Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

Michal Hocko
2020-08-20T11:14:50Z

On Thu 20-08-20 12:55:56, Oleg Nesterov wrote:
> On 08/19, Suren Baghdasaryan wrote:
> >
> > Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely
> > used the additional mutex lock in that path of the clone() syscall should
> > not affect its overall performance. Clearing the MMF_PROC_SHARED flag
> > (when the last process sharing the mm exits) is left out of this patch to
> > keep it simple and because it is believed that this threading model is
> > rare.
>
> vfork() ?

Could you be more specific?

> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
> > if (clone_flags & CLONE_VM) {
> > mmget(oldmm);
> > mm = oldmm;
> > + if (!(clone_flags & CLONE_SIGHAND)) {
>
> I agree with Christian, you need CLONE_THREAD

This was my suggestion to Suren, likely because I've misrememberd which
clone flag is responsible for the signal delivery. But now, after double
checking we do explicitly disallow CLONE_SIGHAND && !CLONE_VM. So
CLONE_THREAD is the right thing to check.

> > + /* We need to synchronize with __set_oom_adj */
> > + mutex_lock(&oom_adj_lock);
> > + set_bit(MMF_PROC_SHARED, &mm->flags);
> > + /* Update the values in case they were changed after copy_signal */
> > + tsk->signal->oom_score_adj = current->signal->oom_score_adj;
> > + tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min;
> > + mutex_unlock(&oom_adj_lock);
>
> I don't understand how this can close the race with __set_oom_adj...
>
> What if __set_oom_adj() is called right after mutex_unlock() ? It will see
> MMF_PROC_SHARED, but for_each_process() won't find the new child until
> copy_process() does list_add_tail_rcu(&p->tasks, &init_task.tasks) ?

Good point. Then we will have to move this thing there.

Thanks!
--
Michal Hocko
SUSE Labs