linux-kernel - Re: [patch V2 09/20] cpumask: Cache num_possible

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Message-ID: <aPpXDLxcRq-TZMxL@yury>
Date: Thu, 23 Oct 2025 12:25:48 -0400
From: Yury Norov <yury.norov@...il.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>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>
Subject: Re: [patch V2 09/20] cpumask: Cache num_possible_cpus()

On Wed, Oct 22, 2025 at 02:55:30PM +0200, 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.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Yury Norov <yury.norov@...il.com>
> ---
> V2: New patch
> ---
>  include/linux/cpumask.h |   10 ++++++++--
>  kernel/cpu.c            |   15 +++++++++++++++
>  2 files changed, 23 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -126,6 +126,7 @@ extern struct cpumask __cpu_dying_mask;
>  #define cpu_dying_mask    ((const struct cpumask *)&__cpu_dying_mask)
>  
>  extern atomic_t __num_online_cpus;
> +extern unsigned int __num_possible_cpus;
>  
>  extern cpumask_t cpus_booted_once_mask;
>  
> @@ -1152,13 +1153,13 @@ void init_cpu_possible(const struct cpum
>  #define __assign_cpu(cpu, mask, val)	\
>  	__assign_bit(cpumask_check(cpu), cpumask_bits(mask), (val))
>  
> -#define set_cpu_possible(cpu, possible)	assign_cpu((cpu), &__cpu_possible_mask, (possible))
>  #define set_cpu_enabled(cpu, enabled)	assign_cpu((cpu), &__cpu_enabled_mask, (enabled))
>  #define set_cpu_present(cpu, present)	assign_cpu((cpu), &__cpu_present_mask, (present))
>  #define set_cpu_active(cpu, active)	assign_cpu((cpu), &__cpu_active_mask, (active))
>  #define set_cpu_dying(cpu, dying)	assign_cpu((cpu), &__cpu_dying_mask, (dying))
>  
>  void set_cpu_online(unsigned int cpu, bool online);
> +void set_cpu_possible(unsigned int cpu, bool possible);
>  
>  /**
>   * to_cpumask - convert a NR_CPUS bitmap to a struct cpumask *
> @@ -1211,7 +1212,12 @@ static __always_inline unsigned int num_
>  {
>  	return raw_atomic_read(&__num_online_cpus);
>  }
> -#define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
> +
> +static __always_inline unsigned int num_possible_cpus(void)
> +{
> +	return __num_possible_cpus;
> +}
> +
>  #define num_enabled_cpus()	cpumask_weight(cpu_enabled_mask)
>  #define num_present_cpus()	cpumask_weight(cpu_present_mask)
>  #define num_active_cpus()	cpumask_weight(cpu_active_mask)
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3108,6 +3108,9 @@ EXPORT_SYMBOL(__cpu_dying_mask);
>  atomic_t __num_online_cpus __read_mostly;
>  EXPORT_SYMBOL(__num_online_cpus);
>  
> +unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
> +EXPORT_SYMBOL(__num_possible_cpus);
> +
>  void init_cpu_present(const struct cpumask *src)
>  {
>  	cpumask_copy(&__cpu_present_mask, src);
> @@ -3116,6 +3119,7 @@ void init_cpu_present(const struct cpuma
>  void init_cpu_possible(const struct cpumask *src)
>  {
>  	cpumask_copy(&__cpu_possible_mask, src);
> +	__num_possible_cpus = cpumask_weight(&__cpu_possible_mask);
>  }
>  
>  void set_cpu_online(unsigned int cpu, bool online)
> @@ -3139,6 +3143,17 @@ void set_cpu_online(unsigned int cpu, bo
>  	}
>  }
>  
> +void set_cpu_possible(unsigned int cpu, bool possible)
> +{
> +	if (possible) {
> +		if (!cpumask_test_and_set_cpu(cpu, &__cpu_possible_mask))
> +			__num_possible_cpus++;
> +	} else {
> +		if (cpumask_test_and_clear_cpu(cpu, &__cpu_possible_mask))
> +			__num_possible_cpus--;
> +	}
> +}

You can save a couple conditionals:

        if (possible)
                __num_possible_cpus += !cpumask_test_and_set_cpu(cpu, &__cpu_possible_mask));
        else
                __num_possible_cpus -= cpumask_test_and_clear_cpu(cpu, &__cpu_possible_mask));

Otherwise,

Reviewed-by: Yury Norov (NVIDIA) <yury.norov@...il.com>