Re: [PATCH 04 of 24] serialize oom killer

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 22 Aug 2007 14:48:51 +0200 Andrea Arcangeli <andrea@suse.de> wrote:

> # HG changeset patch
> # User Andrea Arcangeli <andrea@suse.de>
> # Date 1187778125 -7200
> # Node ID 871b7a4fd566de0811207628b74abea0a73341f6
> # Parent  5566f2af006a171cd47d596c6654f51beca74203
> serialize oom killer
> 
> It's risky and useless to run two oom killers in parallel, let serialize it to
> reduce the probability of spurious oom-killage.
> 
> Signed-off-by: Andrea Arcangeli <andrea@suse.de>
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -401,12 +401,15 @@ void out_of_memory(struct zonelist *zone
>  	unsigned long points = 0;
>  	unsigned long freed = 0;
>  	int constraint;
> +	static DECLARE_MUTEX(OOM_lock);
>  
>  	blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
>  	if (freed > 0)
>  		/* Got some memory back in the last second. */
>  		return;
>  
> +	if (down_trylock(&OOM_lock))
> +		return;
>  	if (printk_ratelimit()) {
>  		printk(KERN_WARNING "%s invoked oom-killer: "
>  			"gfp_mask=0x%x, order=%d, oomkilladj=%d\n",
> @@ -473,4 +476,6 @@ out:
>  	 */
>  	if (!test_thread_flag(TIF_MEMDIE))
>  		schedule_timeout_uninterruptible(1);
> -}
> +
> +	up(&OOM_lock);
> +}

Please use mutexes, not semaphores.  I'll make this change.

I think this patch needs more explanation/justification.

What problems were observed, and what effect did this change have upon the
system behaviour?

What happens to all the tasks which fail to grab the lock?  Do they return
to sleep in congestion_wait() for a bit?  If so, OK.  But are there
scenarios in which they'll go nuts consuming CPU?  Because if there are, a
non-preemptible uniproc kernel could wedge up forever: the task which holds
OOM_lock is not running and the task which is trying to get it never gives
up the CPU?

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