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Subject[PATCH 05/12] mm: balance slab aging
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@mail.ustc.edu.cn>
---

include/linux/mm.h | 4 +
mm/vmscan.c | 118 +++++++++++++++++++++++------------------------------
2 files changed, 55 insertions(+), 67 deletions(-)

--- linux.orig/include/linux/mm.h
+++ linux/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;
};

--- linux.orig/mm/vmscan.c
+++ linux/mm/vmscan.c
@@ -161,6 +161,18 @@ 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
*/
@@ -172,7 +184,9 @@ 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);
@@ -208,80 +222,61 @@ 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.
- *
- * 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.
+ * If the vm pressure is high, shrink the slabs more.
*
* Returns the number of slab objects which we shrunk.
*/
-static int shrink_slab(unsigned long scanned, gfp_t gfp_mask,
- unsigned long lru_pages)
+static int shrink_slab(gfp_t gfp_mask)
{
struct shrinker *shrinker;
- int ret = 0;
-
- if (scanned == 0)
- scanned = SWAP_CLUSTER_MAX;
+ struct pglist_data *pgdat;
+ struct zone *zone;
+ int n;

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;
- }
+ /* find the major zone for the slabs to catch up age with */
+ pgdat = NODE_DATA(numa_node_id());
+ zone = pgdat->node_zones;
+ for (n = 1; n < pgdat->nr_zones; n++) {
+ struct zone *z = pgdat->node_zones + n;

- /*
- * 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;
+ if (zone->present_pages < z->present_pages)
+ zone = z;
+ }

- while (total_scan >= SHRINK_BATCH) {
- long this_scan = SHRINK_BATCH;
- int shrink_ret;
+ n = 0;
+ list_for_each_entry(shrinker, &shrinker_list, list) {
+ while (pages_more_aged(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 <= SHRINK_BATCH * zone->prev_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;
+
+ n += 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,
+ SHRINK_BATCH * shrinker->seeks *
+ zone->prev_priority);

cond_resched();
}
-
- shrinker->nr += total_scan;
}
up_read(&shrinker_rwsem);
- return ret;
+ return n;
}

/* Called without lock on whether page is mapped, so answer is unstable */
@@ -484,11 +479,6 @@ 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;

@@ -933,6 +923,7 @@ 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
@@ -1251,7 +1242,6 @@ 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();
@@ -1269,7 +1259,6 @@ 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 */
@@ -1282,7 +1271,7 @@ 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);
+ shrink_slab(gfp_mask);
if (reclaim_state) {
sc.nr_reclaimed += reclaim_state->reclaimed_slab;
reclaim_state->reclaimed_slab = 0;
@@ -1386,8 +1375,6 @@ 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;
@@ -1431,7 +1418,6 @@ loop_again:
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);

@@ -1440,7 +1426,7 @@ loop_again:
zone->all_unreclaimable = 1;
}
reclaim_state->reclaimed_slab = 0;
- shrink_slab(sc.nr_scanned, GFP_KERNEL, lru_pages);
+ shrink_slab(GFP_KERNEL);
sc.nr_reclaimed += reclaim_state->reclaimed_slab;
total_reclaimed += sc.nr_reclaimed;
total_scanned += sc.nr_scanned;
--
-
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