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 node, 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 node's file LRU lists, a counter for inactive evictions
 145  * and activations is maintained (node->inactive_age).
 146  *
 147  * On eviction, a snapshot of this counter (along with some bits to
 148  * identify the node) 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                          NODES_SHIFT +  \
 157                          MEM_CGROUP_ID_SHIFT)
 158 #define EVICTION_MASK   (~0UL >> EVICTION_SHIFT)
 159
 160 /*
 161  * Eviction timestamps need to be able to cover the full range of
 162  * actionable refaults. However, bits are tight in the radix tree
 163  * entry, and after storing the identifier for the lruvec there might
 164  * not be enough left to represent every single actionable refault. In
 165  * that case, we have to sacrifice granularity for distance, and group
 166  * evictions into coarser buckets by shaving off lower timestamp bits.
 167  */
 168 static unsigned int bucket_order __read_mostly;
 169
 170 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
 171 {
 172         eviction >>= bucket_order;
 173         eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
 174         eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
 175         eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
 176
 177         return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
 178 }
 179
 180 static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
 181                           unsigned long *evictionp)
 182 {
 183         unsigned long entry = (unsigned long)shadow;
 184         int memcgid, nid;
 185
 186         entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
 187         nid = entry & ((1UL << NODES_SHIFT) - 1);
 188         entry >>= NODES_SHIFT;
 189         memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
 190         entry >>= MEM_CGROUP_ID_SHIFT;
 191
 192         *memcgidp = memcgid;
 193         *pgdat = NODE_DATA(nid);
 194         *evictionp = entry << bucket_order;
 195 }
 196
 197 /**
 198  * workingset_eviction - note the eviction of a page from memory
 199  * @mapping: address space the page was backing
 200  * @page: the page being evicted
 201  *
 202  * Returns a shadow entry to be stored in @mapping->page_tree in place
 203  * of the evicted @page so that a later refault can be detected.
 204  */
 205 void *workingset_eviction(struct address_space *mapping, struct page *page)
 206 {
 207         struct mem_cgroup *memcg = page_memcg(page);
 208         struct pglist_data *pgdat = page_pgdat(page);
 209         int memcgid = mem_cgroup_id(memcg);
 210         unsigned long eviction;
 211         struct lruvec *lruvec;
 212
 213         /* Page is fully exclusive and pins page->mem_cgroup */
 214         VM_BUG_ON_PAGE(PageLRU(page), page);
 215         VM_BUG_ON_PAGE(page_count(page), page);
 216         VM_BUG_ON_PAGE(!PageLocked(page), page);
 217
 218         lruvec = mem_cgroup_lruvec(pgdat, memcg);
 219         eviction = atomic_long_inc_return(&lruvec->inactive_age);
 220         return pack_shadow(memcgid, pgdat, eviction);
 221 }
 222
 223 /**
 224  * workingset_refault - evaluate the refault of a previously evicted page
 225  * @shadow: shadow entry of the evicted page
 226  *
 227  * Calculates and evaluates the refault distance of the previously
 228  * evicted page in the context of the node it was allocated in.
 229  *
 230  * Returns %true if the page should be activated, %false otherwise.
 231  */
 232 bool workingset_refault(void *shadow)
 233 {
 234         unsigned long refault_distance;
 235         unsigned long active_file;
 236         struct mem_cgroup *memcg;
 237         unsigned long eviction;
 238         struct lruvec *lruvec;
 239         unsigned long refault;
 240         struct pglist_data *pgdat;
 241         int memcgid;
 242
 243         unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
 244
 245         rcu_read_lock();
 246         /*
 247          * Look up the memcg associated with the stored ID. It might
 248          * have been deleted since the page's eviction.
 249          *
 250          * Note that in rare events the ID could have been recycled
 251          * for a new cgroup that refaults a shared page. This is
 252          * impossible to tell from the available data. However, this
 253          * should be a rare and limited disturbance, and activations
 254          * are always speculative anyway. Ultimately, it's the aging
 255          * algorithm's job to shake out the minimum access frequency
 256          * for the active cache.
 257          *
 258          * XXX: On !CONFIG_MEMCG, this will always return NULL; it
 259          * would be better if the root_mem_cgroup existed in all
 260          * configurations instead.
 261          */
 262         memcg = mem_cgroup_from_id(memcgid);
 263         if (!mem_cgroup_disabled() && !memcg) {
 264                 rcu_read_unlock();
 265                 return false;
 266         }
 267         lruvec = mem_cgroup_lruvec(pgdat, memcg);
 268         refault = atomic_long_read(&lruvec->inactive_age);
 269         active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE);
 270         rcu_read_unlock();
 271
 272         /*
 273          * The unsigned subtraction here gives an accurate distance
 274          * across inactive_age overflows in most cases.
 275          *
 276          * There is a special case: usually, shadow entries have a
 277          * short lifetime and are either refaulted or reclaimed along
 278          * with the inode before they get too old.  But it is not
 279          * impossible for the inactive_age to lap a shadow entry in
 280          * the field, which can then can result in a false small
 281          * refault distance, leading to a false activation should this
 282          * old entry actually refault again.  However, earlier kernels
 283          * used to deactivate unconditionally with *every* reclaim
 284          * invocation for the longest time, so the occasional
 285          * inappropriate activation leading to pressure on the active
 286          * list is not a problem.
 287          */
 288         refault_distance = (refault - eviction) & EVICTION_MASK;
 289
 290         inc_node_state(pgdat, WORKINGSET_REFAULT);
 291
 292         if (refault_distance <= active_file) {
 293                 inc_node_state(pgdat, WORKINGSET_ACTIVATE);
 294                 return true;
 295         }
 296         return false;
 297 }
 298
 299 /**
 300  * workingset_activation - note a page activation
 301  * @page: page that is being activated
 302  */
 303 void workingset_activation(struct page *page)
 304 {
 305         struct mem_cgroup *memcg;
 306         struct lruvec *lruvec;
 307
 308         rcu_read_lock();
 309         /*
 310          * Filter non-memcg pages here, e.g. unmap can call
 311          * mark_page_accessed() on VDSO pages.
 312          *
 313          * XXX: See workingset_refault() - this should return
 314          * root_mem_cgroup even for !CONFIG_MEMCG.
 315          */
 316         memcg = page_memcg_rcu(page);
 317         if (!mem_cgroup_disabled() && !memcg)
 318                 goto out;
 319         lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
 320         atomic_long_inc(&lruvec->inactive_age);
 321 out:
 322         rcu_read_unlock();
 323 }
 324
 325 /*
 326  * Shadow entries reflect the share of the working set that does not
 327  * fit into memory, so their number depends on the access pattern of
 328  * the workload.  In most cases, they will refault or get reclaimed
 329  * along with the inode, but a (malicious) workload that streams
 330  * through files with a total size several times that of available
 331  * memory, while preventing the inodes from being reclaimed, can
 332  * create excessive amounts of shadow nodes.  To keep a lid on this,
 333  * track shadow nodes and reclaim them when they grow way past the
 334  * point where they would still be useful.
 335  */
 336
 337 struct list_lru workingset_shadow_nodes;
 338
 339 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
 340                                         struct shrink_control *sc)
 341 {
 342         unsigned long shadow_nodes;
 343         unsigned long max_nodes;
 344         unsigned long pages;
 345
 346         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 347         local_irq_disable();
 348         shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
 349         local_irq_enable();
 350
 351         if (memcg_kmem_enabled()) {
 352                 pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
 353                                                      LRU_ALL_FILE);
 354         } else {
 355                 pages = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
 356                         node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
 357         }
 358
 359         /*
 360          * Active cache pages are limited to 50% of memory, and shadow
 361          * entries that represent a refault distance bigger than that
 362          * do not have any effect.  Limit the number of shadow nodes
 363          * such that shadow entries do not exceed the number of active
 364          * cache pages, assuming a worst-case node population density
 365          * of 1/8th on average.
 366          *
 367          * On 64-bit with 7 radix_tree_nodes per page and 64 slots
 368          * each, this will reclaim shadow entries when they consume
 369          * ~2% of available memory:
 370          *
 371          * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
 372          */
 373         max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
 374
 375         if (shadow_nodes <= max_nodes)
 376                 return 0;
 377
 378         return shadow_nodes - max_nodes;
 379 }
 380
 381 static enum lru_status shadow_lru_isolate(struct list_head *item,
 382                                           struct list_lru_one *lru,
 383                                           spinlock_t *lru_lock,
 384                                           void *arg)
 385 {
 386         struct address_space *mapping;
 387         struct radix_tree_node *node;
 388         unsigned int i;
 389         int ret;
 390
 391         /*
 392          * Page cache insertions and deletions synchroneously maintain
 393          * the shadow node LRU under the mapping->tree_lock and the
 394          * lru_lock.  Because the page cache tree is emptied before
 395          * the inode can be destroyed, holding the lru_lock pins any
 396          * address_space that has radix tree nodes on the LRU.
 397          *
 398          * We can then safely transition to the mapping->tree_lock to
 399          * pin only the address_space of the particular node we want
 400          * to reclaim, take the node off-LRU, and drop the lru_lock.
 401          */
 402
 403         node = container_of(item, struct radix_tree_node, private_list);
 404         mapping = node->private_data;
 405
 406         /* Coming from the list, invert the lock order */
 407         if (!spin_trylock(&mapping->tree_lock)) {
 408                 spin_unlock(lru_lock);
 409                 ret = LRU_RETRY;
 410                 goto out;
 411         }
 412
 413         list_lru_isolate(lru, item);
 414         spin_unlock(lru_lock);
 415
 416         /*
 417          * The nodes should only contain one or more shadow entries,
 418          * no pages, so we expect to be able to remove them all and
 419          * delete and free the empty node afterwards.
 420          */
 421
 422         BUG_ON(!node->count);
 423         BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
 424
 425         for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 426                 if (node->slots[i]) {
 427                         BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
 428                         node->slots[i] = NULL;
 429                         BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
 430                         node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
 431                         BUG_ON(!mapping->nrexceptional);
 432                         mapping->nrexceptional--;
 433                 }
 434         }
 435         BUG_ON(node->count);
 436         inc_node_state(page_pgdat(virt_to_page(node)), WORKINGSET_NODERECLAIM);
 437         if (!__radix_tree_delete_node(&mapping->page_tree, node))
 438                 BUG();
 439
 440         spin_unlock(&mapping->tree_lock);
 441         ret = LRU_REMOVED_RETRY;
 442 out:
 443         local_irq_enable();
 444         cond_resched();
 445         local_irq_disable();
 446         spin_lock(lru_lock);
 447         return ret;
 448 }
 449
 450 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
 451                                        struct shrink_control *sc)
 452 {
 453         unsigned long ret;
 454
 455         /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
 456         local_irq_disable();
 457         ret =  list_lru_shrink_walk(&workingset_shadow_nodes, sc,
 458                                     shadow_lru_isolate, NULL);
 459         local_irq_enable();
 460         return ret;
 461 }
 462
 463 static struct shrinker workingset_shadow_shrinker = {
 464         .count_objects = count_shadow_nodes,
 465         .scan_objects = scan_shadow_nodes,
 466         .seeks = DEFAULT_SEEKS,
 467         .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
 468 };
 469
 470 /*
 471  * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
 472  * mapping->tree_lock.
 473  */
 474 static struct lock_class_key shadow_nodes_key;
 475
 476 static int __init workingset_init(void)
 477 {
 478         unsigned int timestamp_bits;
 479         unsigned int max_order;
 480         int ret;
 481
 482         BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
 483         /*
 484          * Calculate the eviction bucket size to cover the longest
 485          * actionable refault distance, which is currently half of
 486          * memory (totalram_pages/2). However, memory hotplug may add
 487          * some more pages at runtime, so keep working with up to
 488          * double the initial memory by using totalram_pages as-is.
 489          */
 490         timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
 491         max_order = fls_long(totalram_pages - 1);
 492         if (max_order > timestamp_bits)
 493                 bucket_order = max_order - timestamp_bits;
 494         pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
 495                timestamp_bits, max_order, bucket_order);
 496
 497         ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
 498         if (ret)
 499                 goto err;
 500         ret = register_shrinker(&workingset_shadow_shrinker);
 501         if (ret)
 502                 goto err_list_lru;
 503         return 0;
 504 err_list_lru:
 505         list_lru_destroy(&workingset_shadow_nodes);
 506 err:
 507         return ret;
 508 }
 509 module_init(workingset_init);