Re: [PATCH 3/3] Count the number of times zone_reclaim() scans and fails

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On NUMA machines, the administrator can configure zone_reclaim_mode that
> is a more targetted form of direct reclaim. On machines with large NUMA
> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> unmapped pages will be reclaimed if the zone watermarks are not being met.
> 
> There is a heuristic that determines if the scan is worthwhile but it is
> possible that the heuristic will fail and the CPU gets tied up scanning
> uselessly. Detecting the situation requires some guesswork and experimentation
> so this patch adds a counter "zreclaim_failed" to /proc/vmstat. If during
> high CPU utilisation this counter is increasing rapidly, then the resolution
> to the problem may be to set /proc/sys/vm/zone_reclaim_mode to 0.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> ---
>  include/linux/vmstat.h |    3 +++
>  mm/vmscan.c            |    3 +++
>  mm/vmstat.c            |    3 +++
>  3 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index ff4696c..416f748 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -36,6 +36,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		FOR_ALL_ZONES(PGSTEAL),
>  		FOR_ALL_ZONES(PGSCAN_KSWAPD),
>  		FOR_ALL_ZONES(PGSCAN_DIRECT),
> +#ifdef CONFIG_NUMA
> +		PGSCAN_ZONERECLAIM_FAILED,
> +#endif
>  		PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
>  		PAGEOUTRUN, ALLOCSTALL, PGROTATED,
>  #ifdef CONFIG_HUGETLB_PAGE
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7b8eb3f..42c1013 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2492,6 +2492,9 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	ret = __zone_reclaim(zone, gfp_mask, order);
>  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
>  
> +	if (!ret)
> +		count_vm_event(PGSCAN_ZONERECLAIM_FAILED);
> +
>  	return ret;
>  }
>  #endif
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 1e3aa81..02677d1 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -673,6 +673,9 @@ static const char * const vmstat_text[] = {
>  	TEXTS_FOR_ZONES("pgscan_kswapd")
>  	TEXTS_FOR_ZONES("pgscan_direct")
>  
> +#ifdef CONFIG_NUMA
> +	"zreclaim_failed",
> +#endif
>  	"pginodesteal",
>  	"slabs_scanned",
>  	"kswapd_steal",

Looks good.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>



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