linux-kernel - Re: [PATCH -next RFC 05/16] cpuset: factor out partition

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Message-ID: <649eeb65-8872-4b52-aef9-d61cd282c7ed@redhat.com>
Date: Mon, 20 Oct 2025 15:45:24 -0400
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Waiman Long <llong@...hat.com>,
 tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com
Cc: cgups@...r.kernel.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH -next RFC 05/16] cpuset: factor out partition_update()
 function

On 10/20/25 4:05 AM, Chen Ridong wrote:
>
> On 2025/10/20 10:43, Waiman Long wrote:
>> On 9/28/25 3:12 AM, Chen Ridong wrote:
>>> From: Chen Ridong <chenridong@...wei.com>
>>>
>>> Extract the core partition update logic into a dedicated partition_update()
>>> function. This refactoring centralizes updates to key cpuset data
>>> structures including remote_sibling, effective_xcpus, partition_root_state,
>>> and prs_err.
>>>
>>> The function handles the complete partition update workflow:
>>> - Adding and removing exclusive CPUs via partition_xcpus_add()/del()
>>> - Managing remote sibling relationships
>>> - Synchronizing effective exclusive CPUs mask
>>> - Updating partition state and error status
>>> - Triggering required system updates and workqueue synchronization
>>>
>>> This creates a coherent interface for partition operations and establishes
>>> a foundation for enhanced partition management while maintaining existing
>>> remote partition behavior.
>>>
>>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>>> ---
>>>    kernel/cgroup/cpuset.c | 71 ++++++++++++++++++++++++++++--------------
>>>    1 file changed, 47 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>> index 1944410ae872..0e2f95daf459 100644
>>> --- a/kernel/cgroup/cpuset.c
>>> +++ b/kernel/cgroup/cpuset.c
>>> @@ -1587,6 +1587,49 @@ static void partition_disable(struct cpuset *cs, struct cpuset *parent,
>>>        cpuset_force_rebuild();
>>>    }
>>>    +/**
>>> + * partition_update - Update an existing partition configuration
>>> + * @cs: The cpuset to update
>>> + * @prs: Partition root state (must be positive)
>>> + * @xcpus: New exclusive CPUs mask for the partition (NULL to keep current)
>>> + * @excpus: New effective exclusive CPUs mask
>>> + * @tmp: Temporary masks
>>> + *
>>> + * Updates partition-related fields. The tmp->addmask is the CPU mask that
>>> + * will be added to the subpartitions_cpus and removed from parent's
>>> + * effective_cpus, and the tmp->delmask vice versa.
>>> + */
>>> +static void partition_update(struct cpuset *cs, int prs, struct cpumask *xcpus,
>>> +                  struct cpumask *excpus, struct tmpmasks *tmp)
>>> +{
>>> +    bool isolcpus_updated;
>>> +    bool excl_updated;
>>> +    struct cpuset *parent;
>>> +
>>> +    lockdep_assert_held(&cpuset_mutex);
>>> +    WARN_ON_ONCE(!cpuset_v2());
>>> +    WARN_ON_ONCE(prs <= 0);
>>> +
>>> +    parent = is_remote_partition(cs) ? NULL : parent_cs(cs);
>>> +    excl_updated = !cpumask_empty(tmp->addmask) ||
>>> +            !cpumask_empty(tmp->delmask);
>>> +
>>> +    spin_lock_irq(&callback_lock);
>>> +    isolcpus_updated = partition_xcpus_add(prs, parent, tmp->addmask);
>>> +    isolcpus_updated |= partition_xcpus_del(prs, parent, tmp->delmask);
>> The current partition_xcpus_add/del() functions assume the given cpumas is non-empty. In the new
>> partition_update() helper, you can pass an empty cpumask to them. This will cause useless work to be
>> done. Also isolcpus_update may not be correct because of that causing unneeded work to be done in
>> the workqueue code.
>>
>> -Longman
> Thank you, Longman.
>
> I think we can add a check for empty cpumask inputs in partition_xcpus_add() and
> partition_xcpus_del() to avoid unnecessary operations.
Yes, you should do an empty cpumask check if empty cpumask can be passed 
to the helper.
>
> To clarify, do you mean that passing an empty cpumask to these functions might lead to incorrect
> isolcpus_updated value? or are you referring to other potential logic issues?

My main concern is doing non-useful work. However, I am sure if there 
will be an undesirable side effects as well.

Cheers,
Longman