From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Gushchin Subject: Re: [PATCH v5 4/7] cgroup: cgroup v2 freezer Date: Thu, 20 Dec 2018 21:43:47 +0000 Message-ID: <20181220214338.GA16795@tower.DHCP.thefacebook.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> <20181220161649.GA31865@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : references : in-reply-to : content-type : content-id : content-transfer-encoding : mime-version; s=facebook; bh=0YXgXI7mcWc33l9vb73uIL6NxcZpO2O+MM8onuB+MHU=; b=NqjK4j+YyK8rkmDQ3gQnjRI34H5if3lCb1iEintr/diCawadDcjXPn4ZVLlS2H1FvZf1 llW4yA90exHktqG8JdWO1RABwMYtF8sccA45SdSqhVk0dIIuBGV5h+SPi1avEQYK5oEb gqXSbElDWeeJ9TmPJTHQVax86Q/AfehSV1k= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.onmicrosoft.com; s=selector1-fb-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=0YXgXI7mcWc33l9vb73uIL6NxcZpO2O+MM8onuB+MHU=; b=HLY04jtHPGqBSSNS2ZWxSEDCLZqhFcS0E8cZuKnztwh5oStdcI43P7nUe49oyHWqgjwB9YE6wvulFW2yxznu7oxgkzkzq2hqpfeJrsPSU14BXGX/c9PucXv8d4BwffO3St89yQbAGN5L0BbkaPRAYro1iuWhGAkDXaB4N9O7BW0= In-Reply-To: <20181220161649.GA31865@redhat.com> Content-Language: en-US Content-ID: <833EAB85961EEC41B965B6416EA86CDC@namprd15.prod.outlook.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: To: Oleg Nesterov 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 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!