From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer Date: Thu, 20 Dec 2018 17:16:50 +0100 Message-ID: <20181220161649.GA31865@redhat.com> References: <20181207201531.1665-1-guro@fb.com> <20181207201531.1665-5-guro@fb.com> <20181211162632.GB8504@redhat.com> <20181211184033.GA8971@tower.DHCP.thefacebook.com> <20181212174902.GA30309@redhat.com> <20181218012800.GA29563@tower.DHCP.thefacebook.com> <20181218171230.GA11319@redhat.com> <20181218202701.GA12730@castle.DHCP.thefacebook.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20181218202701.GA12730@castle.DHCP.thefacebook.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Roman Gushchin Cc: Roman Gushchin , Tejun Heo , Dan Carpenter , Mike Rapoport , "cgroups@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Kernel Team 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. > 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. Oleg.