Re: [PATCH 3/6] mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj

[Michal Hocko]

On Mon 30-05-16 15:19:32, Vladimir Davydov wrote:
> On Mon, May 30, 2016 at 01:11:48PM +0200, Michal Hocko wrote:
> > On Mon 30-05-16 13:26:44, Vladimir Davydov wrote:
> > > On Mon, May 30, 2016 at 11:39:50AM +0200, Michal Hocko wrote:
> > [...]
> > > > Yes and that leads me to a suspicion that we can do that. Maybe I should
> > > > just add a note into the log that we are doing that so that people can
> > > > complain? Something like the following
> > > > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > > > index fa0b3ca94dfb..7f3495415719 100644
> > > > --- a/fs/proc/base.c
> > > > +++ b/fs/proc/base.c
> > > > @@ -1104,7 +1104,6 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
> > > >  err_sighand:
> > > >  	unlock_task_sighand(task, &flags);
> > > >  err_put_task:
> > > > -	put_task_struct(task);
> > > >  
> > > >  	if (mm) {
> > > >  		struct task_struct *p;
> > > > @@ -1113,6 +1112,10 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
> > > >  		for_each_process(p) {
> > > >  			task_lock(p);
> > > >  			if (!p->vfork_done && process_shares_mm(p, mm)) {
> > > > +				pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n",
> > > > +						task_pid_nr(p), p->comm,
> > > > +						p->signal->oom_score_adj, oom_adj,
> > > > +						task_pid_nr(task), task->comm);
> > > 
> > > IMO this could be acceptable from userspace pov, but I don't very much
> > > like how vfork is special-cased here and in oom killer code.
> > 
> > Well, the vfork has to be special cased here. We definitely have to
> > support
> > 	vfork()
> > 	set_oom_score_adj()
> > 	exec()
> > 
> > use case. And I do not see other way without adding something to the
> > clone hot paths which sounds like not justifiable considering we are
> > talking about a really rare usecase that basically nobody cares about.
> 
> I don't think that vfork->exec use case is rare. Quite the contrary, I'm
> pretty sure it's used often, because in contrast to fork->exec it avoids
> copying page tables, which can be very expensive for fat processes.

Ohh, yes, the way I put it is ambiguous. What I wanted to say is that
the oom is really unlikely so it doesn't justify hot path changes.

> Frankly, I don't understand why you are so determined not to add
> anything to the fork path.

It is not just the fork path. It would require touching exit path as
well and all that code is quite complex already. I would prefer if the
oom related complexity stay in the oom proper.

> Of course, if the overhead were that
> dramatic, we would have to forget the idea, but if it were say <= 0.1 %
> for a contrived test that calls fork in a loop, IMHO the modification
> would be justified.

But why if the proc handler resp. oom_kill_process paths can handle most
cases and the occasional races should be tolerate able AFAICS.
-- 
Michal Hocko
SUSE Labs