Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer

[Roman Gushchin <guro@fb.com>]

On Thu, Dec 20, 2018 at 05:16:50PM +0100, Oleg Nesterov wrote:
> On 12/18, Roman Gushchin wrote:
> >
> > > > > > --- a/kernel/freezer.c
> > > > > > +++ b/kernel/freezer.c
> > > > > > @@ -134,7 +134,7 @@ bool freeze_task(struct task_struct *p)
> > > > > >                 return false;
> > > > > >
> > > > > >         spin_lock_irqsave(&freezer_lock, flags);
> > > > > > -       if (!freezing(p) || frozen(p)) {
> > > > > > +       if (!freezing(p) || frozen(p) || cgroup_task_frozen()) {
> > > > > >                 spin_unlock_irqrestore(&freezer_lock, flags);
> > > > > >                 return false;
> > > > > >         }
> > > > > >
> > > > > > --
> > > > > >
> > > > > > If the task is already frozen by the cgroup freezer, we don't have to do
> > > > > > anything additionally.
> > > > >
> > > > > I don't think so. A cgroup_task_frozen() task can be killed after
> > > > > try_to_freeze_tasks() succeeds, and the exiting task can close files,
> > > > > do IO, etc. Or it can be thawed by cgroup_freeze_task(false).
> > > > >
> > > > > In short, if try_to_freeze_tasks() succeeds, the caller has all rights
> > > > > to assume that nobody can escape from __refrigerator().
> > > >
> > > > But this is what we do with stopped and ptraced tasks, isn't it?
> > >
> > > No,
> > >
> > > > We do use freezable_schedule() and the system freezer just ignores such tasks.
> > >
> > > 	static inline void freezable_schedule(void)
> > > 	{
> > > 		freezer_do_not_count();
> > > 		schedule();
> > > 		freezer_count();
> > > 	}
> > >
> > > and note that freezer_count() calls try_to_freeze().
> > >
> > > IOW, the task sleeping in freezable_schedule() doesn't really differ from the
> > > task sleeping in __refrigerator(). It is not that "the system freezer just
> > > ignores such tasks", it ignores them because it can safely count them as frozen.
> >
> > Right, so the task is sleeping peacefully, and we know, that it won't get
> > anywhere, because we'll catch it in freezer_count(). We allow it to sleep
> > there, we don't force it to __refrigerator(), and we treat it as frozen.
> >
> > How's that different to cgroup v2 freezer? If the task is frozen by cgroup v2
> > freezer, let it sleep there, and catch if it tries to escape. Exactly as it
> > works for SIGSTOP.
> >
> > Am I missing something?
> 
> Roman, perhaps we misunderstood each other...
> 
> I still think that the cgroup_task_frozen() check in freeze_task() you proposed
> a) is not right, and b) it is not what we do with the STOPPED/TRACED tasks which
> call freezable_schedule(). This is what I tried to say.
> 
> If you meant that freezer v2 can too use freezable_schedule() - I agree.

Sorry for the confusion. Yeah, what I'm saying, is that freezable_schedule()
will work for v2 as well.

> 
> > So, you think that v2 freezer should follow the same approach, and allow tasks
> > sleeping on SIGSTOP, for instance, to be treated as frozen?
> > Hm, maybe. I have to think more here.
> 
> I think this would be nice. Otherwise, say, CGRP_FREEZE can be never reported
> if I read this code correctly. And this looks "symmetrical" with the fact that
> a ->frozen task reacts to SIGSTOP and it is still treated as frozen after that.

Yeah, looks so. I'll try to implement this in v6.

Thanks!