Re: [patch 04/12] mm: oom_kill: remove unnecessary locking in exit_oom_victim()

From: Michal Hocko <mhocko@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Huang Ying <ying.huang@intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Dave Chinner <david@fromorbit.com>,
	"Theodore Ts'o" <tytso@mit.edu>
Subject: Re: [patch 04/12] mm: oom_kill: remove unnecessary locking in exit_oom_victim()
Date: Thu, 26 Mar 2015 13:53:48 +0100	[thread overview]
Message-ID: <20150326125348.GF15257@dhcp22.suse.cz> (raw)
In-Reply-To: <1427264236-17249-5-git-send-email-hannes@cmpxchg.org>

On Wed 25-03-15 02:17:08, Johannes Weiner wrote:
> Disabling the OOM killer needs to exclude allocators from entering,
> not existing victims from exiting.

The idea was that exit_oom_victim doesn't miss a waiter.

exit_oom_victim is doing
	atomic_dec_return(&oom_victims) && oom_killer_disabled)

so there is a full (implicit) memory barrier befor oom_killer_disabled
check. The other part is trickier. oom_killer_disable does:
	oom_killer_disabled = true;
        up_write(&oom_sem);

        wait_event(oom_victims_wait, !atomic_read(&oom_victims));

up_write doesn't guarantee a full memory barrier AFAICS in
Documentation/memory-barriers.txt (although the generic and x86
implementations seem to implement it as a full barrier) but wait_event
implies the full memory barrier (prepare_to_wait_event does spin
lock&unlock) before checking the condition in the slow path. This should
be sufficient and docummented...

	/*
	 * We do not need to hold oom_sem here because oom_killer_disable
	 * guarantees that oom_killer_disabled chage is visible before
	 * the waiter is put into sleep (prepare_to_wait_event) so
	 * we cannot miss a wake up.
	 */

in unmark_oom_victim()

> Right now the only waiter is suspend code, which achieves quiescence
> by disabling the OOM killer.  But later on we want to add waits that
> hold the lock instead to stop new victims from showing up.

It is not entirely clear what you mean by this from the current context.
exit_oom_victim is not called from any context which would be locked by
any OOM internals so it should be safe to use the locking.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I have nothing against the change as it seems correct but it would be
good to get a better clarification and also document the implicit memory
barriers.

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

> ---
>  mm/oom_kill.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 4b9547be9170..88aa9ba40fa5 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -437,14 +437,12 @@ void exit_oom_victim(void)
>  {
>  	clear_thread_flag(TIF_MEMDIE);
>  
> -	down_read(&oom_sem);
>  	/*
>  	 * There is no need to signal the lasst oom_victim if there
>  	 * is nobody who cares.
>  	 */
>  	if (!atomic_dec_return(&oom_victims) && oom_killer_disabled)
>  		wake_up_all(&oom_victims_wait);
> -	up_read(&oom_sem);
>  }
>  
>  /**
> -- 
> 2.3.3
> 

-- 
Michal Hocko
SUSE Labs