[PATCH v2 2/2] memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()

[Haifeng Xu]

Before commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to
the charge path"), all memcg oom killers were delayed to page fault
path. And the explicit wakeup is used in this case:

thread A:
        ...
        if (locked) {           // complete oom-kill, hold the lock
                mem_cgroup_oom_unlock(memcg);
                ...
        }
        ...

thread B:
        ...

        if (locked && !memcg->oom_kill_disable) {
                ...
        } else {
                schedule();     // can't acquire the lock
                ...
        }
        ...

The reason is that thread A kicks off the OOM-killer, which leads to
wakeups from the uncharges of the exiting task. But thread B is not
guaranteed to see them if it enters the OOM path after the OOM kills
but before thread A releases the lock.

Now only oom_kill_disable case is handled from the #PF path. In that
case it is userspace to trigger the wake up not the #PF path itself.
All potential paths to free some charges are responsible to call
memcg_oom_recover() , so the explicit wakeup is not needed in the
mem_cgroup_oom_synchronize() path which doesn't release any memory
itself.

Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
---
v2: split original into two and improve patch description
---
 mm/memcontrol.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fbf4d2bb1003..710ce3e7824f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2003,15 +2003,8 @@ bool mem_cgroup_oom_synchronize(bool handle)
 	mem_cgroup_unmark_under_oom(memcg);
 	finish_wait(&memcg_oom_waitq, &owait.wait);
 
-	if (locked) {
+	if (locked)
 		mem_cgroup_oom_unlock(memcg);
-		/*
-		 * There is no guarantee that an OOM-lock contender
-		 * sees the wakeups triggered by the OOM kill
-		 * uncharges.  Wake any sleepers explicitly.
-		 */
-		memcg_oom_recover(memcg);
-	}
 cleanup:
 	current->memcg_in_oom = NULL;
 	css_put(&memcg->css);
-- 
2.25.1