Re: [PATCH 4/5] vmscan: Have kswapd sleep for a short interval and double check it should be asleep

[KOSAKI Motohiro]

> After kswapd balances all zones in a pgdat, it goes to sleep. In the event
> of no IO congestion, kswapd can go to sleep very shortly after the high
> watermark was reached. If there are a constant stream of allocations from
> parallel processes, it can mean that kswapd went to sleep too quickly and
> the high watermark is not being maintained for sufficient length time.
> 
> This patch makes kswapd go to sleep as a two-stage process. It first
> tries to sleep for HZ/10. If it is woken up by another process or the
> high watermark is no longer met, it's considered a premature sleep and
> kswapd continues work. Otherwise it goes fully to sleep.
> 
> This adds more counters to distinguish between fast and slow breaches of
> watermarks. A "fast" premature sleep is one where the low watermark was
> hit in a very short time after kswapd going to sleep. A "slow" premature
> sleep indicates that the high watermark was breached after a very short
> interval.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>

Why do you submit this patch to mainline? this is debugging patch
no more and no less.


> ---
>  include/linux/vmstat.h |    1 +
>  mm/vmscan.c            |   44 ++++++++++++++++++++++++++++++++++++++++++--
>  mm/vmstat.c            |    2 ++
>  3 files changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 2d0f222..9716003 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -40,6 +40,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		PGSCAN_ZONE_RECLAIM_FAILED,
>  #endif
>  		PGINODESTEAL, SLABS_SCANNED, KSWAPD_STEAL, KSWAPD_INODESTEAL,
> +		KSWAPD_PREMATURE_FAST, KSWAPD_PREMATURE_SLOW,
>  		PAGEOUTRUN, ALLOCSTALL, PGROTATED,

Please don't use the word of "premature" and "fast". it is too hard to understand the meanings.
Plus, please use per-zone stastics (like NUMA_HIT).

>
>  #ifdef CONFIG_HUGETLB_PAGE
>  		HTLB_BUDDY_PGALLOC, HTLB_BUDDY_PGALLOC_FAIL,
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 190bae1..ffa1766 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1904,6 +1904,24 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  }
>  #endif
>  
> +/* is kswapd sleeping prematurely? */
> +static int sleeping_prematurely(int order, long remaining)
> +{
> +	struct zone *zone;
> +
> +	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
> +	if (remaining)
> +		return 1;
> +
> +	/* If after HZ/10, a zone is below the high mark, it's premature */
> +	for_each_populated_zone(zone)
> +		if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
> +								0, 0))
> +			return 1;

for_each_populated_zone() iterate all populated zone. but kswapd shuld't see another node.

> +
> +	return 0;
> +}
> +
>  /*
>   * For kswapd, balance_pgdat() will work across all this node's zones until
>   * they are all at high_wmark_pages(zone).
> @@ -2184,8 +2202,30 @@ static int kswapd(void *p)
>  			 */
>  			order = new_order;
>  		} else {
> -			if (!freezing(current))
> -				schedule();
> +			if (!freezing(current)) {
> +				long remaining = 0;
> +
> +				/* Try to sleep for a short interval */
> +				if (!sleeping_prematurely(order, remaining)) {
> +					remaining = schedule_timeout(HZ/10);
> +					finish_wait(&pgdat->kswapd_wait, &wait);
> +					prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
> +				}
> +
> +				/*
> +				 * After a short sleep, check if it was a
> +				 * premature sleep. If not, then go fully
> +				 * to sleep until explicitly woken up
> +				 */
> +				if (!sleeping_prematurely(order, remaining))
> +					schedule();
> +				else {
> +					if (remaining)
> +						count_vm_event(KSWAPD_PREMATURE_FAST);
> +					else
> +						count_vm_event(KSWAPD_PREMATURE_SLOW);
> +				}
> +			}
>  
>  			order = pgdat->kswapd_max_order;
>  		}
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index c81321f..90b11e4 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -683,6 +683,8 @@ static const char * const vmstat_text[] = {
>  	"slabs_scanned",
>  	"kswapd_steal",
>  	"kswapd_inodesteal",
> +	"kswapd_slept_prematurely_fast",
> +	"kswapd_slept_prematurely_slow",
>  	"pageoutrun",
>  	"allocstall",
>  
> -- 
> 1.6.5
> 



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