Re: [patch 1/2] mm, memcg: avoid oom notification when current needs access to memory reserves

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Dec 03, 2013 at 03:50:41PM -0800, David Rientjes wrote:
> On Tue, 3 Dec 2013, Michal Hocko wrote:
> 
> > 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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