Re: [PATCH 5/5] vmscan: Take order into consideration when deciding if kswapd is in trouble

KOSAKI Motohiro
2009-11-13T18:01:45Z

> This makes a lot of sense. Tests look good and I added stats to make sure
> the logic was triggering. On X86, kswapd avoided a congestion_wait 11723
> times and X86-64 avoided it 5084 times. I think we should hold onto the
> stats temporarily until all these bugs are ironed out.
>
> Would you like to sign off the following?
>
> If you are ok to sign off, this patch should replace my patch 5 in
> the series.

I'm sorry, I found my bug.
Please see below.

>
> ==== CUT HERE ====
>
> vmscan: Stop kswapd waiting on congestion when the min watermark is not being met
>
> If reclaim fails to make sufficient progress, the priority is raised.
> Once the priority is higher, kswapd starts waiting on congestion. However,
> if the zone is below the min watermark then kswapd needs to continue working
> without delay as there is a danger of an increased rate of GFP_ATOMIC
> allocation failure.
>
> This patch changes the conditions under which kswapd waits on
> congestion by only going to sleep if the min watermarks are being met.
>
> [mel@csn.ul.ie: Add stats to track how relevant the logic is]
> Needs-signed-off-by-original-author
>
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 9716003..7d66695 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -41,6 +41,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> #endif
> PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
> KSWAPD_PREMATURE_FAST, KSWAPD_PREMATURE_SLOW,
> + KSWAPD_NO_CONGESTION_WAIT,
> PAGEOUTRUN, ALLOCSTALL, PGROTATED,
> #ifdef CONFIG_HUGETLB_PAGE
> HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ffa1766..70967e1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1966,6 +1966,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
> * free_pages == high_wmark_pages(zone).
> */
> int temp_priority[MAX_NR_ZONES];
> + int has_under_min_watermark_zone = 0;

This is wrong declaration place. It must change to

for (priority = DEF_PRIORITY; priority >= 0; priority--) {
int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
unsigned long lru_pages = 0;
+ int has_under_min_watermark_zone = 0;

because, has_under_min_watermark_zone should be initialized every priority.

> loop_again:
> total_scanned = 0;
> @@ -2085,6 +2086,15 @@ loop_again:
> if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
> sc.may_writepage = 1;
> +
> + /*
> + * We are still under min water mark. it mean we have
> + * GFP_ATOMIC allocation failure risk. Hurry up!
> + */
> + if (!zone_watermark_ok(zone, order, min_wmark_pages(zone),
> + end_zone, 0))
> + has_under_min_watermark_zone = 1;
> +
> }
> if (all_zones_ok)
> break; /* kswapd: all done */
> @@ -2092,8 +2102,13 @@ loop_again:
> * OK, kswapd is getting into trouble. Take a nap, then take
> * another pass across the zones.
> */
> - if (total_scanned && priority < DEF_PRIORITY - 2)
> - congestion_wait(BLK_RW_ASYNC, HZ/10);
> + if (total_scanned && (priority < DEF_PRIORITY - 2)) {
> +

This blank line is unnecesary.

> + if (!has_under_min_watermark_zone)

Probably "if (has_under_min_watermark_zone)" is correct.

> + count_vm_event(KSWAPD_NO_CONGESTION_WAIT);
> + else
> + congestion_wait(BLK_RW_ASYNC, HZ/10);
> + }

Otherthing looks pretty good to me. please feel free to add my s-o-b or reviewed-by.

Thanks.