linux-kernel - Re: [PATCH] cgroup/cpuset: simplify the percpu kthreads check in update_tasks

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Date:   Wed, 5 Jul 2023 10:49:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        tj@...nel.org, hannes@...xchg.org, lizefan.x@...edance.com
Subject: Re: [PATCH] cgroup/cpuset: simplify the percpu kthreads check in
 update_tasks_cpumask()

On 7/5/23 01:56, Miaohe Lin wrote:
> On 2023/7/5 11:14, Waiman Long wrote:
>> On 7/4/23 07:30, Miaohe Lin wrote:
>>> kthread_is_per_cpu() can be called directly without checking whether
>>> PF_KTHREAD is set in task->flags. So remove PF_KTHREAD check to make
>>> code more concise.
>>>
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>    kernel/cgroup/cpuset.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>> index 58e6f18f01c1..601c40da8e03 100644
>>> --- a/kernel/cgroup/cpuset.c
>>> +++ b/kernel/cgroup/cpuset.c
>>> @@ -1230,7 +1230,7 @@ static void update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
>>>                /*
>>>                 * Percpu kthreads in top_cpuset are ignored
>>>                 */
>>> -            if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task))
>>> +            if (kthread_is_per_cpu(task))
>>>                    continue;
>>>                cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus);
>>>            } else {
>> The initial intention was to ignore only percpu kthreads. The current code likely ignore all the kthreads. Removing the PF_KTHREAD flag, however, may introduce unexpected regression at this point. I would like to hold off for now until more investigation are done.
> IMHO, the current code will ignore only percpu kthreads:
>    1.If PF_KTHREAD is set in task->flags, this patch doesn't make any difference.
>    2.If PF_KTHREAD is not set in task->flags, kthread_is_per_cpu will *always return false*. So this patch doesn't make any functional change.
>
>      bool kthread_is_per_cpu(struct task_struct *p)
>      {
>          struct kthread *kthread = __to_kthread(p);
> 	if (!kthread)
> 		return false;
>          ....
>      }
>
>      static inline struct kthread *__to_kthread(struct task_struct *p)
>      {
> 	void *kthread = p->worker_private;
> 	if (kthread && !(p->flags & PF_KTHREAD))
> 			 ^^^^^^^^^^^^^^^^^^^^^^
> 			 PF_KTHREAD is not set, so kthread = NULL.
> 		kthread = NULL;
> 	return kthread;
>      }
>
> Or am I miss something? Thanks for comment and review.

Yes, you are right. I was that conscious when I reviewed the patch last 
night :-)

Reviewed-by: Waiman Long <longman@...hat.com>