linux-kernel - Re: [cgroup/for-6.20 PATCH 3/4] cgroup/cpuset: Don't fail cpuset.cpus change in v2

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Message-ID: <471d1be6-db23-42b2-a5e8-5207fb4dcaeb@redhat.com>
Date: Sat, 27 Dec 2025 02:42:32 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.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 3/4] cgroup/cpuset: Don't fail cpuset.cpus
 change in v2

On 12/25/25 4:30 AM, Chen Ridong wrote:
>
> On 2025/12/25 15:30, 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.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 30 +++++++++++++++---------------
>>   1 file changed, 15 insertions(+), 15 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 850334dbc36a..83bf6b588e5f 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
>> + * @new_xcpus:	new exclusive_cpus in trial cpuset
>>    *
> Can we rename it to xcpus_changed?
>
> The current name new_xcpus gives me the impression that CPUs are being added.
> For example: if exclusive_cpus is 1, and it changes to 1-7, then new_xcpus would be 2-7.

Sure. I will make the change in the next version.

Cheers,
Longman

>
>>    * 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 new_xcpus)
>>   {
>>   	/* 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 (new_xcpus && !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 new_xcpus;
>>   	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;
>> +	new_xcpus = !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, new_xcpus))
>>   			goto out;
>>   		if (mems_excl_conflict(trial, c))
>>   			goto out;