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

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Message-ID: <20251020082250.GF3245006@noisy.programming.kicks-ass.net>
Date: Mon, 20 Oct 2025 10:22:50 +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 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.