From: Roman Gushchin <guro@fb.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Roman Gushchin <guroan@gmail.com>, Tejun Heo <tj@kernel.org>,
Kernel Team <Kernel-team@fb.com>,
"cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 0/7] freezer for cgroup v2
Date: Wed, 20 Feb 2019 22:00:27 +0000 [thread overview]
Message-ID: <20190220220020.GA16335@castle.DHCP.thefacebook.com> (raw)
In-Reply-To: <20190220143748.GA9477@redhat.com>
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!
next prev parent reply other threads:[~2019-02-20 22:01 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-19 22:02 [PATCH v8 0/7] freezer for cgroup v2 Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 1/7] cgroup: rename freezer.c into legacy_freezer.c Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 2/7] cgroup: implement __cgroup_task_count() helper Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 3/7] cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 4/7] cgroup: cgroup v2 freezer Roman Gushchin
2019-02-20 14:42 ` Oleg Nesterov
2019-02-20 22:14 ` Roman Gushchin
2019-02-21 16:44 ` Oleg Nesterov
2019-02-19 22:02 ` [PATCH v8 5/7] kselftests: cgroup: don't fail on cg_kill_all() error in cg_destroy() Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 6/7] kselftests: cgroup: add freezer controller self-tests Roman Gushchin
2019-02-19 22:02 ` [PATCH v8 7/7] cgroup: document cgroup v2 freezer interface Roman Gushchin
2019-02-20 14:37 ` [PATCH v8 0/7] freezer for cgroup v2 Oleg Nesterov
2019-02-20 22:00 ` Roman Gushchin [this message]
2019-02-21 16:29 ` Oleg Nesterov
2019-02-21 17:34 ` Tejun Heo
2019-02-22 16:34 ` Oleg Nesterov
2019-02-22 18:17 ` Tejun Heo
2019-02-25 15:57 ` Oleg Nesterov
2019-03-05 17:27 ` Tejun Heo
2019-02-21 22:43 ` Roman Gushchin
2019-02-22 17:04 ` Oleg Nesterov
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