Re: [PATCH] mm: stop kswapd's infinite loop at high order allocation take2

[MinChan Kim]

Hi, kosaki-san.

I read the previous threads now. It's rather late :(.

I think it's rather awkward that sudden big change of order from 10 to 0.

This problem causes zone_water_mark's fail.
It mean now this zone's proportional free page per order size is not good.
Although order-0 page is very important, Shouldn't we consider other
order allocations ?

So I want to balance zone's proportional free page.
How about following ?

if (nr_reclaimed < SWAP_CLUSTER_MAX) {
   if (order != 0) {
     order -=1;
     sc.order -=1;
   }
}

It prevents infinite loop and do best effort to make zone's
proportional free page per order size good.

It's just my opinion within my knowledge.
If it have a problem, pz, explain me :)


On Thu, Jan 1, 2009 at 11:52 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>
>> >                 /*
>> >                  * Fragmentation may mean that the system cannot be
>> >                  * rebalanced for high-order allocations in all zones.
>> >                  * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
>> >                  * it means the zones have been fully scanned and are still
>> >                  * not balanced. For high-order allocations, there is
>> >                  * little point trying all over again as kswapd may
>> >                  * infinite loop.
>> >                  *
>> >                  * Instead, recheck all watermarks at order-0 as they
>> >                  * are the most important. If watermarks are ok, kswapd will go
>> >                  * back to sleep. High-order users can still direct reclaim
>> >                  * if they wish.
>> >                  */
>> >
>> > ?
>>
>> Excellent. I strongly like this and I hope merge it to my patch.
>> I'll resend new patch.
>
> Done.
>
>
>
> ==
> 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.... kswapd will continue to try to balance order 10 of zone
>  highmem forever (or until someone release a very large chunk of highmem).
>
> For non order-0 allocations, the system may never be balanced due to
> fragmentation but kswapd should not infinitely loop as a result.
>
> Instead, recheck all watermarks at order-0 as they are the most important.
> If watermarks are ok, kswapd will go back to sleep.
>
>
> Reported-by: wassim dagash <wassim.dagash@xxxxxxxxx>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Reviewed-by: Nick Piggin <npiggin@xxxxxxx>
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>,
> ---
>  mm/vmscan.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> Index: b/mm/vmscan.c
> ===================================================================
> --- a/mm/vmscan.c       2008-12-25 08:26:37.000000000 +0900
> +++ b/mm/vmscan.c       2009-01-01 01:56:02.000000000 +0900
> @@ -1872,6 +1872,23 @@ out:
>
>                try_to_freeze();
>
> +               /*
> +                * Fragmentation may mean that the system cannot be
> +                * rebalanced for high-order allocations in all zones.
> +                * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
> +                * it means the zones have been fully scanned and are still
> +                * not balanced. For high-order allocations, there is
> +                * little point trying all over again as kswapd may
> +                * infinite loop.
> +                *
> +                * Instead, recheck all watermarks at order-0 as they
> +                * are the most important. If watermarks are ok, kswapd will go
> +                * back to sleep. High-order users can still direct reclaim
> +                * if they wish.
> +                */
> +               if (nr_reclaimed < SWAP_CLUSTER_MAX)
> +                       order = sc.order = 0;
> +
>                goto loop_again;
>        }
>
>
>
>
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-- 
Kinds regards,
MinChan Kim
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