linux-kernel - Re: [PATCHv3] memcg: fix get_scan

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Date:	Wed, 27 Apr 2011 17:48:18 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Ying Han <yinghan@...gle.com>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"mgorman@...e.de" <mgorman@...e.de>
Subject: Re: [PATCHv3] memcg: fix get_scan_count for small targets

On Wed, Apr 27, 2011 at 4:47 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> At memory reclaim, we determine the number of pages to be scanned
> per zone as
>        (anon + file) >> priority.
> Assume
>        scan = (anon + file) >> priority.
>
> If scan < SWAP_CLUSTER_MAX, the scan will be skipped for this time
> and priority gets higher. This has some problems.
>
>  1. This increases priority as 1 without any scan.
>     To do scan in this priority, amount of pages should be larger than 512M.
>     If pages>>priority < SWAP_CLUSTER_MAX, it's recorded and scan will be
>     batched, later. (But we lose 1 priority.)
>     If memory size is below 16M, pages >> priority is 0 and no scan in
>     DEF_PRIORITY forever.
>
>  2. If zone->all_unreclaimabe==true, it's scanned only when priority==0.
>     So, x86's ZONE_DMA will never be recoverred until the user of pages
>     frees memory by itself.
>
>  3. With memcg, the limit of memory can be small. When using small memcg,
>     it gets priority < DEF_PRIORITY-2 very easily and need to call
>     wait_iff_congested().
>     For doing scan before priorty=9, 64MB of memory should be used.
>
> Then, this patch tries to scan SWAP_CLUSTER_MAX of pages in force...when
>
>  1. the target is enough small.
>  2. it's kswapd or memcg reclaim.
>
> Then we can avoid rapid priority drop and may be able to recover
> all_unreclaimable in a small zones. And this patch removes nr_saved_scan.
> This will allow scanning in this priority even when pages >> priority
> is very small.
>
> Changelog v2->v3
>  - removed nr_saved_scan completely.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

The patch looks good to me but I have a nitpick about just coding style.
How about this? I think below looks better but it's just my private
opinion and I can't insist on my style. If you don't mind it, ignore.

barrios@...rios-desktop:~/linux-2.6$ git diff
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6771ea7..268e7d4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1817,8 +1817,28 @@ out:
                        scan >>= priority;
                        scan = div64_u64(scan * fraction[file], denominator);
                }
-               nr[l] = nr_scan_try_batch(scan,
-                                         &reclaim_stat->nr_saved_scan[l]);
+
+               nr[l] = scan;
+               if (scan)
+                       continue;
+               /*
+                * 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.
+                */
+               if (((anon + file) >> priority) < SWAP_CLUSTER_MAX) {
+                       /* kswapd does zone balancing and need to scan
this zone */
+                       /* memcg may have small limit and need to
avoid priority drop */
+                       if ((scanning_global_lru(sc) && current_is_kswapd())
+                                       || !scanning_global_lru(sc)) {
+                               if (file || !noswap)
+                                       nr[l] = SWAP_CLUSTER_MAX;
+                       }
+               }
        }
 }


-- 
Kind regards,
Minchan Kim
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