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Message-ID: <fd99f1a3-c38c-f344-b581-7df4a3937eef@redhat.com>
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>
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