Message-ID: <d8f7d319-3b72-4531-ab25-a2505cdce828@huaweicloud.com>
Date: Mon, 12 Jan 2026 09:25:06 +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: [PATCH cgroup/for-6.20 v3 3/5] cgroup/cpuset: Don't fail
 cpuset.cpus change in v2



On 2026/1/10 9:32, Waiman Long wrote:
> Commit fe8cd2736e75 ("cgroup/cpuset: Delay setting of CS_CPU_EXCLUSIVE
> until valid partition") introduced a new check to disallow the setting
> of a new cpuset.cpus.exclusive value that is a superset of a sibling's
> cpuset.cpus value so that there will at least be one CPU left in the
> sibling in case the cpuset becomes a valid partition root. This new
> check does have the side effect of failing a cpuset.cpus change that
> make it a subset of a sibling's cpuset.cpus.exclusive value.
> 
> With v2, users are supposed to be allowed to set whatever value they
> want in cpuset.cpus without failure. To maintain this rule, the check
> is now restricted to only when cpuset.cpus.exclusive is being changed
> not when cpuset.cpus is changed.
> 
> The cgroup-v2.rst doc file is also updated to reflect this change.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  8 +++----
>  kernel/cgroup/cpuset.c                  | 30 ++++++++++++-------------
>  2 files changed, 19 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 7f5b59d95fce..510df2461aff 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -2561,10 +2561,10 @@ Cpuset Interface Files
>  	Users can manually set it to a value that is different from
>  	"cpuset.cpus".	One constraint in setting it is that the list of
>  	CPUs must be exclusive with respect to "cpuset.cpus.exclusive"
> -	of its sibling.  If "cpuset.cpus.exclusive" of a sibling cgroup
> -	isn't set, its "cpuset.cpus" value, if set, cannot be a subset
> -	of it to leave at least one CPU available when the exclusive
> -	CPUs are taken away.
> +	and "cpuset.cpus.exclusive.effective" of its siblings.	Another
> +	constraint is that it cannot be a superset of "cpuset.cpus"
> +	of its sibling in order to leave at least one CPU available to
> +	that sibling when the exclusive CPUs are taken away.
>  
>  	For a parent cgroup, any one of its exclusive CPUs can only
>  	be distributed to at most one of its child cgroups.  Having an
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 894131f47f78..4819ab429771 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -609,33 +609,31 @@ static inline bool cpusets_are_exclusive(struct cpuset *cs1, struct cpuset *cs2)
>  
>  /**
>   * cpus_excl_conflict - Check if two cpusets have exclusive CPU conflicts
> - * @cs1: first cpuset to check
> - * @cs2: second cpuset to check
> + * @trial:	the trial cpuset to be checked
> + * @sibling:	a sibling cpuset to be checked against
> + * @xcpus_changed: set if exclusive_cpus has been set
>   *
>   * Returns: true if CPU exclusivity conflict exists, false otherwise
>   *
>   * Conflict detection rules:
>   * 1. If either cpuset is CPU exclusive, they must be mutually exclusive
>   * 2. exclusive_cpus masks cannot intersect between cpusets
> - * 3. The allowed CPUs of one cpuset cannot be a subset of another's exclusive CPUs
> + * 3. The allowed CPUs of a sibling cpuset cannot be a subset of the new exclusive CPUs
>   */
> -static inline bool cpus_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
> +static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibling,
> +				      bool xcpus_changed)
>  {
>  	/* If either cpuset is exclusive, check if they are mutually exclusive */
> -	if (is_cpu_exclusive(cs1) || is_cpu_exclusive(cs2))
> -		return !cpusets_are_exclusive(cs1, cs2);
> +	if (is_cpu_exclusive(trial) || is_cpu_exclusive(sibling))
> +		return !cpusets_are_exclusive(trial, sibling);
>  
>  	/* Exclusive_cpus cannot intersect */
> -	if (cpumask_intersects(cs1->exclusive_cpus, cs2->exclusive_cpus))
> +	if (cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus))
>  		return true;
>  
> -	/* The cpus_allowed of one cpuset cannot be a subset of another cpuset's exclusive_cpus */
> -	if (!cpumask_empty(cs1->cpus_allowed) &&
> -	    cpumask_subset(cs1->cpus_allowed, cs2->exclusive_cpus))
> -		return true;
> -
> -	if (!cpumask_empty(cs2->cpus_allowed) &&
> -	    cpumask_subset(cs2->cpus_allowed, cs1->exclusive_cpus))
> +	/* The cpus_allowed of a sibling cpuset cannot be a subset of the new exclusive_cpus */
> +	if (xcpus_changed && !cpumask_empty(sibling->cpus_allowed) &&
> +	    cpumask_subset(sibling->cpus_allowed, trial->exclusive_cpus))
>  		return true;
>  
>  	return false;
> @@ -672,6 +670,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
>  {
>  	struct cgroup_subsys_state *css;
>  	struct cpuset *c, *par;
> +	bool xcpus_changed;
>  	int ret = 0;
>  
>  	rcu_read_lock();
> @@ -728,10 +727,11 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
>  	 * overlap. exclusive_cpus cannot overlap with each other if set.
>  	 */
>  	ret = -EINVAL;
> +	xcpus_changed = !cpumask_equal(cur->exclusive_cpus, trial->exclusive_cpus);
>  	cpuset_for_each_child(c, css, par) {
>  		if (c == cur)
>  			continue;
> -		if (cpus_excl_conflict(trial, c))
> +		if (cpus_excl_conflict(trial, c, xcpus_changed))
>  			goto out;
>  		if (mems_excl_conflict(trial, c))
>  			goto out;

Reviewed-by: Chen Ridong <chenridong@...wei.com>

-- 
Best regards,
Ridong

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