linux-kernel - Re: [PATCH 2/2] cgroup: make css->refcnt clearing on cgroup removal optional

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Date:	Sun, 1 Apr 2012 06:28:56 +0900
From:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Li Zefan <lizefan@...wei.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, fweisbec@...il.com, rni@...gle.com,
	ctalbott@...gle.com, Vivek Goyal <vgoyal@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/2] cgroup: make css->refcnt clearing on cgroup removal optional

2012年3月31日7:36 Tejun Heo <tj@...nel.org>:
> Currently, cgroup removal tries to drain all css references.  If there
> are active css references, the removal logic waits and retries
> ->pre_detroy() until either all refs drop to zero or removal is
> cancelled.
>
> This semantics is unusual and adds non-trivial complexity to cgroup
> core and IMHO is fundamentally misguided in that it couples internal
> implementation details (references to internal data structure) with
> externally visible operation (rmdir).  To userland, this is a behavior
> peculiarity which is unnecessary and difficult to expect (css refs is
> otherwise invisible from userland), and, to policy implementations,
> this is an unnecessary restriction (e.g. blkcg wants to hold css refs
> for caching purposes but can't as that becomes visible as rmdir hang).
>
> Unfortunately, memcg currently depends on ->pre_destroy() retrials and
> cgroup removal vetoing and can't be immmediately switched to the new
> behavior.  This patch introduces the new behavior of not waiting for
> css refs to drain and maintains the old behavior for subsystems which
> have __DEPRECATED_clear_css_refs set.
>
> Once, memcg is updated, we can drop the code paths for the old
> behavior as proposed in the following patch.  Note that the following
> patch is incorrect in that dput work item is in cgroup and may lose
> some of dputs when multiples css's are released back-to-back, and
> __css_put() triggers check_for_release() when refcnt reaches 0 instead
> of 1; however, it shows what part can be removed.
>
>  http://thread.gmane.org/gmane.linux.kernel.containers/22559/focus=75251
>
> Note that, in not-too-distant future, cgroup core will start emitting
> warning messages for subsys which require the old behavior, so please
> get moving.
>
Ok, I'll update memcg ASAP (after LSF/MM).
I'm now thinking to move all resources to root if use_hierarchy=0.

Thanks,
-Kame
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