Message-ID: <0384dc28-bfff-4a74-9153-66d287888ce7@redhat.com>
Date: Fri, 9 Jan 2026 15:15:57 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Waiman Long <llong@...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 1/5/26 1:29 AM, Chen Ridong wrote:
>
> On 2026/1/5 12:06, Waiman Long wrote:
>> On 1/4/26 10:58 PM, Chen Ridong wrote:
>>> On 2026/1/5 11:50, Waiman Long wrote:
>>>> On 1/4/26 8:15 PM, Chen Ridong wrote:
>>>>> On 2026/1/5 5:25, Waiman Long wrote:
>>>>>> On 1/3/26 9:48 PM, Chen Ridong wrote:
>>>>>>> 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?
>>>>>> update_cpumasks_hier() is called when changes in a cpuset or hotplug affects other cpusets in the
>>>>>> hierarchy. With respect to changes in partition state, it is either from valid to invalid or vice
>>>>>> versa. It will not change from a valid partition to member. The only way new_prs = 0 is when
>>>>>> old_prs
>>>>>> = 0. Even if the affected cpuset is processed again in update_cpumask_hier(), any state change
>>>>>> from
>>>>>> valid partition to member (update_prstate()), reset_partition_data() should have been called
>>>>>> there.
>>>>>> That is why we only care about when new_prs != 0.
>>>>>>
>>>>> Thank you for your patience.
>>>>>
>>>>>> The code isn't wrong here. However I can change the condition to (new_prs <= 0) if it makes it
>>>>>> easier to understand.
>>>>>>
>>>>> I agree there's nothing wrong with the current logic. However, for clarity, I suggest changing the
>>>>> condition to (new_prs <= 0). This allows the function's logic to be fully self-consistent and
>>>>> focused on a single responsibility. This approach would allow us to simplify the code to:
>>>>>
>>>>>       if (new_prs > 0)
>>>>>           compute_excpus(cp, cp->effective_xcpus);
>>>>>       else
>>>>>           reset_partition_data(cp);
>>>>>
>>>>> Since reset_partition_data() already handles cases whether cp->exclusive_cpus is empty or not, this
>>>>> implementation would be more concise while correctly covering all scenarios.
>>>> effective_xcpus should be set when exclusive_cpus is not empty or when the cpuset is a valid
>>>> partition root. So just checking new_prs for compute_excpus() is not enough.
>>>>
>>> If we change the condition to (new_prs <= 0), it will reset the partition data even when we call
>>> compute_excpus (for !cpumask_empty(cp->exclusive_cpus)), so we should still get the same result,
>>> right?
>> Changing the condition to (new_prs <= 0) won't affect the result except for a bit of wasted cpu
>> cycles. That is why I am planning to make the change in the next version to make it easier to
>> understand.
>>
> Sorry, I should have been clearer. If we change the condition, the code would essentially be:
>
> 	if ((new_prs > 0) || !cpumask_empty(cp->exclusive_cpus))
> 		compute_excpus(cp, cp->effective_xcpus);
>          if (new_prs <= 0)
> 		reset_partition_data(cp);
>
> For cases where new_prs <= 0 && !cpumask_empty(cp->exclusive_cpus), both compute_excpus() and
> reset_partition_data() would be called.
>
> Is this functionally equivalent to:
>
> 	if (new_prs > 0)
> 		compute_excpus(cp, cp->effective_xcpus);
>          else (new_prs <= 0)
> 		reset_partition_data(cp);

They are not equivalent because reset_partition_data() won't do a 
compute_excpus(). In fact, one of the tests in test_cpuset_prs.sh will 
fail if we make this change.

Cheers,
Longman

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