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- /*
- * 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 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->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 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);
- rcu_read_unlock();
- /*
- * 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_node_state(pgdat, WORKINGSET_REFAULT);
- if (refault_distance <= active_file) {
- inc_node_state(pgdat, WORKINGSET_ACTIVATE);
- return true;
- }
- 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.
- */
- 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_shrink_count(&workingset_shadow_nodes, sc);
- local_irq_enable();
- if (sc->memcg) {
- pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
- LRU_ALL_FILE);
- } else {
- pages = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
- node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
- }
- /*
- * 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,
- 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 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_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.
- */
- BUG_ON(!workingset_node_shadows(node));
- BUG_ON(workingset_node_pages(node));
- 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;
- workingset_node_shadows_dec(node);
- BUG_ON(!mapping->nrexceptional);
- mapping->nrexceptional--;
- }
- }
- BUG_ON(workingset_node_shadows(node));
- inc_node_state(page_pgdat(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_shrink_walk(&workingset_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
- * mapping->tree_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 = __list_lru_init(&workingset_shadow_nodes, true, &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);
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