linux-kernel - Re: [patch 05/12] mm: oom_kill: generalize OOM progress waitqueue

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Date:	Thu, 26 Mar 2015 14:03:50 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Huang Ying <ying.huang@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [patch 05/12] mm: oom_kill: generalize OOM progress waitqueue

On Wed 25-03-15 02:17:09, Johannes Weiner wrote:
> It turns out that the mechanism to wait for exiting OOM victims is
> less generic than it looks: it won't issue wakeups unless the OOM
> killer is disabled.
> 
> The reason this check was added was the thought that, since only the
> OOM disabling code would wait on this queue, wakeup operations could
> be saved when that specific consumer is known to be absent.
> 
> However, this is quite the handgrenade.  Later attempts to reuse the
> waitqueue for other purposes will lead to completely unexpected bugs
> and the failure mode will appear seemingly illogical.  Generally,
> providers shouldn't make unnecessary assumptions about consumers.
> 
> This could have been replaced with waitqueue_active(), but it only
> saves a few instructions in one of the coldest paths in the kernel.
> Simply remove it.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

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

> ---
>  mm/oom_kill.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 88aa9ba40fa5..d3490b019d46 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -437,11 +437,7 @@ void exit_oom_victim(void)
>  {
>  	clear_thread_flag(TIF_MEMDIE);
>  
> -	/*
> -	 * 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)
> +	if (!atomic_dec_return(&oom_victims))
>  		wake_up_all(&oom_victims_wait);
>  }
>  
> -- 
> 2.3.3
> 

-- 
Michal Hocko
SUSE Labs
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