Re: [patch] mm, oom: make a last minute check to prevent unnecessary memcg oom kills

[David Rientjes]

On Wed, 11 Mar 2020, Tetsuo Handa wrote:

> > The patch certainly prevents unnecessary oom kills when there is a pending 
> > victim that uncharges its memory between invoking the oom killer and 
> > finding MMF_OOM_SKIP in the list of eligible tasks and its much more 
> > common on systems with limited cpu cores.
> 
> I think that it is dump_header() which currently spends much time (due to
> synchronous printing) enough to make "the second memcg oom kill shows usage
> is >40MB below its limit of 100MB" happen. Shouldn't we call dump_header()
> and then do the last check and end with "but did not kill anybody" message?
> 

Lol, I actually did that for internal testing as well :)  I didn't like 
how it spammed the kernel log and then basically said "just kidding, 
nothing was oom killed."

But if you think this would helpful I can propose it as v2.
---
 include/linux/memcontrol.h |  7 +++++++
 mm/memcontrol.c            |  2 +-
 mm/oom_kill.c              | 14 +++++++++++++-
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -445,6 +445,8 @@ void mem_cgroup_iter_break(struct mem_cgroup *, struct mem_cgroup *);
 int mem_cgroup_scan_tasks(struct mem_cgroup *,
 			  int (*)(struct task_struct *, void *), void *);
 
+unsigned long mem_cgroup_margin(struct mem_cgroup *memcg);
+
 static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_disabled())
@@ -945,6 +947,11 @@ static inline int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 	return 0;
 }
 
+static inline unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
+{
+	return 0;
+}
+
 static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
 {
 	return 0;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1286,7 +1286,7 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
  * Returns the maximum amount of memory @mem can be charged with, in
  * pages.
  */
-static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
+unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 {
 	unsigned long margin = 0;
 	unsigned long count;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -934,7 +934,6 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
 	mmdrop(mm);
 	put_task_struct(victim);
 }
-#undef K
 
 /*
  * Kill provided task unless it's secured by setting
@@ -982,6 +981,18 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	 */
 	oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
 
+	/* One last check: do we *really* need to kill? */
+	if (is_memcg_oom(oc)) {
+		unsigned long margin = mem_cgroup_margin(oc->memcg);
+
+		if (unlikely(margin >= (1 << oc->order))) {
+			put_task_struct(victim);
+			pr_info("Suppressed oom kill, %lukB of memory can be charged\n",
+				K(margin));
+			return;
+		}
+	}
+
 	__oom_kill_process(victim, message);
 
 	/*
@@ -994,6 +1005,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 		mem_cgroup_put(oom_group);
 	}
 }
+#undef K
 
 /*
  * Determines whether the kernel must panic because of the panic_on_oom sysctl.