[PATCH 06/13] mm: balance slab aging

[Wu Fengguang]

The current slab shrinking code is way too fragile.
Let it manage aging pace by itself, and provide a simple and robust interface.

The design considerations:
- use the same syncing facilities as that of the zones
- keep the age of slabs in line with that of the largest zone
  this in effect makes aging rate of slabs follow that of the most aged node.

- reserve a minimal number of unused slabs
  the size of reservation depends on vm pressure

- shrink more slab caches only when vm pressure is high
  the old logic, `mmap pages found' - `shrink more caches' - `avoid swapping',
  sounds not quite logical, so the code is removed.

- let sc->nr_scanned record the exact number of cold pages scanned
  it is no longer used by the slab cache shrinking algorithm, but good for other
  algorithms(e.g. the active_list/inactive_list balancing).

Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
---

 fs/drop-pagecache.c |    2 
 include/linux/mm.h  |    7 +--
 mm/vmscan.c         |  106 +++++++++++++++++++++-------------------------------
 3 files changed, 48 insertions(+), 67 deletions(-)

--- linux-2.6.15-rc5-mm1.orig/include/linux/mm.h
+++ linux-2.6.15-rc5-mm1/include/linux/mm.h
@@ -798,7 +798,9 @@ struct shrinker {
 	shrinker_t		shrinker;
 	struct list_head	list;
 	int			seeks;	/* seeks to recreate an obj */
-	long			nr;	/* objs pending delete */
+	unsigned long		aging_total;
+	unsigned long		aging_milestone;
+	unsigned long		page_age;
 	struct shrinker_stats	*s_stats;
 };
 
@@ -1080,8 +1082,7 @@ int in_gate_area_no_task(unsigned long a
 
 int drop_pagecache_sysctl_handler(struct ctl_table *, int, struct file *,
 					void __user *, size_t *, loff_t *);
-int shrink_slab(unsigned long scanned, gfp_t gfp_mask,
-			unsigned long lru_pages);
+int shrink_slab(struct zone *zone, int priority, gfp_t gfp_mask);
 
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
--- linux-2.6.15-rc5-mm1.orig/mm/vmscan.c
+++ linux-2.6.15-rc5-mm1/mm/vmscan.c
@@ -165,6 +165,18 @@ static inline void update_zone_age(struc
 						<< PAGE_AGE_SHIFT) / len;
 }
 
+static inline void update_slab_age(struct shrinker *s,
+					unsigned long len, int nr_scan)
+{
+	s->aging_total += nr_scan;
+
+	if (s->aging_total - s->aging_milestone > len)
+		s->aging_milestone += len;
+
+	s->page_age = ((s->aging_total - s->aging_milestone)
+						<< PAGE_AGE_SHIFT) / len;
+}
+
 /*
  * Add a shrinker callback to be called from the vm
  */
@@ -176,7 +188,9 @@ struct shrinker *set_shrinker(int seeks,
         if (shrinker) {
 	        shrinker->shrinker = theshrinker;
 	        shrinker->seeks = seeks;
-	        shrinker->nr = 0;
+	        shrinker->aging_total = 0;
+	        shrinker->aging_milestone = 0;
+	        shrinker->page_age = 0;
 		shrinker->s_stats = alloc_percpu(struct shrinker_stats);
 		if (!shrinker->s_stats) {
 			kfree(shrinker);
@@ -204,6 +218,7 @@ void remove_shrinker(struct shrinker *sh
 EXPORT_SYMBOL(remove_shrinker);
 
 #define SHRINK_BATCH 128
+#define SLAB_RESERVE 1000
 /*
  * Call the shrink functions to age shrinkable caches
  *
@@ -212,76 +227,49 @@ EXPORT_SYMBOL(remove_shrinker);
  * percentages of the lru and ageable caches.  This should balance the seeks
  * generated by these structures.
  *
- * If the vm encounted mapped pages on the LRU it increase the pressure on
- * slab to avoid swapping.
+ * @priority reflects the vm pressure, the lower the value, the more to
+ * shrink.
  *
- * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
- *
- * `lru_pages' represents the number of on-LRU pages in all the zones which
- * are eligible for the caller's allocation attempt.  It is used for balancing
- * slab reclaim versus page reclaim.
+ * @zone is better to be the least over-scanned one (normally the highest
+ * zone).
  *
  * Returns the number of slab objects which we shrunk.
  */
-int shrink_slab(unsigned long scanned, gfp_t gfp_mask, unsigned long lru_pages)
+int shrink_slab(struct zone *zone, int priority, gfp_t gfp_mask)
 {
 	struct shrinker *shrinker;
 	int ret = 0;
 
-	if (scanned == 0)
-		scanned = SWAP_CLUSTER_MAX;
-
 	if (!down_read_trylock(&shrinker_rwsem))
 		return 1;	/* Assume we'll be able to shrink next time */
 
 	list_for_each_entry(shrinker, &shrinker_list, list) {
-		unsigned long long delta;
-		unsigned long total_scan;
-		unsigned long max_pass = (*shrinker->shrinker)(0, gfp_mask);
-
-		delta = (4 * scanned) / shrinker->seeks;
-		delta *= max_pass;
-		do_div(delta, lru_pages + 1);
-		shrinker->nr += delta;
-		if (shrinker->nr < 0) {
-			printk(KERN_ERR "%s: nr=%ld\n",
-					__FUNCTION__, shrinker->nr);
-			shrinker->nr = max_pass;
-		}
-
-		/*
-		 * Avoid risking looping forever due to too large nr value:
-		 * never try to free more than twice the estimate number of
-		 * freeable entries.
-		 */
-		if (shrinker->nr > max_pass * 2)
-			shrinker->nr = max_pass * 2;
-
-		total_scan = shrinker->nr;
-		shrinker->nr = 0;
-
-		while (total_scan >= SHRINK_BATCH) {
-			long this_scan = SHRINK_BATCH;
-			int shrink_ret;
+		while (!zone || age_gt(zone, shrinker)) {
 			int nr_before;
+			int nr_after;
 
 			nr_before = (*shrinker->shrinker)(0, gfp_mask);
-			shrink_ret = (*shrinker->shrinker)(this_scan, gfp_mask);
-			if (shrink_ret == -1)
+			if (nr_before < SLAB_RESERVE * priority / DEF_PRIORITY)
+				break;
+
+			nr_after = (*shrinker->shrinker)(SHRINK_BATCH, gfp_mask);
+			if (nr_after == -1)
 				break;
-			if (shrink_ret < nr_before) {
-				ret += nr_before - shrink_ret;
-				shrinker_stat_add(shrinker, nr_freed,
-					(nr_before - shrink_ret));
+
+			if (nr_after < nr_before) {
+				int nr_freed = nr_before - nr_after;
+
+				ret += nr_freed;
+				shrinker_stat_add(shrinker, nr_freed, nr_freed);
 			}
-			shrinker_stat_add(shrinker, nr_req, this_scan);
-			mod_page_state(slabs_scanned, this_scan);
-			total_scan -= this_scan;
+			shrinker_stat_add(shrinker, nr_req, SHRINK_BATCH);
+			mod_page_state(slabs_scanned, SHRINK_BATCH);
+			update_slab_age(shrinker, nr_before * DEF_PRIORITY * 2,
+					SHRINK_BATCH * shrinker->seeks *
+					(DEF_PRIORITY + priority));
 
 			cond_resched();
 		}
-
-		shrinker->nr += total_scan;
 	}
 	up_read(&shrinker_rwsem);
 	return ret;
@@ -487,11 +475,6 @@ static int shrink_list(struct list_head 
 
 		BUG_ON(PageActive(page));
 
-		sc->nr_scanned++;
-		/* Double the slab pressure for mapped and swapcache pages */
-		if (page_mapped(page) || PageSwapCache(page))
-			sc->nr_scanned++;
-
 		if (PageWriteback(page))
 			goto keep_locked;
 
@@ -936,6 +919,7 @@ static void shrink_cache(struct zone *zo
 			goto done;
 
 		max_scan -= nr_scan;
+		sc->nr_scanned += nr_scan;
 		if (current_is_kswapd())
 			mod_page_state_zone(zone, pgscan_kswapd, nr_scan);
 		else
@@ -1254,7 +1238,6 @@ int try_to_free_pages(struct zone **zone
 	int total_scanned = 0, total_reclaimed = 0;
 	struct reclaim_state *reclaim_state = current->reclaim_state;
 	struct scan_control sc;
-	unsigned long lru_pages = 0;
 	int i;
 
 	delay_prefetch();
@@ -1272,7 +1255,6 @@ int try_to_free_pages(struct zone **zone
 			continue;
 
 		zone->temp_priority = DEF_PRIORITY;
-		lru_pages += zone->nr_active + zone->nr_inactive;
 	}
 
 	/* The added 10 priorities are for scan rate balancing */
@@ -1285,7 +1267,8 @@ int try_to_free_pages(struct zone **zone
 		if (!priority)
 			disable_swap_token();
 		shrink_caches(zones, &sc);
-		shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
+		if (zone_idx(zones[0]))
+			shrink_slab(zones[0], priority, gfp_mask);
 		if (reclaim_state) {
 			sc.nr_reclaimed += reclaim_state->reclaimed_slab;
 			reclaim_state->reclaimed_slab = 0;
@@ -1381,8 +1364,6 @@ loop_again:
 	}
 
 	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
-		unsigned long lru_pages = 0;
-
 		all_zones_ok = 1;
 		sc.nr_scanned = 0;
 		sc.nr_reclaimed = 0;
@@ -1433,7 +1414,6 @@ scan_swspd:
 			zone->temp_priority = priority;
 			if (zone->prev_priority > priority)
 				zone->prev_priority = priority;
-			lru_pages += zone->nr_active + zone->nr_inactive;
 
 			shrink_zone(zone, &sc);
 
@@ -1442,7 +1422,7 @@ scan_swspd:
 				zone->all_unreclaimable = 1;
 		}
 		reclaim_state->reclaimed_slab = 0;
-		shrink_slab(sc.nr_scanned, GFP_KERNEL, lru_pages);
+		shrink_slab(prev_zone, priority, GFP_KERNEL);
 		sc.nr_reclaimed += reclaim_state->reclaimed_slab;
 		total_reclaimed += sc.nr_reclaimed;
 		total_scanned += sc.nr_scanned;
--- linux-2.6.15-rc5-mm1.orig/fs/drop-pagecache.c
+++ linux-2.6.15-rc5-mm1/fs/drop-pagecache.c
@@ -47,7 +47,7 @@ static void drop_slab(void)
 	int nr_objects;
 
 	do {
-		nr_objects = shrink_slab(1000, GFP_KERNEL, 1000);
+		nr_objects = shrink_slab(NULL, 0, GFP_KERNEL);
 	} while (nr_objects > 10);
 }
 

--
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