linux-kernel - Re: [cgroup/for-6.20 PATCH 1/4] cgroup/cpuset: Streamline rm_siblings_excl

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Message-ID: <c75025d3-17cd-47bb-a222-bde3a156bbbb@huaweicloud.com>
Date: Thu, 25 Dec 2025 17:27:34 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Sun Shaojie <sunshaojie@...inos.cn>
Subject: Re: [cgroup/for-6.20 PATCH 1/4] cgroup/cpuset: Streamline
 rm_siblings_excl_cpus()



On 2025/12/25 15:30, Waiman Long wrote:
> If exclusive_cpus is set, effective_xcpus must be a subset of
> exclusive_cpus. Currently, rm_siblings_excl_cpus() checks both
> exclusive_cpus and effective_xcpus connectively. It is simpler
> to check only exclusive_cpus if non-empty or just effective_xcpus
> otherwise.
> 
> No functional change is expected.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/cgroup/cpuset.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 221da921b4f9..3d2d28f0fd03 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1355,23 +1355,24 @@ static int rm_siblings_excl_cpus(struct cpuset *parent, struct cpuset *cs,
>  	int retval = 0;
>  
>  	if (cpumask_empty(excpus))
> -		return retval;
> +		return 0;
>  
>  	/*
>  	 * Exclude exclusive CPUs from siblings
>  	 */
>  	rcu_read_lock();
>  	cpuset_for_each_child(sibling, css, parent) {
> +		struct cpumask *sibling_xcpus;
> +
>  		if (sibling == cs)
>  			continue;
>  
> -		if (cpumask_intersects(excpus, sibling->exclusive_cpus)) {
> -			cpumask_andnot(excpus, excpus, sibling->exclusive_cpus);
> -			retval++;
> -			continue;
> -		}
> -		if (cpumask_intersects(excpus, sibling->effective_xcpus)) {
> -			cpumask_andnot(excpus, excpus, sibling->effective_xcpus);
> +		sibling_xcpus = cpumask_empty(sibling->exclusive_cpus)
> +			      ? sibling->effective_xcpus
> +			      : sibling->exclusive_cpus;
> +

I'm wondering if this is sufficient?

sibling_xcpus = sibling->effective_xcpus

      p(exclusive_cpus = 1)
   /	  \
 a	b(root, exclusive_cpus=1-7, effective_xcpus=1)

What the sibling's effective exclusive CPUs actually should be is not CPUs 1-7 but CPU 1. So, do we
need to remove CPUs 2-7?

> +		if (cpumask_intersects(excpus, sibling_xcpus)) {
> +			cpumask_andnot(excpus, excpus, sibling_xcpus);
>  			retval++;
>  		}
>  	}

-- 
Best regards,
Ridong