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Date:   Wed, 20 Feb 2019 22:00:27 +0000
From:   Roman Gushchin <guro@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     Roman Gushchin <guroan@...il.com>, Tejun Heo <tj@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 0/7] freezer for cgroup v2

On Wed, Feb 20, 2019 at 03:37:48PM +0100, Oleg Nesterov wrote:
> On 02/19, Roman Gushchin wrote:
> >
> > It provides similar functionality as v1 freezer, but the interface
> > conforms to the cgroup v2 interface design principles, and it
> > provides a better user experience: tasks can be killed, ptrace works,
> 
> I tried to not argue with intent, but to be honest I am more and more
> sceptical... Lets forget about ptrace for the moment.
> 
> Once again, why do we want a killable freezer?
> 
> If a user wants to kill a frozen task from CGRP_FROZEN cgroup he can simply
> 
> 	1. send SIGKILL to that task
> 
> 	2. migrate it to the root cgroup.
> 
> why this doesn't / can't work?

It does work, but it doesn't look as a nice interface to take into
the cgroup v2 world.

It just not clear, why killing a frozen task requires some cgroup-level
operations? It doesn't add anything except some additional complexity
to the userspace. Generally speaking, any process hanging in D-state
for a long time isn't the nicest object from the userspace's point of view.

Exactly as a SIGSTOPped process can be killed without sending SIGCONT,
why a frozen task would require some additional operations?

And I'm not talking about the case, when the process which is sending
SIGKILL has no write access to cgroupfs.


> Why I am starting to argue... The ability to kill a frozen task complicates
> the code, and since cgroup_enter_stopped() (in this version at least) doesn't
> properly interacts with freezable_schedule() leads to other problems.
> 
> From 7/7:
> 
> 	+  cgroup.freeze
> 	+	A read-write single value file which exists on non-root cgroups.
> 	+	Allowed values are "0" and "1". The default is "0".
> 	+
> 	+	Writing "1" to the file causes freezing of the cgroup and all
> 	+	descendant cgroups. This means that all belonging processes will
> 	+	be stopped and will not run until the cgroup will be explicitly
> 	+	unfrozen. Freezing of the cgroup may take some time;
>                                                  ^^^^^^^^^^^^^^^^^^
> it may take infinite time.
> 
> Just suppose that a task does vfork() and this races with cgroup_do_freeze(true).
> If the new child notices JOBCTL_TRAP_FREEZE before exit/exec the cgroup will be
> never frozen.

Hm, why? cgroup_update_frozen() called from cgroup_post_fork() should bring
the cgroup into the frozen state. If it's not true (I'm missing some race here),
it's a bug, but I don't see why it's not possible in general.

> 
> If I read the current kernel/cgroup/freezer.c correctly, CGROUP_FREEZING should
> "always" work (unless a task hangs in D state) and to me this looks more important
> than kill/ptrace support...

Again, I don't see a case, when cgroup v1 freezer will work and the proposed
v2 freezer won't work in general.

> 
> > there is no separate controller, which has to be enabled, etc.
> 
> agreed, this is nice.

Thanks!

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