Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster when high-order watermarks are being hit

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

On Tue, 27 Oct 2009 13:40:33 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> When a high-order allocation fails, kswapd is kicked so that it reclaims
> at a higher-order to avoid direct reclaimers stall and to help GFP_ATOMIC
> allocations. Something has changed in recent kernels that affect the timing
> where high-order GFP_ATOMIC allocations are now failing with more frequency,
> particularly under pressure. This patch forces kswapd to notice sooner that
> high-order allocations are occuring.
> 

"something has changed"?  Shouldn't we find out what that is?

> ---
>  mm/vmscan.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 64e4388..7eceb02 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2016,6 +2016,15 @@ loop_again:
>  					priority != DEF_PRIORITY)
>  				continue;
>  
> +			/*
> +			 * Exit the function now and have kswapd start over
> +			 * if it is known that higher orders are required
> +			 */
> +			if (pgdat->kswapd_max_order > order) {
> +				all_zones_ok = 1;
> +				goto out;
> +			}
> +
>  			if (!zone_watermark_ok(zone, order,
>  					high_wmark_pages(zone), end_zone, 0))
>  				all_zones_ok = 0;

So this handles the case where some concurrent thread or interrupt
increases pgdat->kswapd_max_order while kswapd was running
balance_pgdat(), yes?

Does that actually happen much?  Enough for this patch to make any
useful difference?

If one where to whack a printk in that `if' block, how often would it
trigger, and under what circumstances?


If the -stable maintainers were to ask me "why did you send this" then
right now my answer would have to be "I have no idea".  Help.