linux-kernel - Re: [patch V3 09/20] cpumask: Cache num_possible

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Message-ID: <87tszbw8ik.ffs@tglx>
Date: Mon, 03 Nov 2025 14:28:03 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: 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>, Florian Weimer <fweimer@...hat.com>, Tim Chen
 <tim.c.chen@...el.com>, Yury Norov <yury.norov@...il.com>
Subject: Re: [patch V3 09/20] cpumask: Cache num_possible_cpus()

On Mon, Nov 03 2025 at 15:36, Shrikanth Hegde wrote:
> On 10/29/25 6:39 PM, Thomas Gleixner wrote:
>> Reevaluating num_possible_cpus() over and over does not make sense. That
>> becomes a constant after init as cpu_possible_mask is marked ro_after_init.
>> 
>> Cache the value during initialization and provide that for consumption.
>> 
>
> Reviewed-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
>
>
> Observation:
> I see below usage which could be improved too?
> kernel/irq/affinity.c:		set_vecs = cpumask_weight(cpu_possible_mask);
> lib/tests/cpumask_kunit.c:	KUNIT_EXPECT_EQ_MSG(test, nr_cpu_ids, cpumask_weight(cpu_possible_mask),
>
> Specially irq_calc_affinity_vectors, it seems to take cpus_read_lock, but I don't think
> that lock is protecting possible cpus. possible cpus can't change after boot. No?

It can't. So yes the cpus_read_lock() is pointless.