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Diffstat (limited to 'mm/workingset.c')
| -rw-r--r-- | mm/workingset.c | 539 |
1 files changed, 539 insertions, 0 deletions
diff --git a/mm/workingset.c b/mm/workingset.c new file mode 100644 index 000000000..4516dd790 --- /dev/null +++ b/mm/workingset.c @@ -0,0 +1,539 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Workingset detection + * + * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner + */ + +#include <linux/memcontrol.h> +#include <linux/writeback.h> +#include <linux/shmem_fs.h> +#include <linux/pagemap.h> +#include <linux/atomic.h> +#include <linux/module.h> +#include <linux/swap.h> +#include <linux/dax.h> +#include <linux/fs.h> +#include <linux/mm.h> + +/* + * 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) + return SHRINK_EMPTY; + + 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) +{ + /* list_lru lock nests inside the IRQ-safe i_pages lock */ + return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate, + NULL); +} + +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, + &workingset_shadow_shrinker); + 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); |