linux-kernel - Re: [PATCH v3 7/7] lib/group_cpus: simplify grp_spread_init

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Date:   Wed, 13 Dec 2023 09:06:12 +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 v3 7/7] lib/group_cpus: simplify grp_spread_init_one()
 for more

On Mon, Dec 11, 2023 at 08:21:07PM -0800, Yury Norov wrote:
> The outer and inner loops of grp_spread_init_one() do the same thing -
> move a bit from nmsk to irqmsk.
> 
> The inner loop iterates the sibling group, which includes the CPU picked
> by outer loop. And it means that we can drop the part that moves the bit
> in the outer loop.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  lib/group_cpus.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index 664a56171a1b..7aa7a6289355 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -18,14 +18,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  	int cpu, sibl;
>  
>  	for_each_cpu(cpu, nmsk) {
> -		__cpumask_clear_cpu(cpu, nmsk);
> -		__cpumask_set_cpu(cpu, irqmsk);
> -		if (cpus_per_grp-- == 0)
> -			return;
> -
> -		/* If the cpu has siblings, use them first */
>  		siblmsk = topology_sibling_cpumask(cpu);
> -		sibl = cpu + 1;
> +		sibl = cpu;
>  
>  		for_each_cpu_and_from(sibl, siblmsk, nmsk) {
>  			__cpumask_clear_cpu(sibl, nmsk);

Correctness of the above change requires that 'cpu' has to be included
into topology_sibling_cpumask(cpu), however, not sure it is always true,
see the following comment in Documentation/arch/x86/topology.rst

`
  - topology_sibling_cpumask():

    The cpumask contains all online threads in the core to which a thread
    belongs.
`

Thanks, 
Ming