linux-kernel - Re: [PATCH v3 4/7] cgroup: cgroup v2 freezer

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Message-ID: <20181120164349.GB21462@tower.DHCP.thefacebook.com>
Date:   Tue, 20 Nov 2018 16:43:52 +0000
From:   Roman Gushchin <guro@...com>
To:     Tejun Heo <tj@...nel.org>
CC:     Roman Gushchin <guroan@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 4/7] cgroup: cgroup v2 freezer

On Tue, Nov 20, 2018 at 08:36:04AM -0800, Tejun Heo wrote:
> On Tue, Nov 20, 2018 at 04:33:11PM +0000, Roman Gushchin wrote:
> > > If a non-frozen task is being moved into a frozen cgroup, shouldn't
> > > that also trigger frozen state update?
> > 
> > It does! Just below these lines:
> > 
> > 	/*
> > 	 * If the task isn't in the desired state, force it to it.
> > 	 */
> > 	if (task->frozen != test_bit(CGRP_FREEZE, &dst->flags))
> > 		cgroup_freeze_task(task, test_bit(CGRP_FREEZE, &dst->flags));
> 
> But that wouldn't udpate the cgroup's frozen state and generate
> notifications, right?

Why? The task will be eventually trapped into cgroup_enter_frozen(),
and from there cgroup_update_frozen() will be called.

You are right, that notification will not be issued, because the cgroup
is not changing its state (frozen->frozen). I'm not sure that it makes
sense to change the cgroup state back and forth in this case. Are there
any reasons I'm missing?

Thanks!