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

[Michal Hocko]

We've had a report about soft lockups caused by lock bouncing in the
soft reclaim path:

[331404.849734] BUG: soft lockup - CPU#0 stuck for 22s! [kav4proxy-kavic:3128]
[331404.849920] RIP: 0010:[<ffffffff81469798>]  [<ffffffff81469798>] _raw_spin_lock+0x18/0x20
[331404.849997] Call Trace:
[331404.850010]  [<ffffffff811557ea>] mem_cgroup_soft_limit_reclaim+0x25a/0x280
[331404.850020]  [<ffffffff8111041d>] shrink_zones+0xed/0x200
[331404.850027]  [<ffffffff81111a94>] do_try_to_free_pages+0x74/0x320
[331404.850034]  [<ffffffff81112072>] try_to_free_pages+0x112/0x180
[331404.850042]  [<ffffffff81104a6f>] __alloc_pages_slowpath+0x3ff/0x820
[331404.850049]  [<ffffffff81105079>] __alloc_pages_nodemask+0x1e9/0x200
[331404.850056]  [<ffffffff81141e01>] alloc_pages_vma+0xe1/0x290
[331404.850064]  [<ffffffff8112402f>] do_wp_page+0x19f/0x840
[331404.850071]  [<ffffffff811257cd>] handle_pte_fault+0x1cd/0x230
[331404.850079]  [<ffffffff8146d3ed>] do_page_fault+0x1fd/0x4c0
[331404.850087]  [<ffffffff81469ec5>] page_fault+0x25/0x30

There are no memcgs created so there cannot be any in the soft limit
excess obviously:
[...]
memory  0       1       1

so all this just seems to be mem_cgroup_largest_soft_limit_node
trying to get spin_lock_irq(&mctz->lock) just to find out that the soft
limit excess tree is empty. This is just pointless waisting of cycles
and cache line bouncing during heavy parallel reclaim on large machines.
The particular machine wasn't very healthy and most probably suffering
from a memory leak which just caused the memory reclaim to trash
heavily. But bouncing on the lock certainly didn't help...

Introduce soft_limit_tree_empty which does the optimistic lockless check
and bail out early if the tree is empty. This is theoretically racy but
that shouldn't matter all that much. First of all soft limit is a best
effort feature and it is slowly getting deprecated and its usage should
be really scarce. Bouncing on a lock without a good reason is surely
much bigger problem, especially on large CPU machines.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/memcontrol.c | 8 ++++++++
 1 file changed, 8 insertions(+)

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);
+}
+
 unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 					    gfp_t gfp_mask,
 					    unsigned long *total_scanned)
@@ -2559,6 +2564,9 @@ 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))
+		return 0;
+
 	/*
 	 * This loop can run a while, specially if mem_cgroup's continuously
 	 * keep exceeding their soft limit and putting the system under
-- 
2.8.1

-- 
Michal Hocko
SUSE Labs