linux-kernel - Re: [cgroup/for-6.20 PATCH v2 2/4] cgroup/cpuset: Consistently compute effective_xcpus in update_cpumasks

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Message-ID: <758f42df-52c2-4660-8ef7-1cbacb9323d2@huaweicloud.com>
Date: Sun, 4 Jan 2026 10:48:11 +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>, Jonathan Corbet <corbet@....net>,
 Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
 Sun Shaojie <sunshaojie@...inos.cn>
Subject: Re: [cgroup/for-6.20 PATCH v2 2/4] cgroup/cpuset: Consistently
 compute effective_xcpus in update_cpumasks_hier()



On 2026/1/2 3:15, Waiman Long wrote:
> Since commit f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check()
> & make update_cpumasks_hier() handle remote partition"), the
> compute_effective_exclusive_cpumask() helper was extended to
> strip exclusive CPUs from siblings when computing effective_xcpus
> (cpuset.cpus.exclusive.effective). This helper was later renamed to
> compute_excpus() in commit 86bbbd1f33ab ("cpuset: Refactor exclusive
> CPU mask computation logic").
> 
> This helper is supposed to be used consistently to compute
> effective_xcpus. However, there is an exception within the callback
> critical section in update_cpumasks_hier() when exclusive_cpus of a
> valid partition root is empty. This can cause effective_xcpus value to
> differ depending on where exactly it is last computed. Fix this by using
> compute_excpus() in this case to give a consistent result.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/cgroup/cpuset.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index da2b3b51630e..37d118a9ad4d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2168,17 +2168,13 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>  		spin_lock_irq(&callback_lock);
>  		cpumask_copy(cp->effective_cpus, tmp->new_cpus);
>  		cp->partition_root_state = new_prs;
> -		if (!cpumask_empty(cp->exclusive_cpus) && (cp != cs))
> -			compute_excpus(cp, cp->effective_xcpus);
> -
>  		/*
> -		 * Make sure effective_xcpus is properly set for a valid
> -		 * partition root.
> +		 * Need to compute effective_xcpus if either exclusive_cpus
> +		 * is non-empty or it is a valid partition root.
>  		 */
> -		if ((new_prs > 0) && cpumask_empty(cp->exclusive_cpus))
> -			cpumask_and(cp->effective_xcpus,
> -				    cp->cpus_allowed, parent->effective_xcpus);
> -		else if (new_prs < 0)
> +		if ((new_prs > 0) || !cpumask_empty(cp->exclusive_cpus))
> +			compute_excpus(cp, cp->effective_xcpus);
> +		if (new_prs < 0)
>  			reset_partition_data(cp);
>  		spin_unlock_irq(&callback_lock);
>  

The code resets partition data only for new_prs < 0. My understanding is that a partition is invalid
when new_prs <= 0. Shouldn't reset_partition_data() also be called when new_prs = 0? Is there a
specific reason to skip the reset in that case?

-- 
Best regards,
Ridong