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

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Message-ID: <87v7k8xgbt.ffs@tglx>
Date: Tue, 21 Oct 2025 20:25:26 +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 Mon, Oct 20 2025 at 10:22, Peter Zijlstra wrote:
> On Sun, Oct 19, 2025 at 10:32:47PM +0200, Thomas Gleixner wrote:
>> 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.
>
> So the obvious idea would be to grow hotplug hooks, such that you can
> do:
>
>   nr_cpus_allowed == num_online_cpus()
>
> But then hotplug will have to iterate all mm's. Doable, but not really
> nice.

Right, but that can be done once the dust settled and if there is
actually a need for it.

Thanks,

        tglx