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Date:   Wed, 19 Apr 2023 09:38:41 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Haifeng Xu <haifeng.xu@shopee.com>
Cc:     hannes@cmpxchg.org, roman.gushchin@linux.dev, shakeelb@google.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] memcg, oom: remove explicit wakeup in
 mem_cgroup_oom_synchronize()
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References: <ZDUxVG2otm5i12o2@dhcp22.suse.cz>
 <20230419030739.115845-2-haifeng.xu@shopee.com>
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On Wed 19-04-23 03:07:39, Haifeng Xu wrote:
> 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@shopee.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>

I hope I haven't missed anything but this looks good to me. One way to
test this would be a parallel OOM triggering workload which uses page
faults and an automatic user space driven oom killer (detect under_oom
and send SIGKILL to a random task after a random timeout). Objective is
that no task gets stuck in mem_cgroup_oom_synchronize. I am pretty sure
this could be easily turned into a selftest.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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

-- 
Michal Hocko
SUSE Labs