Re: [PATCH] cgroup: use an ordered workqueue for cgroup destruction

[Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>]

On Thu 06-02-14 15:56:01, 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().
> 
> Just use an ordered workqueue for cgroup_destroy_wq.

Hmm, interesting. Markus has seen hangs even with mem_cgroup_css_offline
and the referenced cgroup fixes, maybe this is the the right one
finally.

> Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
> Suggested-by: Filipe Brandenburger <filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Signed-off-by: Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org # 3.10+

Reviewed-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>

e5fca243abae was marked for 3.9 stable but I do not see it in the Greg's
3.9 stable branch so 3.10+ seems to be sufficient.

> ---
> 
>  kernel/cgroup.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- 3.14-rc1/kernel/cgroup.c	2014-02-02 18:49:07.737302111 -0800
> +++ linux/kernel/cgroup.c	2014-02-06 15:20:35.548904965 -0800
> @@ -4845,12 +4845,12 @@ static int __init cgroup_wq_init(void)
>  	/*
>  	 * There isn't much point in executing destruction path in
>  	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
> -	 * Use 1 for @max_active.
> +	 * Must be ordered to make sure parent is offlined after children.
>  	 *
>  	 * We would prefer to do this in cgroup_init() above, but that
>  	 * is called before init_workqueues(): so leave this until after.
>  	 */
> -	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
> +	cgroup_destroy_wq = alloc_ordered_workqueue("cgroup_destroy", 0);
>  	BUG_ON(!cgroup_destroy_wq);
>  
>  	/*

-- 
Michal Hocko
SUSE Labs