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 static void *pack_shadow(unsigned long eviction, struct zone *zone)
 156 {
 157         eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone);
 158         eviction = (eviction << ZONES_SHIFT) | zone_idx(zone);
 159         eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
 160
 161         return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
 162 }
 163
 164 static void unpack_shadow(void *shadow,
 165                           struct zone **zone,
 166                           unsigned long *distance)
 167 {
 168         unsigned long entry = (unsigned long)shadow;
 169         unsigned long eviction;
 170         unsigned long refault;
 171         unsigned long mask;
 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         eviction = entry;
 180
 181         *zone = NODE_DATA(nid)->node_zones + zid;
 182
 183         refault = atomic_long_read(&(*zone)->inactive_age);
 184         mask = ~0UL >> (NODES_SHIFT + ZONES_SHIFT +
 185                         RADIX_TREE_EXCEPTIONAL_SHIFT);
 186         /*
 187          * The unsigned subtraction here gives an accurate distance
 188          * across inactive_age overflows in most cases.
 189          *
 190          * There is a special case: usually, shadow entries have a
 191          * short lifetime and are either refaulted or reclaimed along
 192          * with the inode before they get too old.  But it is not
 193          * impossible for the inactive_age to lap a shadow entry in
 194          * the field, which can then can result in a false small
 195          * refault distance, leading to a false activation should this
 196          * old entry actually refault again.  However, earlier kernels
 197          * used to deactivate unconditionally with *every* reclaim
 198          * invocation for the longest time, so the occasional
 199          * inappropriate activation leading to pressure on the active
 200          * list is not a problem.
 201          */
 202         *distance = (refault - eviction) & mask;
 203 }
 204
 205 /**
 206  * workingset_eviction - note the eviction of a page from memory
 207  * @mapping: address space the page was backing
 208  * @page: the page being evicted
 209  *
 210  * Returns a shadow entry to be stored in @mapping->page_tree in place
 211  * of the evicted @page so that a later refault can be detected.
 212  */
 213 void *workingset_eviction(struct address_space *mapping, struct page *page)
 214 {
 215         struct zone *zone = page_zone(page);
 216         unsigned long eviction;
 217
 218         eviction = atomic_long_inc_return(&zone->inactive_age);
 219         return pack_shadow(eviction, zone);
 220 }
 221
 222 /**
 223  * workingset_refault - evaluate the refault of a previously evicted page
 224  * @shadow: shadow entry of the evicted page
 225  *
 226  * Calculates and evaluates the refault distance of the previously
 227  * evicted page in the context of the zone it was allocated in.
 228  *
 229  * Returns %true if the page should be activated, %false otherwise.
 230  */
 231 bool workingset_refault(void *shadow)
 232 {
 233         unsigned long refault_distance;
 234         struct zone *zone;
 235
 236         unpack_shadow(shadow, &zone, &refault_distance);
 237         inc_zone_state(zone, WORKINGSET_REFAULT);
 238
 239         if (refault_distance <= zone_page_state(zone, NR_ACTIVE_FILE)) {
 240                 inc_zone_state(zone, WORKINGSET_ACTIVATE);
 241                 return true;
 242         }
 243         return false;
 244 }
 245
 246 /**
 247  * workingset_activation - note a page activation
 248  * @page: page that is being activated
 249  */
 250 void workingset_activation(struct page *page)
 251 {
 252         atomic_long_inc(&page_zone(page)->inactive_age);
 253 }
 254
 255 /*
 256  * Shadow entries reflect the share of the working set that does not
 257  * fit into memory, so their number depends on the access pattern of
 258  * the workload.  In most cases, they will refault or get reclaimed
 259  * along with the inode, but a (malicious) workload that streams
 260  * through files with a total size several times that of available
 261  * memory, while preventing the inodes from being reclaimed, can
 262  * create excessive amounts of shadow nodes.  To keep a lid on this,
 263  * track shadow nodes and reclaim them when they grow way past the
 264  * point where they would still be useful.
 265  */
 266
 267 struct list_lru workingset_shadow_nodes;
 268
 269 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
 270                                         struct shrink_control *sc)
 271 {
 272         unsigned long shadow_nodes;
 273         unsigned long max_nodes;
 274         unsigned long pages;
 275
 276         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 277         local_irq_disable();
 278         shadow_nodes = list_lru_count_node(&workingset_shadow_nodes, sc->nid);
 279         local_irq_enable();
 280
 281         pages = node_present_pages(sc->nid);
 282         /*
 283          * Active cache pages are limited to 50% of memory, and shadow
 284          * entries that represent a refault distance bigger than that
 285          * do not have any effect.  Limit the number of shadow nodes
 286          * such that shadow entries do not exceed the number of active
 287          * cache pages, assuming a worst-case node population density
 288          * of 1/8th on average.
 289          *
 290          * On 64-bit with 7 radix_tree_nodes per page and 64 slots
 291          * each, this will reclaim shadow entries when they consume
 292          * ~2% of available memory:
 293          *
 294          * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
 295          */
 296         max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
 297
 298         if (shadow_nodes <= max_nodes)
 299                 return 0;
 300
 301         return shadow_nodes - max_nodes;
 302 }
 303
 304 static enum lru_status shadow_lru_isolate(struct list_head *item,
 305                                           spinlock_t *lru_lock,
 306                                           void *arg)
 307 {
 308         struct address_space *mapping;
 309         struct radix_tree_node *node;
 310         unsigned int i;
 311         int ret;
 312
 313         /*
 314          * Page cache insertions and deletions synchroneously maintain
 315          * the shadow node LRU under the mapping->tree_lock and the
 316          * lru_lock.  Because the page cache tree is emptied before
 317          * the inode can be destroyed, holding the lru_lock pins any
 318          * address_space that has radix tree nodes on the LRU.
 319          *
 320          * We can then safely transition to the mapping->tree_lock to
 321          * pin only the address_space of the particular node we want
 322          * to reclaim, take the node off-LRU, and drop the lru_lock.
 323          */
 324
 325         node = container_of(item, struct radix_tree_node, private_list);
 326         mapping = node->private_data;
 327
 328         /* Coming from the list, invert the lock order */
 329         if (!spin_trylock(&mapping->tree_lock)) {
 330                 spin_unlock(lru_lock);
 331                 ret = LRU_RETRY;
 332                 goto out;
 333         }
 334
 335         list_del_init(item);
 336         spin_unlock(lru_lock);
 337
 338         /*
 339          * The nodes should only contain one or more shadow entries,
 340          * no pages, so we expect to be able to remove them all and
 341          * delete and free the empty node afterwards.
 342          */
 343
 344         BUG_ON(!node->count);
 345         BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
 346
 347         for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 348                 if (node->slots[i]) {
 349                         BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
 350                         node->slots[i] = NULL;
 351                         BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
 352                         node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
 353                         BUG_ON(!mapping->nrshadows);
 354                         mapping->nrshadows--;
 355                 }
 356         }
 357         BUG_ON(node->count);
 358         inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
 359         if (!__radix_tree_delete_node(&mapping->page_tree, node))
 360                 BUG();
 361
 362         spin_unlock(&mapping->tree_lock);
 363         ret = LRU_REMOVED_RETRY;
 364 out:
 365         local_irq_enable();
 366         cond_resched();
 367         local_irq_disable();
 368         spin_lock(lru_lock);
 369         return ret;
 370 }
 371
 372 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
 373                                        struct shrink_control *sc)
 374 {
 375         unsigned long ret;
 376
 377         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 378         local_irq_disable();
 379         ret =  list_lru_walk_node(&workingset_shadow_nodes, sc->nid,
 380                                   shadow_lru_isolate, NULL, &sc->nr_to_scan);
 381         local_irq_enable();
 382         return ret;
 383 }
 384
 385 static struct shrinker workingset_shadow_shrinker = {
 386         .count_objects = count_shadow_nodes,
 387         .scan_objects = scan_shadow_nodes,
 388         .seeks = DEFAULT_SEEKS,
 389         .flags = SHRINKER_NUMA_AWARE,
 390 };
 391
 392 /*
 393  * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
 394  * mapping->tree_lock.
 395  */
 396 static struct lock_class_key shadow_nodes_key;
 397
 398 static int __init workingset_init(void)
 399 {
 400         int ret;
 401
 402         ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
 403         if (ret)
 404                 goto err;
 405         ret = register_shrinker(&workingset_shadow_shrinker);
 406         if (ret)
 407                 goto err_list_lru;
 408         return 0;
 409 err_list_lru:
 410         list_lru_destroy(&workingset_shadow_nodes);
 411 err:
 412         return ret;
 413 }
 414 module_init(workingset_init);