On Tue, Dec 03, 2013 at 03:50:41PM -0800, David Rientjes wrote:

On Tue, 3 Dec 2013, Michal Hocko wrote:

quoted

OK, as it seems that the notification part is too controversial, how
would you like the following? It reverts the notification part and still
solves the fault on exit path. I will prepare the full patch with the
changelog if this looks reasonable:

Um, no, that's not satisfactory because it obviously does the check after 
mem_cgroup_oom_notify().  There is absolutely no reason why userspace 
should be woken up when current simply needs access to memory reserves to 
exit.  You can already get such notification by memory thresholds at the 
memcg limit.

I'll repeat: Section 10 of Documentation/cgroups/memory.txt specifies what 
userspace should do when waking up; one of those options is not "check if 
the memcg is still actually oom in a short period of time once a charging 
task with a pending SIGKILL or in the exit path has been able to exit."  
Users of this interface typically also disable the memcg oom killer 
through the same file, it's ludicrous to put the responsibility on 
userspace to determine if the wakeup is actionable and requires it to 
intervene in one of the methods listed in section 10.

Kind of a bummer that you haven't read anything I wrote...

But here is a patch that defers wakeups until we know for sure that
userspace action is required:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f1a0ae6..cc6adac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2254,8 +2254,17 @@ bool mem_cgroup_oom_synchronize(bool handle)
 
 	locked = mem_cgroup_oom_trylock(memcg);
 
+#if 0
+	/*
+	 * XXX: An unrelated task in the group might exit at any time,
+	 * making the OOM kill unnecessary.  We don't want to wake up
+	 * the userspace handler unless we are certain it needs to
+	 * intervene, so disable notifications until we solve the
+	 * halting problem.
+	 */
 	if (locked)
 		mem_cgroup_oom_notify(memcg);
+#endif
 
 	if (locked && !memcg->oom_kill_disable) {
 		mem_cgroup_unmark_under_oom(memcg);

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