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

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Message-ID: <87v7kdznxq.ffs@tglx>
Date: Fri, 17 Oct 2025 14:56:49 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Florian Weimer <fweimer@...hat.com>
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

On Fri, Oct 17 2025 at 13:31, Florian Weimer wrote:
> * 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?

get_nprocs_conf(3), which reads /sys/devices/system/cpu/possible