// SPDX-License-Identifier: GPL-2.0 /* * Workingset detection * * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner */ #include #include #include #include #include #include #include #include #include #include /* * Double CLOCK lists * * Per node, 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 node's file LRU lists, a counter for inactive evictions * and activations is maintained (node->inactive_age). * * On eviction, a snapshot of this counter (along with some bits to * identify the node) 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. */ #define EVICTION_SHIFT (RADIX_TREE_EXCEPTIONAL_ENTRY + \ NODES_SHIFT + \ MEM_CGROUP_ID_SHIFT) #define EVICTION_MASK (~0UL >> EVICTION_SHIFT) /* * Eviction timestamps need to be able to cover the full range of * actionable refaults. However, bits are tight in the radix tree * entry, and after storing the identifier for the lruvec there might * not be enough left to represent every single actionable refault. In * that case, we have to sacrifice granularity for distance, and group * evictions into coarser buckets by shaving off lower timestamp bits. */ static unsigned int bucket_order __read_mostly; static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction) { eviction >>= bucket_order; eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid; eviction = (eviction << NODES_SHIFT) | pgdat->node_id; eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT); return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY); } static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat, unsigned long *evictionp) { unsigned long entry = (unsigned long)shadow; int memcgid, nid; entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT; nid = entry & ((1UL << NODES_SHIFT) - 1); entry >>= NODES_SHIFT; memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1); entry >>= MEM_CGROUP_ID_SHIFT; *memcgidp = memcgid; *pgdat = NODE_DATA(nid); *evictionp = entry << bucket_order; } /** * 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->i_pages 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 mem_cgroup *memcg = page_memcg(page); struct pglist_data *pgdat = page_pgdat(page); int memcgid = mem_cgroup_id(memcg); unsigned long eviction; struct lruvec *lruvec; /* Page is fully exclusive and pins page->mem_cgroup */ VM_BUG_ON_PAGE(PageLRU(page), page); VM_BUG_ON_PAGE(page_count(page), page); VM_BUG_ON_PAGE(!PageLocked(page), page); lruvec = mem_cgroup_lruvec(pgdat, memcg); eviction = atomic_long_inc_return(&lruvec->inactive_age); return pack_shadow(memcgid, pgdat, eviction); } /** * 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 node it was allocated in. * * Returns %true if the page should be activated, %false otherwise. */ bool workingset_refault(void *shadow) { unsigned long refault_distance; unsigned long active_file; struct mem_cgroup *memcg; unsigned long eviction; struct lruvec *lruvec; unsigned long refault; struct pglist_data *pgdat; int memcgid; unpack_shadow(shadow, &memcgid, &pgdat, &eviction); rcu_read_lock(); /* * Look up the memcg associated with the stored ID. It might * have been deleted since the page's eviction. * * Note that in rare events the ID could have been recycled * for a new cgroup that refaults a shared page. This is * impossible to tell from the available data. However, this * should be a rare and limited disturbance, and activations * are always speculative anyway. Ultimately, it's the aging * algorithm's job to shake out the minimum access frequency * for the active cache. * * XXX: On !CONFIG_MEMCG, this will always return NULL; it * would be better if the root_mem_cgroup existed in all * configurations instead. */ memcg = mem_cgroup_from_id(memcgid); if (!mem_cgroup_disabled() && !memcg) { rcu_read_unlock(); return false; } lruvec = mem_cgroup_lruvec(pgdat, memcg); refault = atomic_long_read(&lruvec->inactive_age); active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES); /* * 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. */ refault_distance = (refault - eviction) & EVICTION_MASK; inc_lruvec_state(lruvec, WORKINGSET_REFAULT); if (refault_distance <= active_file) { inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE); rcu_read_unlock(); return true; } rcu_read_unlock(); return false; } /** * workingset_activation - note a page activation * @page: page that is being activated */ void workingset_activation(struct page *page) { struct mem_cgroup *memcg; struct lruvec *lruvec; rcu_read_lock(); /* * Filter non-memcg pages here, e.g. unmap can call * mark_page_accessed() on VDSO pages. * * XXX: See workingset_refault() - this should return * root_mem_cgroup even for !CONFIG_MEMCG. */ memcg = page_memcg_rcu(page); if (!mem_cgroup_disabled() && !memcg) goto out; lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg); atomic_long_inc(&lruvec->inactive_age); out: rcu_read_unlock(); } /* * 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. */ static struct list_lru shadow_nodes; void workingset_update_node(struct radix_tree_node *node) { /* * Track non-empty nodes that contain only shadow entries; * unlink those that contain pages or are being freed. * * Avoid acquiring the list_lru lock when the nodes are * already where they should be. The list_empty() test is safe * as node->private_list is protected by the i_pages lock. */ if (node->count && node->count == node->exceptional) { if (list_empty(&node->private_list)) list_lru_add(&shadow_nodes, &node->private_list); } else { if (!list_empty(&node->private_list)) list_lru_del(&shadow_nodes, &node->private_list); } } static unsigned long count_shadow_nodes(struct shrinker *shrinker, struct shrink_control *sc) { unsigned long max_nodes; unsigned long nodes; unsigned long cache; nodes = list_lru_shrink_count(&shadow_nodes, sc); /* * Approximate a reasonable limit for the radix tree nodes * containing shadow entries. We don't need to keep more * shadow entries than possible pages on the active list, * since refault distances bigger than that are dismissed. * * The size of the active list converges toward 100% of * overall page cache as memory grows, with only a tiny * inactive list. Assume the total cache size for that. * * Nodes might be sparsely populated, with only one shadow * entry in the extreme case. Obviously, we cannot keep one * node for every eligible shadow entry, so compromise on a * worst-case density of 1/8th. Below that, not all eligible * refaults can be detected anymore. * * On 64-bit with 7 radix_tree_nodes per page and 64 slots * each, this will reclaim shadow entries when they consume * ~1.8% of available memory: * * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE */ if (sc->memcg) { cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid, LRU_ALL_FILE); } else { cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) + node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE); } max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3); if (nodes <= max_nodes) return 0; return nodes - max_nodes; } static enum lru_status shadow_lru_isolate(struct list_head *item, struct list_lru_one *lru, 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 i_pages 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 i_pages 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 = container_of(node->root, struct address_space, i_pages); /* Coming from the list, invert the lock order */ if (!xa_trylock(&mapping->i_pages)) { spin_unlock_irq(lru_lock); ret = LRU_RETRY; goto out; } list_lru_isolate(lru, 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. */ if (WARN_ON_ONCE(!node->exceptional)) goto out_invalid; if (WARN_ON_ONCE(node->count != node->exceptional)) goto out_invalid; for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) { if (node->slots[i]) { if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i]))) goto out_invalid; if (WARN_ON_ONCE(!node->exceptional)) goto out_invalid; if (WARN_ON_ONCE(!mapping->nrexceptional)) goto out_invalid; node->slots[i] = NULL; node->exceptional--; node->count--; mapping->nrexceptional--; } } if (WARN_ON_ONCE(node->exceptional)) goto out_invalid; inc_lruvec_page_state(virt_to_page(node), WORKINGSET_NODERECLAIM); __radix_tree_delete_node(&mapping->i_pages, node, workingset_lookup_update(mapping)); out_invalid: xa_unlock_irq(&mapping->i_pages); ret = LRU_REMOVED_RETRY; out: cond_resched(); spin_lock_irq(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 the IRQ-safe i_pages lock */ local_irq_disable(); ret = list_lru_shrink_walk(&shadow_nodes, sc, shadow_lru_isolate, NULL); 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 | SHRINKER_MEMCG_AWARE, }; /* * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe * i_pages lock. */ static struct lock_class_key shadow_nodes_key; static int __init workingset_init(void) { unsigned int timestamp_bits; unsigned int max_order; int ret; BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT); /* * Calculate the eviction bucket size to cover the longest * actionable refault distance, which is currently half of * memory (totalram_pages/2). However, memory hotplug may add * some more pages at runtime, so keep working with up to * double the initial memory by using totalram_pages as-is. */ timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT; max_order = fls_long(totalram_pages - 1); if (max_order > timestamp_bits) bucket_order = max_order - timestamp_bits; pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n", timestamp_bits, max_order, bucket_order); ret = prealloc_shrinker(&workingset_shadow_shrinker); if (ret) goto err; ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key); if (ret) goto err_list_lru; register_shrinker_prepared(&workingset_shadow_shrinker); return 0; err_list_lru: free_prealloced_shrinker(&workingset_shadow_shrinker); err: return ret; } module_init(workingset_init);