/* * Workingset detection * * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner */ #include <linux/memcontrol.h> #include <linux/writeback.h> #include <linux/pagemap.h> #include <linux/atomic.h> #include <linux/module.h> #include <linux/swap.h> #include <linux/fs.h> #include <linux/mm.h> /* * Double CLOCK lists * * Per zone, two clock lists are maintained for file pages: the * inactive and the active list. Freshly faulted pages start out at * the head of the inactive list and page reclaim scans pages from the * tail. Pages that are accessed multiple times on the inactive list * are promoted to the active list, to protect them from reclaim, * whereas active pages are demoted to the inactive list when the * active list grows too big. * * fault ------------------------+ * | * +--------------+ | +-------------+ * reclaim <- | inactive | <-+-- demotion | active | <--+ * +--------------+ +-------------+ | * | | * +-------------- promotion ------------------+ * * * Access frequency and refault distance * * A workload is thrashing when its pages are frequently used but they * are evicted from the inactive list every time before another access * would have promoted them to the active list. * * In cases where the average access distance between thrashing pages * is bigger than the size of memory there is nothing that can be * done - the thrashing set could never fit into memory under any * circumstance. * * However, the average access distance could be bigger than the * inactive list, yet smaller than the size of memory. In this case, * the set could fit into memory if it weren't for the currently * active pages - which may be used more, hopefully less frequently: * * +-memory available to cache-+ * | | * +-inactive------+-active----+ * a b | c d e f g h i | J K L M N | * +---------------+-----------+ * * It is prohibitively expensive to accurately track access frequency * of pages. But a reasonable approximation can be made to measure * thrashing on the inactive list, after which refaulting pages can be * activated optimistically to compete with the existing active pages. * * Approximating inactive page access frequency - Observations: * * 1. When a page is accessed for the first time, it is added to the * head of the inactive list, slides every existing inactive page * towards the tail by one slot, and pushes the current tail page * out of memory. * * 2. When a page is accessed for the second time, it is promoted to * the active list, shrinking the inactive list by one slot. This * also slides all inactive pages that were faulted into the cache * more recently than the activated page towards the tail of the * inactive list. * * Thus: * * 1. The sum of evictions and activations between any two points in * time indicate the minimum number of inactive pages accessed in * between. * * 2. Moving one inactive page N page slots towards the tail of the * list requires at least N inactive page accesses. * * Combining these: * * 1. When a page is finally evicted from memory, the number of * inactive pages accessed while the page was in cache is at least * the number of page slots on the inactive list. * * 2. In addition, measuring the sum of evictions and activations (E) * at the time of a page's eviction, and comparing it to another * reading (R) at the time the page faults back into memory tells * the minimum number of accesses while the page was not cached. * This is called the refault distance. * * Because the first access of the page was the fault and the second * access the refault, we combine the in-cache distance with the * out-of-cache distance to get the complete minimum access distance * of this page: * * NR_inactive + (R - E) * * And knowing the minimum access distance of a page, we can easily * tell if the page would be able to stay in cache assuming all page * slots in the cache were available: * * NR_inactive + (R - E) <= NR_inactive + NR_active * * which can be further simplified to * * (R - E) <= NR_active * * Put into words, the refault distance (out-of-cache) can be seen as * a deficit in inactive list space (in-cache). If the inactive list * had (R - E) more page slots, the page would not have been evicted * in between accesses, but activated instead. And on a full system, * the only thing eating into inactive list space is active pages. * * * Activating refaulting pages * * All that is known about the active list is that the pages have been * accessed more than once in the past. This means that at any given * time there is actually a good chance that pages on the active list * are no longer in active use. * * So when a refault distance of (R - E) is observed and there are at * least (R - E) active pages, the refaulting page is activated * optimistically in the hope that (R - E) active pages are actually * used less frequently than the refaulting page - or even not used at * all anymore. * * If this is wrong and demotion kicks in, the pages which are truly * used more frequently will be reactivated while the less frequently * used once will be evicted from memory. * * But if this is right, the stale pages will be pushed out of memory * and the used pages get to stay in cache. * * * Implementation * * For each zone's file LRU lists, a counter for inactive evictions * and activations is maintained (zone->inactive_age). * * On eviction, a snapshot of this counter (along with some bits to * identify the zone) is stored in the now empty page cache radix tree * slot of the evicted page. This is called a shadow entry. * * On cache misses for which there are shadow entries, an eligible * refault distance will immediately activate the refaulting page. */ static void *pack_shadow(unsigned long eviction, struct zone *zone) { eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone); eviction = (eviction << ZONES_SHIFT) | zone_idx(zone); eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT); return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY); } static void unpack_shadow(void *shadow, struct zone **zone, unsigned long *distance) { unsigned long entry = (unsigned long)shadow; unsigned long eviction; unsigned long refault; unsigned long mask; int zid, nid; entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT; zid = entry & ((1UL << ZONES_SHIFT) - 1); entry >>= ZONES_SHIFT; nid = entry & ((1UL << NODES_SHIFT) - 1); entry >>= NODES_SHIFT; eviction = entry; *zone = NODE_DATA(nid)->node_zones + zid; refault = atomic_long_read(&(*zone)->inactive_age); mask = ~0UL >> (NODES_SHIFT + ZONES_SHIFT + RADIX_TREE_EXCEPTIONAL_SHIFT); /* * The unsigned subtraction here gives an accurate distance * across inactive_age overflows in most cases. * * There is a special case: usually, shadow entries have a * short lifetime and are either refaulted or reclaimed along * with the inode before they get too old. But it is not * impossible for the inactive_age to lap a shadow entry in * the field, which can then can result in a false small * refault distance, leading to a false activation should this * old entry actually refault again. However, earlier kernels * used to deactivate unconditionally with *every* reclaim * invocation for the longest time, so the occasional * inappropriate activation leading to pressure on the active * list is not a problem. */ *distance = (refault - eviction) & mask; } /** * workingset_eviction - note the eviction of a page from memory * @mapping: address space the page was backing * @page: the page being evicted * * Returns a shadow entry to be stored in @mapping->page_tree in place * of the evicted @page so that a later refault can be detected. */ void *workingset_eviction(struct address_space *mapping, struct page *page) { struct zone *zone = page_zone(page); unsigned long eviction; eviction = atomic_long_inc_return(&zone->inactive_age); return pack_shadow(eviction, zone); } /** * workingset_refault - evaluate the refault of a previously evicted page * @shadow: shadow entry of the evicted page * * Calculates and evaluates the refault distance of the previously * evicted page in the context of the zone it was allocated in. * * Returns %true if the page should be activated, %false otherwise. */ bool workingset_refault(void *shadow) { unsigned long refault_distance; struct zone *zone; unpack_shadow(shadow, &zone, &refault_distance); inc_zone_state(zone, WORKINGSET_REFAULT); if (refault_distance <= zone_page_state(zone, NR_ACTIVE_FILE)) { inc_zone_state(zone, WORKINGSET_ACTIVATE); return true; } return false; } /** * workingset_activation - note a page activation * @page: page that is being activated */ void workingset_activation(struct page *page) { atomic_long_inc(&page_zone(page)->inactive_age); } /* * Shadow entries reflect the share of the working set that does not * fit into memory, so their number depends on the access pattern of * the workload. In most cases, they will refault or get reclaimed * along with the inode, but a (malicious) workload that streams * through files with a total size several times that of available * memory, while preventing the inodes from being reclaimed, can * create excessive amounts of shadow nodes. To keep a lid on this, * track shadow nodes and reclaim them when they grow way past the * point where they would still be useful. */ struct list_lru workingset_shadow_nodes; static unsigned long count_shadow_nodes(struct shrinker *shrinker, struct shrink_control *sc) { unsigned long shadow_nodes; unsigned long max_nodes; unsigned long pages; /* list_lru lock nests inside IRQ-safe mapping->tree_lock */ local_irq_disable(); shadow_nodes = list_lru_count_node(&workingset_shadow_nodes, sc->nid); local_irq_enable(); pages = node_present_pages(sc->nid); /* * Active cache pages are limited to 50% of memory, and shadow * entries that represent a refault distance bigger than that * do not have any effect. Limit the number of shadow nodes * such that shadow entries do not exceed the number of active * cache pages, assuming a worst-case node population density * of 1/8th on average. * * On 64-bit with 7 radix_tree_nodes per page and 64 slots * each, this will reclaim shadow entries when they consume * ~2% of available memory: * * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE */ max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3); if (shadow_nodes <= max_nodes) return 0; return shadow_nodes - max_nodes; } static enum lru_status shadow_lru_isolate(struct list_head *item, spinlock_t *lru_lock, void *arg) { struct address_space *mapping; struct radix_tree_node *node; unsigned int i; int ret; /* * Page cache insertions and deletions synchroneously maintain * the shadow node LRU under the mapping->tree_lock and the * lru_lock. Because the page cache tree is emptied before * the inode can be destroyed, holding the lru_lock pins any * address_space that has radix tree nodes on the LRU. * * We can then safely transition to the mapping->tree_lock to * pin only the address_space of the particular node we want * to reclaim, take the node off-LRU, and drop the lru_lock. */ node = container_of(item, struct radix_tree_node, private_list); mapping = node->private_data; /* Coming from the list, invert the lock order */ if (!spin_trylock(&mapping->tree_lock)) { spin_unlock(lru_lock); ret = LRU_RETRY; goto out; } list_del_init(item); spin_unlock(lru_lock); /* * The nodes should only contain one or more shadow entries, * no pages, so we expect to be able to remove them all and * delete and free the empty node afterwards. */ BUG_ON(!node->count); BUG_ON(node->count & RADIX_TREE_COUNT_MASK); for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) { if (node->slots[i]) { BUG_ON(!radix_tree_exceptional_entry(node->slots[i])); node->slots[i] = NULL; BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT)); node->count -= 1U << RADIX_TREE_COUNT_SHIFT; BUG_ON(!mapping->nrshadows); mapping->nrshadows--; } } BUG_ON(node->count); inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM); if (!__radix_tree_delete_node(&mapping->page_tree, node)) BUG(); spin_unlock(&mapping->tree_lock); ret = LRU_REMOVED_RETRY; out: local_irq_enable(); cond_resched(); local_irq_disable(); spin_lock(lru_lock); return ret; } static unsigned long scan_shadow_nodes(struct shrinker *shrinker, struct shrink_control *sc) { unsigned long ret; /* list_lru lock nests inside IRQ-safe mapping->tree_lock */ local_irq_disable(); ret = list_lru_walk_node(&workingset_shadow_nodes, sc->nid, shadow_lru_isolate, NULL, &sc->nr_to_scan); local_irq_enable(); return ret; } static struct shrinker workingset_shadow_shrinker = { .count_objects = count_shadow_nodes, .scan_objects = scan_shadow_nodes, .seeks = DEFAULT_SEEKS, .flags = SHRINKER_NUMA_AWARE, }; /* * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe * mapping->tree_lock. */ static struct lock_class_key shadow_nodes_key; static int __init workingset_init(void) { int ret; ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key); if (ret) goto err; ret = register_shrinker(&workingset_shadow_shrinker); if (ret) goto err_list_lru; return 0; err_list_lru: list_lru_destroy(&workingset_shadow_nodes); err: return ret; } module_init(workingset_init);