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

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Message-ID: <11f4e2ca-955e-45a6-adc5-d1d8c85e4579@huaweicloud.com>
Date: Sat, 27 Dec 2025 18:14:50 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...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 2025/12/27 15:40, Waiman Long wrote:
> On 12/25/25 4:27 AM, Chen Ridong wrote:
>>
>> 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?
> 
> By definition, exclusive_cpus have to be exclusive within the same child cpuset level even if some
> of the CPUs cannot be granted from the parent. So other siblings cannot use any of the CPUs 1-7 in
> its exclusive_cpus list or the writing will fail. In the case of cpuset.cpus defined partitions,
> those CPUs will be removed from its effective_xcpus list.
> 
> Cheers,
> Longman
> 

Thank you for the clarification.

Looks good to me.

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

-- 
Best regards,
Ridong