Re: [PATCH] memcg: put soft limit reclaim out of way if the excess tree is empty

[Michal Hocko]

On Mon 01-08-16 11:03:43, Johannes Weiner wrote:
> On Mon, Aug 01, 2016 at 04:12:28PM +0200, Michal Hocko wrote:
[...]
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c265212bec8c..c0b57b6a194e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2543,6 +2543,11 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
> >  	return ret;
> >  }
> >  
> > +static inline bool soft_limit_tree_empty(struct mem_cgroup_tree_per_node *mctz)
> > +{
> > +	return RB_EMPTY_ROOT(&mctz->rb_root);
> > +}
> 
> Can you please fold this into the caller? It should be obvious enough.

OK, fair enough. There will probably be no other callers. I've added
comment as well

> Other than that, this patch makes sense to me.
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks!

If the following sounds good I will resend v2.
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c0b57b6a194e..e56d6a0f92ac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2543,11 +2543,6 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
 	return ret;
 }
 
-static inline bool soft_limit_tree_empty(struct mem_cgroup_tree_per_node *mctz)
-{
-	return RB_EMPTY_ROOT(&mctz->rb_root);
-}
-
 unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 					    gfp_t gfp_mask,
 					    unsigned long *total_scanned)
@@ -2564,7 +2559,13 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 		return 0;
 
 	mctz = soft_limit_tree_node(pgdat->node_id);
-	if (soft_limit_tree_empty(mctz))
+
+	/*
+	 * Do not even bother to check the largest node if the node
+	 * is empty. Do it lockless to prevent lock bouncing. Races
+	 * are acceptable as soft limit is best effort anyway.
+	 */
+	if (RB_EMPTY_ROOT(&mctz->rb_root))
 		return 0;
 
 	/*

-- 
Michal Hocko
SUSE Labs