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

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Message-ID: <87jz0qzl74.ffs@tglx>
Date: Sun, 19 Oct 2025 22:32:47 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
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 20:19, Peter Zijlstra wrote:
> On Fri, Oct 17, 2025 at 07:58:53PM +0200, Peter Zijlstra wrote:
>> 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.
>
> Put another way, nr_cpus_allowed == nr_cpu_ids only work when none of
> the masks involved have holes. The moment anything {possible, present,
> online} has holes in, it goes sideways.

You're right. I was too narrowly focussed on the normal x86 case, where
nr_cpu_ids == num_possible_cpus ....

Let me think about that.