Re: [PATCH 2/2] cgroup: bring back kill_cnt to order css destruction

[Tejun Heo]

Hello, Hugh.

On Wed, Feb 12, 2014 at 03:06:26PM -0800, Hugh Dickins wrote:
> Sometimes the cleanup after memcg hierarchy testing gets stuck in
> mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0.
> 
> There may turn out to be several causes, but a major cause is this: the
> workitem to offline parent can get run before workitem to offline child;
> parent's mem_cgroup_reparent_charges() circles around waiting for the
> child's pages to be reparented to its lrus, but it's holding cgroup_mutex
> which prevents the child from reaching its mem_cgroup_reparent_charges().
> 
> Further testing showed that an ordered workqueue for cgroup_destroy_wq
> is not always good enough: percpu_ref_kill_and_confirm's call_rcu_sched
> stage on the way can mess up the order before reaching the workqueue.
> 
> Instead bring back v3.11's css kill_cnt, repurposing it to make sure
> that offline_css() is not called for parent before it has been called
> for all children.
> 
> Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Reviewed-by: Filipe Brandenburger <filbranden@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v3.10+ (but will need extra care)
> ---
> This is an alternative to Filipe's 1/2: there's no need for both,
> but each has its merits.  I prefer Filipe's, which is much easier to
> understand: this one made more sense in v3.11, when it was just a matter
> of extending the use of css_kill_cnt; but might be preferred if offlining
> children before parent is thought to be a good idea generally.

Not that your implementation is bad or anything but the patch itself
somehow makes me cringe a bit.  It's probably just because it has to
add to the already overly complicated offline path.  Guaranteeing
strict offline ordering might be a good idea but at least for the
immediate bug fix, I agree that the memcg specific fix seems better
suited.  Let's apply that one and reconsider this one if it turns out
we do need strict offline reordering.

Thanks a lot!

-- 
tejun
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