linux-kernel - Re: [patch 00/19] sched: Rewrite MM CID management

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Message-ID: <lhuzf9plq78.fsf@oldenburg.str.redhat.com>
Date: Fri, 17 Oct 2025 13:31:39 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,  Peter Zijlstra
 <peterz@...radead.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>,  Tim Chen <tim.c.chen@...el.com>,  TCMalloc Team
 <tcmalloc-eng@...gle.com>
Subject: Re: [patch 00/19] sched: Rewrite MM CID management

* Thomas Gleixner:

> The CID space compaction itself is not a functional correctness
> requirement, it is only a useful optimization mechanism to reduce the
> memory foot print in unused user space pools.
>
> The optimal CID space is:
>
>     min(nr_tasks, nr_cpus_allowed);
>
> Where @nr_tasks is the number of actual user space threads associated to
> the mm.
>
> @nr_cpus_allowed is the superset of all task affinities. It is growth
> only as it would be insane to take a racy snapshot of all task
> affinities when the affinity of one task changes just do redo it 2
> milliseconds later when the next task changes its affinity.

How can userspace obtain the maximum possible nr_cpus_allowed value?

Thanks,
Florian