linux-kernel - Re: [patch 2/2] mm: vmscan: drop nr_force_scan[] from get_scan

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Date:	Fri, 12 Aug 2011 08:44:34 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, Ying Han <yinghan@...gle.com>,
	Balbir Singh <bsingharora@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [patch 2/2] mm: vmscan: drop nr_force_scan[] from get_scan_count

On Fri, Aug 12, 2011 at 5:31 AM, Johannes Weiner <jweiner@...hat.com> wrote:
> The nr_force_scan[] tuple holds the effective scan numbers for anon
> and file pages in case the situation called for a forced scan and the
> regularly calculated scan numbers turned out zero.
>
> However, the effective scan number can always be assumed to be
> SWAP_CLUSTER_MAX right before the division into anon and file.  The
> numerators and denominator are properly set up for all cases, be it
> force scan for just file, just anon, or both, to do the right thing.
>
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>

Reviewed-by: Minchan Kim <minchan.kim@...il.com>

There is a nitpick at below.

> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Ying Han <yinghan@...gle.com>
> Cc: Balbir Singh <bsingharora@...il.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Cc: Mel Gorman <mel@....ul.ie>
> ---
>  mm/vmscan.c |   24 ++----------------------
>  1 files changed, 2 insertions(+), 22 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 96061d7..45f0986 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1831,7 +1831,6 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
>        enum lru_list l;
>        int noswap = 0;
>        bool force_scan = false;
> -       unsigned long nr_force_scan[2];
>
>        /* kswapd does zone balancing and need to scan this zone */
>        if (scanning_global_lru(sc) && current_is_kswapd())
> @@ -1846,8 +1845,6 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
>                fraction[0] = 0;
>                fraction[1] = 1;
>                denominator = 1;
> -               nr_force_scan[0] = 0;
> -               nr_force_scan[1] = SWAP_CLUSTER_MAX;
>                goto out;
>        }
>
> @@ -1864,8 +1861,6 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
>                        fraction[0] = 1;
>                        fraction[1] = 0;
>                        denominator = 1;
> -                       nr_force_scan[0] = SWAP_CLUSTER_MAX;
> -                       nr_force_scan[1] = 0;
>                        goto out;
>                }
>        }
> @@ -1914,11 +1909,6 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
>        fraction[0] = ap;
>        fraction[1] = fp;
>        denominator = ap + fp + 1;
> -       if (force_scan) {
> -               unsigned long scan = SWAP_CLUSTER_MAX;
> -               nr_force_scan[0] = div64_u64(scan * ap, denominator);
> -               nr_force_scan[1] = div64_u64(scan * fp, denominator);
> -       }
>  out:
>        for_each_evictable_lru(l) {
>                int file = is_file_lru(l);
> @@ -1927,20 +1917,10 @@ out:
>                scan = zone_nr_lru_pages(zone, sc, l);
>                if (priority || noswap) {
>                        scan >>= priority;
> +                       if (!scan && force_scan)
> +                               scan = SWAP_CLUSTER_MAX;
>                        scan = div64_u64(scan * fraction[file], denominator);
>                }
> -
> -               /*
> -                * If zone is small or memcg is small, nr[l] can be 0.
> -                * This results no-scan on this priority and priority drop down.
> -                * For global direct reclaim, it can visit next zone and tend
> -                * not to have problems. For global kswapd, it's for zone
> -                * balancing and it need to scan a small amounts. When using
> -                * memcg, priority drop can cause big latency. So, it's better
> -                * to scan small amount. See may_noscan above.
> -                */

Please move this comment with tidy-up at where making force_scan true.
Of course, we can find it by git log[246e87a9393] but as I looked the
git log, it explain this comment indirectly and it's not
understandable to newbies. I think this comment is more understandable
than changelog in git.



-- 
Kind regards,
Minchan Kim
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