> Since be44562613851 ("cgroup: remove synchronize_rcu() from
> cgroup_diput()"), cgroup destruction path makes use of workqueue. css
> freeing is performed from a work item from that point on and a later
> commit, ea15f8ccdb430 ("cgroup: split cgroup destruction into two
> steps"), moves css offlining to workqueue too.
>
> As cgroup destruction isn't depended upon for memory reclaim, the
> destruction work items were put on the system_wq; unfortunately, some
> controller may block in the destruction path for considerable duration
> while holding cgroup_mutex. As large part of destruction path is
> synchronized through cgroup_mutex, when combined with high rate of
> cgroup removals, this has potential to fill up system_wq's max_active
> of 256.
>
> Also, it turns out that memcg's css destruction path ends up queueing
> and waiting for work items on system_wq through work_on_cpu(). If
> such operation happens while system_wq is fully occupied by cgroup
> destruction work items, work_on_cpu() can't make forward progress
> because system_wq is full and other destruction work items on
> system_wq can't make forward progress because the work item waiting
> for work_on_cpu() is holding cgroup_mutex, leading to deadlock.
>
> This can be fixed by queueing destruction work items on a separate
> workqueue. This patch creates a dedicated workqueue -
> cgroup_destroy_wq - for this purpose. As these work items shouldn't
> have inter-dependencies and mostly serialized by cgroup_mutex anyway,
> giving high concurrency level doesn't buy anything and the workqueue's
> @max_active is set to 1 so that destruction work items are executed
> one by one on each CPU.
>
> Hugh Dickins: Because cgroup_init() is run before init_workqueues(),
> cgroup_destroy_wq can't be allocated from cgroup_init(). Do it from a
> separate core_initcall(). In the future, we probably want to reorder
> so that workqueue init happens before cgroup_init().
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Reported-by: Shawn Bohrer <shawn.bohrer@xxxxxxxxx>
> Link: http://lkml.kernel.org/r/20131111220626.GA7509@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Link: http://lkml.kernel.org/g/alpine.LNX.2.00.1310301606080.2333@eggly.anvils
> Cc: stable@xxxxxxxxxxxxxxx # v3.9+
Acked-by: Li Zefan <lizefan@xxxxxxxxxx>
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