a CPU come to think of it.

Same is true if the cpumask is sparse.

Anyway, just saying, checking against nr_cpu_ids might not be the best
shortcut here.