cregit-Linux how code gets into the kernel

Release 4.8 mm/workingset.c

Directory: mm
 * 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.

                         NODES_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); }


johannes weinerjohannes weiner5892.06%375.00%
mel gormanmel gorman57.94%125.00%

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; }


johannes weinerjohannes weiner9696.97%480.00%
mel gormanmel gorman33.03%120.00%

/** * 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); }


johannes weinerjohannes weiner10793.86%250.00%
mel gormanmel gorman76.14%250.00%

/** * 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); 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; }


johannes weinerjohannes weiner14593.55%360.00%
mel gormanmel gorman106.45%240.00%

/** * 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(); }


johannes weinerjohannes weiner6291.18%375.00%
mel gormanmel gorman68.82%125.00%

/* * 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 (memcg_kmem_enabled()) { 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; }


johannes weinerjohannes weiner8064.52%116.67%
vladimir davydovvladimir davydov3225.81%350.00%
mel gormanmel gorman129.68%233.33%

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; }


johannes weinerjohannes weiner24095.24%240.00%
vladimir davydovvladimir davydov83.17%120.00%
ross zwislerross zwisler20.79%120.00%
mel gormanmel gorman20.79%120.00%

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; }


johannes weinerjohannes weiner4497.78%150.00%
vladimir davydovvladimir davydov12.22%150.00%

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_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; }


johannes weinerjohannes weiner11499.13%266.67%
anton blanchardanton blanchard10.87%133.33%


Overall Contributors

johannes weinerjohannes weiner103091.64%844.44%
mel gormanmel gorman484.27%422.22%
vladimir davydovvladimir davydov433.83%422.22%
ross zwislerross zwisler20.18%15.56%
anton blanchardanton blanchard10.09%15.56%
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