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

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Message-ID: <4cdfd743-3cfd-4317-8e24-0f94e6fa8d39@linux.ibm.com>
Date: Mon, 3 Nov 2025 15:36:35 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>, 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 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?

> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Reviewed-by: Yury Norov <yury.norov@...il.com>
> ---