memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()

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.

Link: https://lkml.kernel.org/r/20230419030739.115845-2-haifeng.xu@shopee.com
Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e8aead97454bd8ea80defecac348be275cb64ea6..d31fb1e2cb3309819b1997d35978d7481efd21a0 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2028,15 +2028,8 @@ 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);