Re: [PATCH] mm/vmscan.c: use DIV_ROUND_UP for calculation of zone's balance_gap and correct comments.

[Rafael Aquini <aquini@redhat.com>]

On Mon, May 19, 2014 at 12:08:30PM +0800, Jianyu Zhan wrote:
> Currently, we use (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> KSWAPD_ZONE_BALANCE_GAP_RATIO to avoid a zero gap value. It's better to
> use DIV_ROUND_UP macro for neater code and clear meaning.
> 
> Besides, the gap value is calculated against the per-zone "managed pages",
> not "present pages". This patch also corrects the comment and do some
> rephrasing.
> 
> Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
> ---
Acked-by: Rafael Aquini <aquini@redhat.com>

>  include/linux/swap.h |  8 ++++----
>  mm/vmscan.c          | 10 ++++------
>  2 files changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 5a14b92..58e1696 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -166,10 +166,10 @@ enum {
>  #define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
>  
>  /*
> - * Ratio between the present memory in the zone and the "gap" that
> - * we're allowing kswapd to shrink in addition to the per-zone high
> - * wmark, even for zones that already have the high wmark satisfied,
> - * in order to provide better per-zone lru behavior. We are ok to
> + * Ratio between zone->managed_pages and the "gap" that above the per-zone
> + * "high_wmark". While balancing nodes, We allow kswapd to shrink zones that
> + * do not meet the (high_wmark + gap) watermark, even which already met the
> + * high_wmark, in order to provide better per-zone lru behavior. We are ok to
>   * spend not more than 1% of the memory for this zone balancing "gap".
>   */
>  #define KSWAPD_ZONE_BALANCE_GAP_RATIO 100
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 32c661d..9ef9f6c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2268,9 +2268,8 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
>  	 * there is a buffer of free pages available to give compaction
>  	 * a reasonable chance of completing and allocating the page
>  	 */
> -	balance_gap = min(low_wmark_pages(zone),
> -		(zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> -			KSWAPD_ZONE_BALANCE_GAP_RATIO);
> +	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
> +			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
>  	watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
>  	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);
>  
> @@ -2891,9 +2890,8 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  	 * high wmark plus a "gap" where the gap is either the low
>  	 * watermark or 1% of the zone, whichever is smaller.
>  	 */
> -	balance_gap = min(low_wmark_pages(zone),
> -		(zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> -		KSWAPD_ZONE_BALANCE_GAP_RATIO);
> +	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
> +			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
>  
>  	/*
>  	 * If there is no low memory pressure or the zone is balanced then no
> -- 
> 2.0.0-rc3
> 

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