Date:	Wed, 31 Dec 2008 12:16:47 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	wassim dagash <wassim.dagash@...il.com>
Subject: Re: [PATCH] mm: stop kswapd's infinite loop at high order allocation

On Wed, Dec 31, 2008 at 11:06:19AM +0000, Mel Gorman wrote:
> On Wed, Dec 31, 2008 at 02:32:33AM +0100, Nick Piggin wrote:
> > On Tue, Dec 30, 2008 at 06:59:19PM +0000, Mel Gorman wrote:
> > > On Tue, Dec 30, 2008 at 07:55:47PM +0900, KOSAKI Motohiro wrote:
> > > kswapd gets a sc.order when it is known there is a process trying to get
> > > high-order pages so it can reclaim at that order in an attempt to prevent
> > > future direct reclaim at a high-order. Your patch does not appear to depend on
> > > GFP_KERNEL at all so I found the comment misleading. Furthermore, asking it to
> > > loop again at order-0 means it may scan and reclaim more memory unnecessarily
> > > seeing as all_zones_ok was calculated based on a high-order value, not order-0.
> > 
> > It shouldn't, because it should check all that.
> > 
> 
> Ok, with KOSAKI's patch we
> 
> 1. Set order to 0 (and stop kswapd doing what it was asked to do)
> 2. goto loop_again
> 3. nr_reclaimed gets set to 0 (meaning we lose that value, but no biggie
>    as it doesn't get used by the caller anyway)
> 4. Reset all priorities
> 5. Do something like the following
> 
> 	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
> 		...
> 		all_zones_ok = 1;
> 		for (i = pgdat->nr_zones - 1; i >= 0; i--) {
> 			...
> 			if (inactive_anon_is_low(zone)) {
> 				shrink_active_list(SWAP_CLUSTER_MAX, zone,
> 					&sc, priority, 0);
> 			}
> 
> 			if (!zone_watermark_ok(zone, order, zone->pages_high,
> 					0, 0)) {
> 				end_zone = i;
> 				break;
> 			}
> 		}
> 	}
> 
>   So, by looping around, we could end up shrinking the active list again
>   before we recheck the zone watermarks depending on the size of the
>   inactive lists.

If this is a problem, it is a problem with that code, because kswapd
can be woken up for any zone at any time anyway.


> > >                 cond_resched();
> > > 
> > >                 try_to_freeze();
> > > 
> > >                 goto loop_again;
> > >         }
> > > 
> > > I used PAGE_ALLOC_COSTLY_ORDER instead of sc.order == 0 because we are
> > > expected to support allocations up to that order in a fairly reliable fashion.
> > 
> > I actually think it's better to do it for all orders, because that
> > constant is more or less arbitrary.
> 
> i.e.
> 
> if (!all_zones_ok && sc.order == 0) {
> 
> ? or something else

Well, I jus tdon't see what's wrong with the original patch.

 
> What I did miss was that we have 
> 
>                 if (nr_reclaimed >= SWAP_CLUSTER_MAX)
>                         break;
> 
> so with my patch, kswapd is bailing out early without trying to reclaim for
> high-orders that hard. That was not what I intended as it means we only ever
> really rebalance the full system for order-0 pages and for everything else we
> do relatively light scanning. The impact is that high-order users will direct
> reclaim rather than depending on kswapd scanning very heavily. Arguably,
> this is a good thing.
> 
> However, it also means that KOSAKI's and my patches only differs in that mine
> bails early and KOSAKI rechecks everything at order-0, possibly reclaiming
> more. If the comment was not so misleading, I'd have been a lot happier.

Rechecking everything is fine by me; order-0 is going to be the most
common and most important. If higher order allocations sometimes have
to enter direct reclaim or kick off kswapd again, it isn't a big deal.


> > IOW, I don't see a big downside, and there is a real upside.
> > 
> > I think the patch is good.
> > 
> 
> Which one, KOSAKI's or my one?
> 
> Here is my one again which bails out for any high-order allocation after
> just light scanning.
> 
> ====
> 
> >From 0e09fe002d8e9956de227b880ef8458842b71ca9 Mon Sep 17 00:00:00 2001
> From: Mel Gorman <mel@....ul.ie>
> Date: Tue, 30 Dec 2008 18:53:23 +0000
> Subject: [PATCH] mm: stop kswapd's infinite loop at high order allocation
> 
> Wassim Dagash reported the following (editted) kswapd infinite loop problem.
> 
>   kswapd runs in some infinite loop trying to swap until order 10 of zone
>   highmem is OK.... kswapd will continue to try to balance order 10 of zone
>   highmem forever (or until someone release a very large chunk of highmem).
> 
> For costly high-order allocations, the system may never be balanced due to
> fragmentation but kswapd should not infinitely loop as a result. The
> following patch lets kswapd stop reclaiming in the event it cannot
> balance zones and the order is high-order.

This one bails out if it was a higher order reclaim, but there is still
an order-0 shortage. I prefer to run the loop again at order==0 to avoid
that condition. A higher kswapd reclaim order shouldn't weaken kswapd
postcondition for order-0 memory.

> 
> Reported-by: wassim dagash <wassim.dagash@...il.com>
> Signed-off-by: Mel Gorman <mel@....ul.ie>
> 
> ---
>  mm/vmscan.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 62e7f62..7b0f412 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1867,7 +1867,16 @@ out:
>  
>  		zone->prev_priority = temp_priority[i];
>  	}
> -	if (!all_zones_ok) {
> +
> +	/*
> +	 * If zones are still not balanced, loop again and continue attempting
> +	 * to rebalance the system. For high-order allocations, fragmentation
> +	 * can prevent the zones being rebalanced no matter how hard kswapd
> +	 * works, particularly on systems with little or no swap. For
> +	 * high-orders, just give up and assume interested processes will
> +	 * either direct reclaim or wake up kswapd again as necessary.
> +	 */
> +	if (!all_zones_ok && sc.order == 0) {
>  		cond_resched();
>  
>  		try_to_freeze();
--
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