linux-kernel - Re: [PATCH v2 2/6] lib/group_cpus: relax atomicity requirement in grp_spread_init

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Message-ID: <ZXJx72/YOGn0l4pI@fedora>
Date:   Fri, 8 Dec 2023 09:31:27 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2 2/6] lib/group_cpus: relax atomicity requirement in
 grp_spread_init_one()

On Thu, Dec 07, 2023 at 12:38:56PM -0800, Yury Norov wrote:
> Because nmsk and irqmsk are stable, extra atomicity is not required.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  lib/group_cpus.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index ee272c4cefcc..8eb18c6bbf3b 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -24,8 +24,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  		if (cpu >= nr_cpu_ids)
>  			return;
>  
> -		cpumask_clear_cpu(cpu, nmsk);
> -		cpumask_set_cpu(cpu, irqmsk);
> +		__cpumask_clear_cpu(cpu, nmsk);
> +		__cpumask_set_cpu(cpu, irqmsk);
>  		cpus_per_grp--;
>  
>  		/* If the cpu has siblings, use them first */
> @@ -34,9 +34,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  			sibl = cpumask_next(sibl, siblmsk);
>  			if (sibl >= nr_cpu_ids)
>  				break;
> -			if (!cpumask_test_and_clear_cpu(sibl, nmsk))
> -				continue;
> -			cpumask_set_cpu(sibl, irqmsk);
> +			__cpumask_clear_cpu(sibl, nmsk);
> +			__cpumask_set_cpu(sibl, irqmsk);
>  			cpus_per_grp--;

Here the change isn't simply to remove atomicity, and the test
part of cpumask_test_and_clear_cpu() is removed, so logic is changed,
I feel the correct change should be:

	if (cpumask_test_cpu(sibl, nmsk)) {
		__cpumask_clear_cpu(sibl, nmsk);
		__cpumask_set_cpu(sibl, irqmsk);
  		cpus_per_grp--;
	}



Thanks,
Ming