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From:   Roman Gushchin <guro@fb.com>
To:     Oleg Nesterov <oleg@redhat.com>
CC:     Roman Gushchin <guroan@gmail.com>, Tejun Heo <tj@kernel.org>,
        "Dan Carpenter" <dan.carpenter@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer
Thread-Topic: [PATCH v5 4/7] cgroup: cgroup v2 freezer
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Date:   Thu, 20 Dec 2018 21:43:47 +0000
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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 h=
ave 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 fi=
les,
> > > > > 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 r=
ights
> > > > > 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 g=
et
> > anywhere, because we'll catch it in freezer_count(). We allow it to sle=
ep
> > 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 cgr=
oup v2
> > freezer, let it sleep there, and catch if it tries to escape. Exactly a=
s it
> > works for SIGSTOP.
> >
> > Am I missing something?
>=20
> Roman, perhaps we misunderstood each other...
>=20
> I still think that the cgroup_task_frozen() check in freeze_task() you pr=
oposed
> a) is not right, and b) it is not what we do with the STOPPED/TRACED task=
s which
> call freezable_schedule(). This is what I tried to say.
>=20
> 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.

>=20
> > So, you think that v2 freezer should follow the same approach, and allo=
w tasks
> > sleeping on SIGSTOP, for instance, to be treated as frozen?
> > Hm, maybe. I have to think more here.
>=20
> I think this would be nice. Otherwise, say, CGRP_FREEZE can be never repo=
rted
> 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!