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Diffstat (limited to '')
| -rw-r--r-- | mm/workingset.c | 629 |
1 files changed, 629 insertions, 0 deletions
diff --git a/mm/workingset.c b/mm/workingset.c new file mode 100644 index 000000000..975a4d2dd --- /dev/null +++ b/mm/workingset.c @@ -0,0 +1,629 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Workingset detection + * + * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner + */ + +#include <linux/memcontrol.h> +#include <linux/mm_inline.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. + * + * + * Refaulting inactive 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. + * + * That means if inactive cache is refaulting with a suitable refault + * distance, we assume the cache workingset is transitioning and put + * pressure on the current active list. + * + * 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. + * + * Refaulting active pages + * + * If on the other hand the refaulting pages have recently been + * deactivated, it means that the active list is no longer protecting + * actively used cache from reclaim. The cache is NOT transitioning to + * a different workingset; the existing workingset is thrashing in the + * space allocated to the page cache. + * + * + * Implementation + * + * For each node's LRU lists, a counter for inactive evictions and + * activations is maintained (node->nonresident_age). + * + * On eviction, a snapshot of this counter (along with some bits to + * identify the node) is stored in the now empty page cache + * 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 ((BITS_PER_LONG - BITS_PER_XA_VALUE) + \ + 1 + 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 xarray + * 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, + bool workingset) +{ + eviction >>= bucket_order; + eviction &= EVICTION_MASK; + eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid; + eviction = (eviction << NODES_SHIFT) | pgdat->node_id; + eviction = (eviction << 1) | workingset; + + return xa_mk_value(eviction); +} + +static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat, + unsigned long *evictionp, bool *workingsetp) +{ + unsigned long entry = xa_to_value(shadow); + int memcgid, nid; + bool workingset; + + workingset = entry & 1; + entry >>= 1; + 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; + *workingsetp = workingset; +} + +/** + * workingset_age_nonresident - age non-resident entries as LRU ages + * @lruvec: the lruvec that was aged + * @nr_pages: the number of pages to count + * + * As in-memory pages are aged, non-resident pages need to be aged as + * well, in order for the refault distances later on to be comparable + * to the in-memory dimensions. This function allows reclaim and LRU + * operations to drive the non-resident aging along in parallel. + */ +void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages) +{ + /* + * Reclaiming a cgroup means reclaiming all its children in a + * round-robin fashion. That means that each cgroup has an LRU + * order that is composed of the LRU orders of its child + * cgroups; and every page has an LRU position not just in the + * cgroup that owns it, but in all of that group's ancestors. + * + * So when the physical inactive list of a leaf cgroup ages, + * the virtual inactive lists of all its parents, including + * the root cgroup's, age as well. + */ + do { + atomic_long_add(nr_pages, &lruvec->nonresident_age); + } while ((lruvec = parent_lruvec(lruvec))); +} + +/** + * workingset_eviction - note the eviction of a page from memory + * @target_memcg: the cgroup that is causing the reclaim + * @page: the page being evicted + * + * Returns a shadow entry to be stored in @page->mapping->i_pages in place + * of the evicted @page so that a later refault can be detected. + */ +void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg) +{ + struct pglist_data *pgdat = page_pgdat(page); + unsigned long eviction; + struct lruvec *lruvec; + int memcgid; + + /* 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(target_memcg, pgdat); + workingset_age_nonresident(lruvec, thp_nr_pages(page)); + /* XXX: target_memcg can be NULL, go through lruvec */ + memcgid = mem_cgroup_id(lruvec_memcg(lruvec)); + eviction = atomic_long_read(&lruvec->nonresident_age); + return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page)); +} + +/** + * workingset_refault - evaluate the refault of a previously evicted page + * @page: the freshly allocated replacement 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 and the memcg whose memory + * pressure caused the eviction. + */ +void workingset_refault(struct page *page, void *shadow) +{ + bool file = page_is_file_lru(page); + struct mem_cgroup *eviction_memcg; + struct lruvec *eviction_lruvec; + unsigned long refault_distance; + unsigned long workingset_size; + struct pglist_data *pgdat; + struct mem_cgroup *memcg; + unsigned long eviction; + struct lruvec *lruvec; + unsigned long refault; + bool workingset; + int memcgid; + + unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset); + + 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. + */ + eviction_memcg = mem_cgroup_from_id(memcgid); + if (!mem_cgroup_disabled() && !eviction_memcg) + goto out; + eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat); + refault = atomic_long_read(&eviction_lruvec->nonresident_age); + + /* + * Calculate the refault distance + * + * The unsigned subtraction here gives an accurate distance + * across nonresident_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 + * nonresident_age to lap a shadow entry in the field, which + * can then 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; + + /* + * The activation decision for this page is made at the level + * where the eviction occurred, as that is where the LRU order + * during page reclaim is being determined. + * + * However, the cgroup that will own the page is the one that + * is actually experiencing the refault event. + */ + memcg = page_memcg(page); + lruvec = mem_cgroup_lruvec(memcg, pgdat); + + inc_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file); + + /* + * Compare the distance to the existing workingset size. We + * don't activate pages that couldn't stay resident even if + * all the memory was available to the workingset. Whether + * workingset competition needs to consider anon or not depends + * on having swap. + */ + workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE); + if (!file) { + workingset_size += lruvec_page_state(eviction_lruvec, + NR_INACTIVE_FILE); + } + if (mem_cgroup_get_nr_swap_pages(memcg) > 0) { + workingset_size += lruvec_page_state(eviction_lruvec, + NR_ACTIVE_ANON); + if (file) { + workingset_size += lruvec_page_state(eviction_lruvec, + NR_INACTIVE_ANON); + } + } + if (refault_distance > workingset_size) + goto out; + + SetPageActive(page); + workingset_age_nonresident(lruvec, thp_nr_pages(page)); + inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE_BASE + file); + + /* Page was active prior to eviction */ + if (workingset) { + SetPageWorkingset(page); + /* XXX: Move to lru_cache_add() when it supports new vs putback */ + spin_lock_irq(&page_pgdat(page)->lru_lock); + lru_note_cost_page(page); + spin_unlock_irq(&page_pgdat(page)->lru_lock); + inc_lruvec_state(lruvec, WORKINGSET_RESTORE_BASE + file); + } +out: + rcu_read_unlock(); +} + +/** + * 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_page_lruvec(page, page_pgdat(page)); + workingset_age_nonresident(lruvec, thp_nr_pages(page)); +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 xa_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. + */ + VM_WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */ + + if (node->count && node->count == node->nr_values) { + if (list_empty(&node->private_list)) { + list_lru_add(&shadow_nodes, &node->private_list); + __inc_lruvec_slab_state(node, WORKINGSET_NODES); + } + } else { + if (!list_empty(&node->private_list)) { + list_lru_del(&shadow_nodes, &node->private_list); + __dec_lruvec_slab_state(node, WORKINGSET_NODES); + } + } +} + +static unsigned long count_shadow_nodes(struct shrinker *shrinker, + struct shrink_control *sc) +{ + unsigned long max_nodes; + unsigned long nodes; + unsigned long pages; + + nodes = list_lru_shrink_count(&shadow_nodes, sc); + + /* + * Approximate a reasonable limit for the 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 xa_nodes per page and 64 slots + * each, this will reclaim shadow entries when they consume + * ~1.8% of available memory: + * + * PAGE_SIZE / xa_nodes / node_entries * 8 / PAGE_SIZE + */ +#ifdef CONFIG_MEMCG + if (sc->memcg) { + struct lruvec *lruvec; + int i; + + lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid)); + for (pages = 0, i = 0; i < NR_LRU_LISTS; i++) + pages += lruvec_page_state_local(lruvec, + NR_LRU_BASE + i); + pages += lruvec_page_state_local( + lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT; + pages += lruvec_page_state_local( + lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT; + } else +#endif + pages = node_present_pages(sc->nid); + + max_nodes = pages >> (XA_CHUNK_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) __must_hold(lru_lock) +{ + struct xa_node *node = container_of(item, struct xa_node, private_list); + struct address_space *mapping; + int ret; + + /* + * Page cache insertions and deletions synchronously 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 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. + */ + + mapping = container_of(node->array, 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); + __dec_lruvec_slab_state(node, WORKINGSET_NODES); + + 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->nr_values)) + goto out_invalid; + if (WARN_ON_ONCE(node->count != node->nr_values)) + goto out_invalid; + mapping->nrexceptional -= node->nr_values; + xa_delete_node(node, workingset_update_node); + __inc_lruvec_slab_state(node, WORKINGSET_NODERECLAIM); + +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 = 0, /* ->count reports only fully expendable nodes */ + .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); |