Re: [PATCH 1/2] memcg: reparent charges of children before processing parent

[Hugh Dickins]

On Wed, 5 Mar 2014, Markus Blank-Burian wrote:

> I wanted to give you small feedback, that this patch successfully fixes the 
> problem with reparent_charges on our cluster. Thank you very much for finding 
> and fixing this one!
> 

That's great to hear, Markus: thanks a lot for letting us know.
All credit to Filipe, who had the idea of what might be going wrong,
and the best patch to fix it.

Hugh

> 
> On Wednesday 12 February 2014 15:03:31 Hugh Dickins wrote:
> > From: Filipe Brandenburger <filbranden@xxxxxxxxxx>
> > 
> > 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, when offlining a memcg, call mem_cgroup_reparent_charges() on
> > all its children (and grandchildren, in the correct order) to have their
> > charges reparented first.
> > 
> > Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup
> > destruction") Signed-off-by: Filipe Brandenburger <filbranden@xxxxxxxxxx>
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # v3.10+ (but will need extra care)
> > ---
> > Or, you may prefer my alternative cgroup.c approach in 2/2:
> > there's no need for both.  Please note that neither of these patches
> > attempts to handle the unlikely case of racy charges made to child
> > after its offline, but parent's offline coming before child's free:
> > mem_cgroup_css_free()'s backstop call to mem_cgroup_reparent_charges()
> > cannot help in that case, with or without these patches.  Fixing that
> > would have to be a separate effort - Michal's?
> > 
> >  mm/memcontrol.c |   10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > --- 3.14-rc2/mm/memcontrol.c	2014-02-02 18:49:07.897302115 -0800
> > +++ linux/mm/memcontrol.c	2014-02-11 17:48:07.604582963 -0800
> > @@ -6595,6 +6595,7 @@ static void mem_cgroup_css_offline(struc
> >  {
> >  	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> >  	struct mem_cgroup_event *event, *tmp;
> > +	struct cgroup_subsys_state *iter;
> > 
> >  	/*
> >  	 * Unregister events and notify userspace.
> > @@ -6611,7 +6612,14 @@ static void mem_cgroup_css_offline(struc
> >  	kmem_cgroup_css_offline(memcg);
> > 
> >  	mem_cgroup_invalidate_reclaim_iterators(memcg);
> > -	mem_cgroup_reparent_charges(memcg);
> > +
> > +	/*
> > +	 * This requires that offlining is serialized.  Right now that is
> > +	 * guaranteed because css_killed_work_fn() holds the cgroup_mutex.
> > +	 */
> > +	css_for_each_descendant_post(iter, css)
> > +		mem_cgroup_reparent_charges(mem_cgroup_from_css(iter));
> > +
> >  	mem_cgroup_destroy_all_caches(memcg);
> >  	vmpressure_cleanup(&memcg->vmpressure);
> >  }
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