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

[Li Zefan]

Tejun Heo wrote:

> 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.
> 


I don't like this either.

> 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.
> 
> Signed-off-by: Tejun Heo <tj at kernel.org>


Both patches look good.

Acked-by: Li Zefan <lizefan at huawei.com>

and I'd like to see code shrink with memcg updates ASAP.

> Cc: Vivek Goyal <vgoyal at redhat.com>
> Cc: Johannes Weiner <hannes at cmpxchg.org>
> Cc: Michal Hocko <mhocko at suse.cz>
> Cc: Balbir Singh <bsingharora at gmail.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>