Re: [RFC] memcg, oom: clean up mem_cgroup_oom_synchronize

[Michal Hocko]

On Wed 15-03-23 07:03:02, Haifeng Xu wrote:
> Since commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to
> the charge path"), only oom_kill_disable is set, oom killer will
> be delayed to page fault path. In the charge patch, even if the
> oom_lock in memcg can't be acquired, the oom handing can also be
> invoked. In order to keep the behavior consistent with it, remove
> the lock check, just leave oom_kill_disable check behind in the
> page fault path.

I do not understand the actual problem you are trying to deal with here.

> Furthermore, the lock contender won't be scheduled out, this doesn't
> fit the sixth description in commit fb2a6fc56be66 ("mm: memcg:
> rework and document OOM waiting and wakeup"). So remove the explicit
> wakeup for the lock holder.
> 
> Fixes: fb2a6fc56be6 ("mm: memcg: rework and document OOM waiting and wakeup")

The subject mentions a clean up but the fixes tag would indicate an
acutal fix.

> Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
> ---
>  mm/memcontrol.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..360fa7cf7879 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1999,7 +1999,7 @@ bool mem_cgroup_oom_synchronize(bool handle)
>  	if (locked)
>  		mem_cgroup_oom_notify(memcg);
>  
> -	if (locked && !memcg->oom_kill_disable) {
> +	if (!memcg->oom_kill_disable) {
>  		mem_cgroup_unmark_under_oom(memcg);
>  		finish_wait(&memcg_oom_waitq, &owait.wait);
>  		mem_cgroup_out_of_memory(memcg, current->memcg_oom_gfp_mask,

Now looking at the actual code I suspect you in fact want to simplify
the logic here as mem_cgroup_oom_synchronize is only ever triggered whe
oom_kill_disable == true because current->memcg_in_oom is never non NULL
otherwise. So the check is indeed unnecessary. Your patch, however
doesn't really simplify the code much. 

Did you want this instead?
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 12559c08d976..a77dc88cfa12 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1999,16 +1999,9 @@ bool mem_cgroup_oom_synchronize(bool handle)
 	if (locked)
 		mem_cgroup_oom_notify(memcg);
 
-	if (locked && !READ_ONCE(memcg->oom_kill_disable)) {
-		mem_cgroup_unmark_under_oom(memcg);
-		finish_wait(&memcg_oom_waitq, &owait.wait);
-		mem_cgroup_out_of_memory(memcg, current->memcg_oom_gfp_mask,
-					 current->memcg_oom_order);
-	} else {
-		schedule();
-		mem_cgroup_unmark_under_oom(memcg);
-		finish_wait(&memcg_oom_waitq, &owait.wait);
-	}
+	schedule();
+	mem_cgroup_unmark_under_oom(memcg);
+	finish_wait(&memcg_oom_waitq, &owait.wait);
 
 	if (locked) {
 		mem_cgroup_oom_unlock(memcg);

> @@ -2010,15 +2010,8 @@ bool mem_cgroup_oom_synchronize(bool handle)
>  		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);
> -	}

Hmm, so this seems unneded as well for the oom_kill_disable case as
well. Rather than referring to fb2a6fc56be66 it would be better to
why the explicit recovery is not really needed anymore.

>  cleanup:
>  	current->memcg_in_oom = NULL;
>  	css_put(&memcg->css);

-- 
Michal Hocko
SUSE Labs