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

[Christian Brauner]

On Thu, Aug 20, 2020 at 01:13:49PM +0200, Michal Hocko wrote:
> 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?

vfork() implies CLONE_VM but !CLONE_THREAD. The way this patch is
written the mutex lock will be taken every time you do a vfork().

(It's honestly also debatable whether it's that rare. For one, userspace
stuff I maintain uses it too (see [1]).
[1]: https://github.com/lxc/lxc/blob/9d3b7c97f0443adc9f0b0438437657ab42f5a1c3/src/lxc/start.c#L1676
)

> 
> > > --- 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.

I was toying with moving this into sm like:

static inline copy_oom_score(unsigned long flags, struct task_struct *tsk)

trying to rely on set_bit() and test_bit() in copy_mm() being atomic and
then calling it where Oleg said after the point of no return.

Christian