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

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Message-ID: <20251017111223.GD4067720@noisy.programming.kicks-ass.net>
Date: Fri, 17 Oct 2025 13:12:23 +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 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).