linux-kernel - Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom

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Message-ID: <20200820111349.GE5033@dhcp22.suse.cz>
Date:   Thu, 20 Aug 2020 13:13:49 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        christian.brauner@...ntu.com, mingo@...nel.org,
        peterz@...radead.org, tglx@...utronix.de, esyr@...hat.com,
        christian@...lner.me, areber@...hat.com, shakeelb@...gle.com,
        cyphar@...har.com, adobriyan@...il.com, akpm@...ux-foundation.org,
        ebiederm@...ssion.com, gladkov.alexey@...il.com, walken@...gle.com,
        daniel.m.jordan@...cle.com, avagin@...il.com,
        bernd.edlinger@...mail.de, john.johansen@...onical.com,
        laoar.shao@...il.com, timmurray@...gle.com, minchan@...nel.org,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in
 __set_oom_adj when not necessary

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