mm, oom: remove sleep from under oom_lock

Tetsuo has pointed out that since 27ae357fa82b ("mm, oom: fix concurrent
munlock and oom reaper unmap, v3") we have a strong synchronization
between the oom_killer and victim's exiting because both have to take
the oom_lock.  Therefore the original heuristic to sleep for a short
time in out_of_memory doesn't serve the original purpose.

Moreover Tetsuo has noticed that the short sleep can be more harmful
than actually useful.  Hammering the system with many processes can lead
to a starvation when the task holding the oom_lock can block for a long
time (minutes) and block any further progress because the oom_reaper
depends on the oom_lock as well.

Drop the short sleep from out_of_memory when we hold the lock.  Keep the
sleep when the trylock fails to throttle the concurrent OOM paths a bit.
This should be solved in a more reasonable way (e.g.  sleep proportional
to the time spent in the active reclaiming etc.) but this is much more
complex thing to achieve.  This is a quick fixup to remove a stale code.

Link: http://lkml.kernel.org/r/20180709074706.30635-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -1077,15 +1077,9 @@ bool out_of_memory(struct oom_control *oc)
 		dump_header(oc, NULL);
 		panic("Out of memory and no killable processes...\n");
 	}
-	if (oc->chosen && oc->chosen != (void *)-1UL) {
+	if (oc->chosen && oc->chosen != (void *)-1UL)
 		oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
 				 "Memory cgroup out of memory");
-		/*
-		 * Give the killed process a good chance to exit before trying
-		 * to allocate memory again.
-		 */
-		schedule_timeout_killable(1);
-	}
 	return !!oc->chosen;
 }