From: Oleg Nesterov <oleg@redhat.com>
To: Roman Gushchin <guroan@gmail.com>
Cc: Tejun Heo <tj@kernel.org>,
kernel-team@fb.com, cgroups@vger.kernel.org,
linux-kernel@vger.kernel.org, Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH v8 0/7] freezer for cgroup v2
Date: Wed, 20 Feb 2019 15:37:48 +0100 [thread overview]
Message-ID: <20190220143748.GA9477@redhat.com> (raw)
In-Reply-To: <20190219220252.4906-1-guro@fb.com>
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?
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.
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...
> there is no separate controller, which has to be enabled, etc.
agreed, this is nice.
Oleg.
next prev parent reply other threads:[~2019-02-20 14:37 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 ` Oleg Nesterov [this message]
2019-02-20 22:00 ` [PATCH v8 0/7] freezer for cgroup v2 Roman Gushchin
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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