Hi,
I am sending the patch below as an RFC because I am not entirely happy
about myself and maybe somebody can come up with a different approach
which would be less hackish.
As a background, I have noticed that memcg OOM killer kills a wrong
tasks while playing with memory.swappiness==0 in a small group (e.g.
50M). I have multiple anon mem eaters which fault in more than the hard
limit. OOM killer kills the last executed task:
# mem_eater spawns one process per parameter, mmaps the given size and
# faults memory in in parallel (all of them are synced to start together)
./mem_eater anon:50M anon:20M anon:20M anon:20M
10571: anon_eater for 20971520B
10570: anon_eater for 52428800B
10573: anon_eater for 20971520B
10572: anon_eater for 20971520B
10573: done with status 9
10571: done with status 0
10572: done with status 9
10570: done with status 9
[ pid ] uid tgid total_vm rss nr_ptes swapents oom_score_adj name
[ 5706] 0 5706 4955 556 13 0 0 bash
[10569] 0 10569 1015 134 6 0 0 mem_eater
[10570] 0 10570 13815 4118 15 0 0 mem_eater
[10571] 0 10571 6135 5140 16 0 0 mem_eater
[10572] 0 10572 6135 22 7 0 0 mem_eater
[10573] 0 10573 6135 3541 14 0 0 mem_eater
Memory cgroup out of memory: Kill process 10573 (mem_eater) score 0 or sacrifice child
Killed process 10573 (mem_eater) total-vm:24540kB, anon-rss:14028kB, file-rss:136kB
[...]
[ pid ] uid tgid total_vm rss nr_ptes swapents oom_score_adj name
[ 5706] 0 5706 4955 556 13 0 0 bash
[10569] 0 10569 1015 134 6 0 0 mem_eater
[10570] 0 10570 13815 10267 27 0 0 mem_eater
[10572] 0 10572 6135 2519 12 0 0 mem_eater
Memory cgroup out of memory: Kill process 10572 (mem_eater) score 0 or sacrifice child
Killed process 10572 (mem_eater) total-vm:24540kB, anon-rss:9940kB, file-rss:136kB
[...]
[ pid ] uid tgid total_vm rss nr_ptes swapents oom_score_adj name
[ 5706] 0 5706 4955 556 13 0 0 bash
[10569] 0 10569 1015 134 6 0 0 mem_eater
[10570] 0 10570 13815 12773 31 0 0 mem_eater
Memory cgroup out of memory: Kill process 10570 (mem_eater) score 2 or sacrifice child
Killed process 10570 (mem_eater) total-vm:55260kB, anon-rss:50956kB, file-rss:136kB
As you can see 50M (pid:10570) is killed as the last one while 20M ones
are killed first. See the patch for more details about the problem.
As I state in the changelog the very same issue is present in the global
oom killer as well but it is much less probable as the amount of swap is
usualy much smaller than the available RAM and I think it is not worth
considering.
---
>From 445c2ced957cd77cbfca44d0e3f5056fed252a34 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxx>
Date: Wed, 10 Oct 2012 15:46:54 +0200
Subject: [PATCH] memcg: oom: fix totalpages calculation for swappiness==0
oom_badness takes totalpages argument which says how many pages are
available and it uses it as a base for the score calculation. The value
is calculated by mem_cgroup_get_limit which considers both limit and
total_swap_pages (resp. memsw portion of it).
This is usually correct but since fe35004f (mm: avoid swapping out
with swappiness==0) we do not swap when swappiness is 0 which means
that we cannot really use up all the totalpages pages. This in turn
confuses oom score calculation if the memcg limit is much smaller
than the available swap because the used memory (capped by the limit)
is negligible comparing to totalpages so the resulting score is too
small. A wrong process might be selected as result.
The same issue exists for the global oom killer as well but it is not
that problematic as the amount of the RAM is usually much bigger than
the swap space.
The problem can be worked around by checking swappiness==0 and not
considering swap at all.
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
mm/memcontrol.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7acf43b..93a7e36 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1452,17 +1452,26 @@ static int mem_cgroup_count_children(struct mem_cgroup *memcg)
static u64 mem_cgroup_get_limit(struct mem_cgroup *memcg)
{
u64 limit;
- u64 memsw;
limit = res_counter_read_u64(&memcg->res, RES_LIMIT);
- limit += total_swap_pages << PAGE_SHIFT;
- memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
/*
- * If memsw is finite and limits the amount of swap space available
- * to this memcg, return that limit.
+ * Do not consider swap space if we cannot swap due to swappiness
*/
- return min(limit, memsw);
+ if (mem_cgroup_swappiness(memcg)) {
+ u64 memsw;
+
+ limit += total_swap_pages << PAGE_SHIFT;
+ memsw = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
+
+ /*
+ * If memsw is finite and limits the amount of swap space
+ * available to this memcg, return that limit.
+ */
+ limit = min(limit, memsw);
+ }
+
+ return limit;
}
void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
--
1.7.10.4
--
Michal Hocko
SUSE Labs
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