[PATCH] oom: do not live lock on frozen tasks

[Michal Hocko]

[WARNING untested]

OOM can end up in a live lock if select_bad_process picks up a frozen
task. On the other hand we cannot mark such processes as unkillable
because we could panic the system even though there is a chance that
somebody could thaw the process so we can make a forward process (e.g.
a process from another cpuset or with a different nodemask).

Let's give all frozen tasks a bonus (OOM_SCORE_ADJ_MAX/2) so that we do
not consider them unless really necessary and if we really pick up one
then thaw its threads before we try to kill it.

TODO
- given bonus might be too big?
- aren't we racing with try_to_freeze_tasks?
---
 mm/oom_kill.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 626303b..fd194bc 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -32,6 +32,7 @@
 #include <linux/mempolicy.h>
 #include <linux/security.h>
 #include <linux/ptrace.h>
+#include <linux/freezer.h>
 
 int sysctl_panic_on_oom;
 int sysctl_oom_kill_allocating_task;
@@ -214,6 +215,14 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
 	points += p->signal->oom_score_adj;
 
 	/*
+	 * Do not try to kill frozen tasks unless there is nothing else to kill.
+	 * We do not want to give it 1 point because we still want to select a good
+	 * candidate among all frozen tasks. Let's give it a reasonable bonus.
+	 */
+	if (frozen(p))
+		points -= OOM_SCORE_ADJ_MAX/2;
+
+	/*
 	 * 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%.
@@ -450,6 +459,10 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
 			pr_err("Kill process %d (%s) sharing same memory\n",
 				task_pid_nr(q), q->comm);
 			task_unlock(q);
+
+			if (frozen(q))
+				thaw_process(q);
+
 			force_sig(SIGKILL, q);
 		}
 
-- 
1.7.5.4

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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