Re: [PATCH] mm: stop kswapd's infinite loop at high order allocation

[Nick Piggin]

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:
> > 
> > ok, wassim confirmed this patch works well.
> > 
> > 
> > ==
> > From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > Subject: [PATCH] mm: kswapd stop infinite loop at high order allocation
> > 
> > Wassim Dagash reported following kswapd infinite loop problem.
> > 
> >   kswapd runs in some infinite loop trying to swap until order 10 of zone
> >   highmem is OK, While zone higmem (as I understand) has nothing to do
> >   with contiguous memory (cause there is no 1-1 mapping) which means
> >   kswapd will continue to try to balance order 10 of zone highmem
> >   forever (or until someone release a very large chunk of highmem).
> > 
> > He proposed remove contenious checking on highmem at all.
> > However hugepage on highmem need contenious highmem page.
> > 
> 
> I'm lacking the original problem report, but contiguous order-10 pages are
> indeed required for hugepages in highmem and reclaiming for them should not
> be totally disabled at any point. While no 1-1 mapping exists for the kernel,
> contiguity is still required.

This doesn't totally disable them. It disables asynchronous reclaim for them
until the next time kswapd kicks is kicked by a higher order allocator. The
guy who kicked us off this time should go into direct reclaim.


> 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.


> While constantly looping trying to balance for high-orders is indeed bad,
> I'm unconvinced this is the correct change. As we have already gone through
> a priorities and scanned everything at the high-order, would it not make
> more sense to do just give up with something like the following?
> 
>        /*
>         * 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 costly
>         * orders, just give up and assume interested processes will either
>         * direct reclaim or wake up kswapd as necessary.
>         */
>         if (!all_zones_ok && sc.order <= PAGE_ALLOC_COSTLY_ORDER) {
>                 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. It is possible a zone might become
too fragmented to support this, but the allocating process has been OOMed or
had their allocation satisfied from another zone. kswapd would have no way
out of the loop even if the system no longer requires higher order allocations.

IOW, I don't see a big downside, and there is a real upside.

I think the patch is good.

> 
> > To add infinite loop stopper is simple and good.
> > 
> > 
> > 
> > Reported-by: wassim dagash <wassim.dagash@xxxxxxxxx>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> 
> For the moment
> 
> NAKed-by: Mel Gorman <mel@xxxxxxxxx>
> 
> How about the following (compile-tested-only) patch?
> 
> =============
> From: Mel Gorman <mel@xxxxxxxxx>
> Subject: [PATCH] mm: stop kswapd's infinite loop at high order allocation
> 
>   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.
> 
> Reported-by: wassim dagash <wassim.dagash@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 62e7f62..03ed9a0 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 costly
> +	 * orders, just give up and assume interested processes will either
> +	 * direct reclaim or wake up kswapd as necessary.
> +	 */
> +	if (!all_zones_ok && sc.order <= PAGE_ALLOC_COSTLY_ORDER) {
>  		cond_resched();
>  
>  		try_to_freeze();
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