oom: always return a badness score of non-zero for eligible tasks

A task's badness score is roughly a proportion of its rss and swap
compared to the system's capacity.  The scale ranges from 0 to 1000 with
the highest score chosen for kill.  Thus, this scale operates on a
resolution of 0.1% of RAM + swap.  Admin tasks are also given a 3% bonus,
so the badness score of an admin task using 3% of memory, for example,
would still be 0.

It's possible that an exceptionally large number of tasks will combine to
exhaust all resources but never have a single task that uses more than
0.1% of RAM and swap (or 3.0% for admin tasks).

This patch ensures that the badness score of any eligible task is never 0
so the machine doesn't unnecessarily panic because it cannot find a task
to kill.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index fc81cb22869e..859250c7dc06 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -208,8 +208,13 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
          */
         points += p->signal->oom_score_adj;
 
-        if (points < 0)
-                return 0;
+        /*
+         * Never return 0 for an eligible task that may be killed since it's
+         * possible that no single user task uses more than 0.1% of memory and
+         * no single admin tasks uses more than 3.0%.
+         */
+        if (points <= 0)
+                return 1;
         return (points < 1000) ? points : 1000;
 }