mm/workingset.c

   1 /*
   2  * Workingset detection
   3  *
   4  * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner
   5  */
   6
   7 #include <linux/memcontrol.h>
   8 #include <linux/writeback.h>
   9 #include <linux/pagemap.h>
  10 #include <linux/atomic.h>
  11 #include <linux/module.h>
  12 #include <linux/swap.h>
  13 #include <linux/fs.h>
  14 #include <linux/mm.h>
  15
  16 /*
  17  *              Double CLOCK lists
  18  *
  19  * Per zone, two clock lists are maintained for file pages: the
  20  * inactive and the active list.  Freshly faulted pages start out at
  21  * the head of the inactive list and page reclaim scans pages from the
  22  * tail.  Pages that are accessed multiple times on the inactive list
  23  * are promoted to the active list, to protect them from reclaim,
  24  * whereas active pages are demoted to the inactive list when the
  25  * active list grows too big.
  26  *
  27  *   fault ------------------------+
  28  *                                 |
  29  *              +--------------+   |            +-------------+
  30  *   reclaim <- |   inactive   | <-+-- demotion |    active   | <--+
  31  *              +--------------+                +-------------+    |
  32  *                     |                                           |
  33  *                     +-------------- promotion ------------------+
  34  *
  35  *
  36  *              Access frequency and refault distance
  37  *
  38  * A workload is thrashing when its pages are frequently used but they
  39  * are evicted from the inactive list every time before another access
  40  * would have promoted them to the active list.
  41  *
  42  * In cases where the average access distance between thrashing pages
  43  * is bigger than the size of memory there is nothing that can be
  44  * done - the thrashing set could never fit into memory under any
  45  * circumstance.
  46  *
  47  * However, the average access distance could be bigger than the
  48  * inactive list, yet smaller than the size of memory.  In this case,
  49  * the set could fit into memory if it weren't for the currently
  50  * active pages - which may be used more, hopefully less frequently:
  51  *
  52  *      +-memory available to cache-+
  53  *      |                           |
  54  *      +-inactive------+-active----+
  55  *  a b | c d e f g h i | J K L M N |
  56  *      +---------------+-----------+
  57  *
  58  * It is prohibitively expensive to accurately track access frequency
  59  * of pages.  But a reasonable approximation can be made to measure
  60  * thrashing on the inactive list, after which refaulting pages can be
  61  * activated optimistically to compete with the existing active pages.
  62  *
  63  * Approximating inactive page access frequency - Observations:
  64  *
  65  * 1. When a page is accessed for the first time, it is added to the
  66  *    head of the inactive list, slides every existing inactive page
  67  *    towards the tail by one slot, and pushes the current tail page
  68  *    out of memory.
  69  *
  70  * 2. When a page is accessed for the second time, it is promoted to
  71  *    the active list, shrinking the inactive list by one slot.  This
  72  *    also slides all inactive pages that were faulted into the cache
  73  *    more recently than the activated page towards the tail of the
  74  *    inactive list.
  75  *
  76  * Thus:
  77  *
  78  * 1. The sum of evictions and activations between any two points in
  79  *    time indicate the minimum number of inactive pages accessed in
  80  *    between.
  81  *
  82  * 2. Moving one inactive page N page slots towards the tail of the
  83  *    list requires at least N inactive page accesses.
  84  *
  85  * Combining these:
  86  *
  87  * 1. When a page is finally evicted from memory, the number of
  88  *    inactive pages accessed while the page was in cache is at least
  89  *    the number of page slots on the inactive list.
  90  *
  91  * 2. In addition, measuring the sum of evictions and activations (E)
  92  *    at the time of a page's eviction, and comparing it to another
  93  *    reading (R) at the time the page faults back into memory tells
  94  *    the minimum number of accesses while the page was not cached.
  95  *    This is called the refault distance.
  96  *
  97  * Because the first access of the page was the fault and the second
  98  * access the refault, we combine the in-cache distance with the
  99  * out-of-cache distance to get the complete minimum access distance
 100  * of this page:
 101  *
 102  *      NR_inactive + (R - E)
 103  *
 104  * And knowing the minimum access distance of a page, we can easily
 105  * tell if the page would be able to stay in cache assuming all page
 106  * slots in the cache were available:
 107  *
 108  *   NR_inactive + (R - E) <= NR_inactive + NR_active
 109  *
 110  * which can be further simplified to
 111  *
 112  *   (R - E) <= NR_active
 113  *
 114  * Put into words, the refault distance (out-of-cache) can be seen as
 115  * a deficit in inactive list space (in-cache).  If the inactive list
 116  * had (R - E) more page slots, the page would not have been evicted
 117  * in between accesses, but activated instead.  And on a full system,
 118  * the only thing eating into inactive list space is active pages.
 119  *
 120  *
 121  *              Activating refaulting pages
 122  *
 123  * All that is known about the active list is that the pages have been
 124  * accessed more than once in the past.  This means that at any given
 125  * time there is actually a good chance that pages on the active list
 126  * are no longer in active use.
 127  *
 128  * So when a refault distance of (R - E) is observed and there are at
 129  * least (R - E) active pages, the refaulting page is activated
 130  * optimistically in the hope that (R - E) active pages are actually
 131  * used less frequently than the refaulting page - or even not used at
 132  * all anymore.
 133  *
 134  * If this is wrong and demotion kicks in, the pages which are truly
 135  * used more frequently will be reactivated while the less frequently
 136  * used once will be evicted from memory.
 137  *
 138  * But if this is right, the stale pages will be pushed out of memory
 139  * and the used pages get to stay in cache.
 140  *
 141  *
 142  *              Implementation
 143  *
 144  * For each zone's file LRU lists, a counter for inactive evictions
 145  * and activations is maintained (zone->inactive_age).
 146  *
 147  * On eviction, a snapshot of this counter (along with some bits to
 148  * identify the zone) is stored in the now empty page cache radix tree
 149  * slot of the evicted page.  This is called a shadow entry.
 150  *
 151  * On cache misses for which there are shadow entries, an eligible
 152  * refault distance will immediately activate the refaulting page.
 153  */
 154
 155 #define EVICTION_SHIFT  (RADIX_TREE_EXCEPTIONAL_ENTRY + \
 156                          ZONES_SHIFT + NODES_SHIFT)
 157 #define EVICTION_MASK   (~0UL >> EVICTION_SHIFT)
 158
 159 static void *pack_shadow(unsigned long eviction, struct zone *zone)
 160 {
 161         eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone);
 162         eviction = (eviction << ZONES_SHIFT) | zone_idx(zone);
 163         eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
 164
 165         return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
 166 }
 167
 168 static void unpack_shadow(void *shadow, struct zone **zonep,
 169                           unsigned long *evictionp)
 170 {
 171         unsigned long entry = (unsigned long)shadow;
 172         int zid, nid;
 173
 174         entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
 175         zid = entry & ((1UL << ZONES_SHIFT) - 1);
 176         entry >>= ZONES_SHIFT;
 177         nid = entry & ((1UL << NODES_SHIFT) - 1);
 178         entry >>= NODES_SHIFT;
 179
 180         *zonep = NODE_DATA(nid)->node_zones + zid;
 181         *evictionp = entry;
 182 }
 183
 184 /**
 185  * workingset_eviction - note the eviction of a page from memory
 186  * @mapping: address space the page was backing
 187  * @page: the page being evicted
 188  *
 189  * Returns a shadow entry to be stored in @mapping->page_tree in place
 190  * of the evicted @page so that a later refault can be detected.
 191  */
 192 void *workingset_eviction(struct address_space *mapping, struct page *page)
 193 {
 194         struct zone *zone = page_zone(page);
 195         unsigned long eviction;
 196
 197         eviction = atomic_long_inc_return(&zone->inactive_age);
 198         return pack_shadow(eviction, zone);
 199 }
 200
 201 /**
 202  * workingset_refault - evaluate the refault of a previously evicted page
 203  * @shadow: shadow entry of the evicted page
 204  *
 205  * Calculates and evaluates the refault distance of the previously
 206  * evicted page in the context of the zone it was allocated in.
 207  *
 208  * Returns %true if the page should be activated, %false otherwise.
 209  */
 210 bool workingset_refault(void *shadow)
 211 {
 212         unsigned long refault_distance;
 213         unsigned long eviction;
 214         unsigned long refault;
 215         struct zone *zone;
 216
 217         unpack_shadow(shadow, &zone, &eviction);
 218
 219         refault = atomic_long_read(&zone->inactive_age);
 220
 221         /*
 222          * The unsigned subtraction here gives an accurate distance
 223          * across inactive_age overflows in most cases.
 224          *
 225          * There is a special case: usually, shadow entries have a
 226          * short lifetime and are either refaulted or reclaimed along
 227          * with the inode before they get too old.  But it is not
 228          * impossible for the inactive_age to lap a shadow entry in
 229          * the field, which can then can result in a false small
 230          * refault distance, leading to a false activation should this
 231          * old entry actually refault again.  However, earlier kernels
 232          * used to deactivate unconditionally with *every* reclaim
 233          * invocation for the longest time, so the occasional
 234          * inappropriate activation leading to pressure on the active
 235          * list is not a problem.
 236          */
 237         refault_distance = (refault - eviction) & EVICTION_MASK;
 238
 239         inc_zone_state(zone, WORKINGSET_REFAULT);
 240
 241         if (refault_distance <= zone_page_state(zone, NR_ACTIVE_FILE)) {
 242                 inc_zone_state(zone, WORKINGSET_ACTIVATE);
 243                 return true;
 244         }
 245         return false;
 246 }
 247
 248 /**
 249  * workingset_activation - note a page activation
 250  * @page: page that is being activated
 251  */
 252 void workingset_activation(struct page *page)
 253 {
 254         atomic_long_inc(&page_zone(page)->inactive_age);
 255 }
 256
 257 /*
 258  * Shadow entries reflect the share of the working set that does not
 259  * fit into memory, so their number depends on the access pattern of
 260  * the workload.  In most cases, they will refault or get reclaimed
 261  * along with the inode, but a (malicious) workload that streams
 262  * through files with a total size several times that of available
 263  * memory, while preventing the inodes from being reclaimed, can
 264  * create excessive amounts of shadow nodes.  To keep a lid on this,
 265  * track shadow nodes and reclaim them when they grow way past the
 266  * point where they would still be useful.
 267  */
 268
 269 struct list_lru workingset_shadow_nodes;
 270
 271 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
 272                                         struct shrink_control *sc)
 273 {
 274         unsigned long shadow_nodes;
 275         unsigned long max_nodes;
 276         unsigned long pages;
 277
 278         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 279         local_irq_disable();
 280         shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
 281         local_irq_enable();
 282
 283         pages = node_present_pages(sc->nid);
 284         /*
 285          * Active cache pages are limited to 50% of memory, and shadow
 286          * entries that represent a refault distance bigger than that
 287          * do not have any effect.  Limit the number of shadow nodes
 288          * such that shadow entries do not exceed the number of active
 289          * cache pages, assuming a worst-case node population density
 290          * of 1/8th on average.
 291          *
 292          * On 64-bit with 7 radix_tree_nodes per page and 64 slots
 293          * each, this will reclaim shadow entries when they consume
 294          * ~2% of available memory:
 295          *
 296          * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
 297          */
 298         max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
 299
 300         if (shadow_nodes <= max_nodes)
 301                 return 0;
 302
 303         return shadow_nodes - max_nodes;
 304 }
 305
 306 static enum lru_status shadow_lru_isolate(struct list_head *item,
 307                                           struct list_lru_one *lru,
 308                                           spinlock_t *lru_lock,
 309                                           void *arg)
 310 {
 311         struct address_space *mapping;
 312         struct radix_tree_node *node;
 313         unsigned int i;
 314         int ret;
 315
 316         /*
 317          * Page cache insertions and deletions synchroneously maintain
 318          * the shadow node LRU under the mapping->tree_lock and the
 319          * lru_lock.  Because the page cache tree is emptied before
 320          * the inode can be destroyed, holding the lru_lock pins any
 321          * address_space that has radix tree nodes on the LRU.
 322          *
 323          * We can then safely transition to the mapping->tree_lock to
 324          * pin only the address_space of the particular node we want
 325          * to reclaim, take the node off-LRU, and drop the lru_lock.
 326          */
 327
 328         node = container_of(item, struct radix_tree_node, private_list);
 329         mapping = node->private_data;
 330
 331         /* Coming from the list, invert the lock order */
 332         if (!spin_trylock(&mapping->tree_lock)) {
 333                 spin_unlock(lru_lock);
 334                 ret = LRU_RETRY;
 335                 goto out;
 336         }
 337
 338         list_lru_isolate(lru, item);
 339         spin_unlock(lru_lock);
 340
 341         /*
 342          * The nodes should only contain one or more shadow entries,
 343          * no pages, so we expect to be able to remove them all and
 344          * delete and free the empty node afterwards.
 345          */
 346
 347         BUG_ON(!node->count);
 348         BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
 349
 350         for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 351                 if (node->slots[i]) {
 352                         BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
 353                         node->slots[i] = NULL;
 354                         BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
 355                         node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
 356                         BUG_ON(!mapping->nrexceptional);
 357                         mapping->nrexceptional--;
 358                 }
 359         }
 360         BUG_ON(node->count);
 361         inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
 362         if (!__radix_tree_delete_node(&mapping->page_tree, node))
 363                 BUG();
 364
 365         spin_unlock(&mapping->tree_lock);
 366         ret = LRU_REMOVED_RETRY;
 367 out:
 368         local_irq_enable();
 369         cond_resched();
 370         local_irq_disable();
 371         spin_lock(lru_lock);
 372         return ret;
 373 }
 374
 375 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
 376                                        struct shrink_control *sc)
 377 {
 378         unsigned long ret;
 379
 380         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 381         local_irq_disable();
 382         ret =  list_lru_shrink_walk(&workingset_shadow_nodes, sc,
 383                                     shadow_lru_isolate, NULL);
 384         local_irq_enable();
 385         return ret;
 386 }
 387
 388 static struct shrinker workingset_shadow_shrinker = {
 389         .count_objects = count_shadow_nodes,
 390         .scan_objects = scan_shadow_nodes,
 391         .seeks = DEFAULT_SEEKS,
 392         .flags = SHRINKER_NUMA_AWARE,
 393 };
 394
 395 /*
 396  * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
 397  * mapping->tree_lock.
 398  */
 399 static struct lock_class_key shadow_nodes_key;
 400
 401 static int __init workingset_init(void)
 402 {
 403         int ret;
 404
 405         ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
 406         if (ret)
 407                 goto err;
 408         ret = register_shrinker(&workingset_shadow_shrinker);
 409         if (ret)
 410                 goto err_list_lru;
 411         return 0;
 412 err_list_lru:
 413         list_lru_destroy(&workingset_shadow_nodes);
 414 err:
 415         return ret;
 416 }
 417 module_init(workingset_init);