Commit f8629631 authored by Wu Fengguang's avatar Wu Fengguang Committed by Linus Torvalds
Browse files

mm: do batched scans for mem_cgroup 



For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1, in
which case shrink_list() _still_ calls isolate_pages() with the much
larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list scan
rate by up to 32 times.

For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
So when shrink_zone() expects to scan 4 pages in the active/inactive list,
the active list will be scanned 4 pages, while the inactive list will be
(over) scanned SWAP_CLUSTER_MAX=32 pages in effect.  And that could break
the balance between the two lists.

It can further impact the scan of anon active list, due to the anon
active/inactive ratio rebalance logic in balance_pgdat()/shrink_zone():

inactive anon list over scanned => inactive_anon_is_low() == TRUE
                                => shrink_active_list()
                                => active anon list over scanned

So the end result may be

- anon inactive  => over scanned
- anon active    => over scanned (maybe not as much)
- file inactive  => over scanned
- file active    => under scanned (relatively)

The accesses to nr_saved_scan are not lock protected and so not 100%
accurate, however we can tolerate small errors and the resulted small
imbalanced scan rates between zones.

Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1a8670a2

include/linux/mmzone.h

+5 −1
Original line number Diff line number Diff line
@@ -273,6 +273,11 @@ struct zone_reclaim_stat { 
	 */

	unsigned long		recent_rotated[2];

	unsigned long		recent_scanned[2];



	/*

	 * accumulated for batching

	 */

	unsigned long		nr_saved_scan[NR_LRU_LISTS];

};



struct zone {
@@ -327,7 +332,6 @@ struct zone { 
	spinlock_t		lru_lock;	

	struct zone_lru {

		struct list_head list;

		unsigned long nr_saved_scan;	/* accumulated for batching */

	} lru[NR_LRU_LISTS];



	struct zone_reclaim_stat reclaim_stat;

mm/page_alloc.c

+1 −1
Original line number Diff line number Diff line
@@ -3809,7 +3809,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat, 
		zone_pcp_init(zone);

		for_each_lru(l) {

			INIT_LIST_HEAD(&zone->lru[l].list);

			zone->lru[l].nr_saved_scan = 0;

			zone->reclaim_stat.nr_saved_scan[l] = 0;

		}

		zone->reclaim_stat.recent_rotated[0] = 0;

		zone->reclaim_stat.recent_rotated[1] = 0;

mm/vmscan.c

+11 −9
Original line number Diff line number Diff line
@@ -1586,6 +1586,7 @@ static void shrink_zone(int priority, struct zone *zone, 
	enum lru_list l;

	unsigned long nr_reclaimed = sc->nr_reclaimed;

	unsigned long swap_cluster_max = sc->swap_cluster_max;

	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);

	int noswap = 0;



	/* If we have no swap space, do not bother scanning anon pages. */
@@ -1605,12 +1606,9 @@ static void shrink_zone(int priority, struct zone *zone, 
			scan >>= priority;

			scan = (scan * percent[file]) / 100;

		}

		if (scanning_global_lru(sc))

		nr[l] = nr_scan_try_batch(scan,

						  &zone->lru[l].nr_saved_scan,

					  &reclaim_stat->nr_saved_scan[l],

					  swap_cluster_max);

		else

			nr[l] = scan;

	}



	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
@@ -2220,6 +2218,7 @@ static void shrink_all_zones(unsigned long nr_pages, int prio, 
{

	struct zone *zone;

	unsigned long nr_reclaimed = 0;

	struct zone_reclaim_stat *reclaim_stat;



	for_each_populated_zone(zone) {

		enum lru_list l;
@@ -2236,11 +2235,14 @@ static void shrink_all_zones(unsigned long nr_pages, int prio, 
						l == LRU_ACTIVE_FILE))

				continue;



			zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1;

			if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) {

			reclaim_stat = get_reclaim_stat(zone, sc);

			reclaim_stat->nr_saved_scan[l] +=

						(lru_pages >> prio) + 1;

			if (reclaim_stat->nr_saved_scan[l]

						>= nr_pages || pass > 3) {

				unsigned long nr_to_scan;



				zone->lru[l].nr_saved_scan = 0;

				reclaim_stat->nr_saved_scan[l] = 0;

				nr_to_scan = min(nr_pages, lru_pages);

				nr_reclaimed += shrink_list(l, nr_to_scan, zone,

								sc, prio);

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