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/shmem_fs.h> 10 #include <linux/pagemap.h> 11 #include <linux/atomic.h> 12 #include <linux/module.h> 13 #include <linux/swap.h> 14 #include <linux/dax.h> 15 #include <linux/fs.h> 16 #include <linux/mm.h> 17 18 /* 19 * Double CLOCK lists 20 * 21 * Per node, two clock lists are maintained for file pages: the 22 * inactive and the active list. Freshly faulted pages start out at 23 * the head of the inactive list and page reclaim scans pages from the 24 * tail. Pages that are accessed multiple times on the inactive list 25 * are promoted to the active list, to protect them from reclaim, 26 * whereas active pages are demoted to the inactive list when the 27 * active list grows too big. 28 * 29 * fault ------------------------+ 30 * | 31 * +--------------+ | +-------------+ 32 * reclaim <- | inactive | <-+-- demotion | active | <--+ 33 * +--------------+ +-------------+ | 34 * | | 35 * +-------------- promotion ------------------+ 36 * 37 * 38 * Access frequency and refault distance 39 * 40 * A workload is thrashing when its pages are frequently used but they 41 * are evicted from the inactive list every time before another access 42 * would have promoted them to the active list. 43 * 44 * In cases where the average access distance between thrashing pages 45 * is bigger than the size of memory there is nothing that can be 46 * done - the thrashing set could never fit into memory under any 47 * circumstance. 48 * 49 * However, the average access distance could be bigger than the 50 * inactive list, yet smaller than the size of memory. In this case, 51 * the set could fit into memory if it weren't for the currently 52 * active pages - which may be used more, hopefully less frequently: 53 * 54 * +-memory available to cache-+ 55 * | | 56 * +-inactive------+-active----+ 57 * a b | c d e f g h i | J K L M N | 58 * +---------------+-----------+ 59 * 60 * It is prohibitively expensive to accurately track access frequency 61 * of pages. But a reasonable approximation can be made to measure 62 * thrashing on the inactive list, after which refaulting pages can be 63 * activated optimistically to compete with the existing active pages. 64 * 65 * Approximating inactive page access frequency - Observations: 66 * 67 * 1. When a page is accessed for the first time, it is added to the 68 * head of the inactive list, slides every existing inactive page 69 * towards the tail by one slot, and pushes the current tail page 70 * out of memory. 71 * 72 * 2. When a page is accessed for the second time, it is promoted to 73 * the active list, shrinking the inactive list by one slot. This 74 * also slides all inactive pages that were faulted into the cache 75 * more recently than the activated page towards the tail of the 76 * inactive list. 77 * 78 * Thus: 79 * 80 * 1. The sum of evictions and activations between any two points in 81 * time indicate the minimum number of inactive pages accessed in 82 * between. 83 * 84 * 2. Moving one inactive page N page slots towards the tail of the 85 * list requires at least N inactive page accesses. 86 * 87 * Combining these: 88 * 89 * 1. When a page is finally evicted from memory, the number of 90 * inactive pages accessed while the page was in cache is at least 91 * the number of page slots on the inactive list. 92 * 93 * 2. In addition, measuring the sum of evictions and activations (E) 94 * at the time of a page's eviction, and comparing it to another 95 * reading (R) at the time the page faults back into memory tells 96 * the minimum number of accesses while the page was not cached. 97 * This is called the refault distance. 98 * 99 * Because the first access of the page was the fault and the second 100 * access the refault, we combine the in-cache distance with the 101 * out-of-cache distance to get the complete minimum access distance 102 * of this page: 103 * 104 * NR_inactive + (R - E) 105 * 106 * And knowing the minimum access distance of a page, we can easily 107 * tell if the page would be able to stay in cache assuming all page 108 * slots in the cache were available: 109 * 110 * NR_inactive + (R - E) <= NR_inactive + NR_active 111 * 112 * which can be further simplified to 113 * 114 * (R - E) <= NR_active 115 * 116 * Put into words, the refault distance (out-of-cache) can be seen as 117 * a deficit in inactive list space (in-cache). If the inactive list 118 * had (R - E) more page slots, the page would not have been evicted 119 * in between accesses, but activated instead. And on a full system, 120 * the only thing eating into inactive list space is active pages. 121 * 122 * 123 * Activating refaulting pages 124 * 125 * All that is known about the active list is that the pages have been 126 * accessed more than once in the past. This means that at any given 127 * time there is actually a good chance that pages on the active list 128 * are no longer in active use. 129 * 130 * So when a refault distance of (R - E) is observed and there are at 131 * least (R - E) active pages, the refaulting page is activated 132 * optimistically in the hope that (R - E) active pages are actually 133 * used less frequently than the refaulting page - or even not used at 134 * all anymore. 135 * 136 * If this is wrong and demotion kicks in, the pages which are truly 137 * used more frequently will be reactivated while the less frequently 138 * used once will be evicted from memory. 139 * 140 * But if this is right, the stale pages will be pushed out of memory 141 * and the used pages get to stay in cache. 142 * 143 * 144 * Implementation 145 * 146 * For each node's file LRU lists, a counter for inactive evictions 147 * and activations is maintained (node->inactive_age). 148 * 149 * On eviction, a snapshot of this counter (along with some bits to 150 * identify the node) is stored in the now empty page cache radix tree 151 * slot of the evicted page. This is called a shadow entry. 152 * 153 * On cache misses for which there are shadow entries, an eligible 154 * refault distance will immediately activate the refaulting page. 155 */ 156 157 #define EVICTION_SHIFT (RADIX_TREE_EXCEPTIONAL_ENTRY + \ 158 NODES_SHIFT + \ 159 MEM_CGROUP_ID_SHIFT) 160 #define EVICTION_MASK (~0UL >> EVICTION_SHIFT) 161 162 /* 163 * Eviction timestamps need to be able to cover the full range of 164 * actionable refaults. However, bits are tight in the radix tree 165 * entry, and after storing the identifier for the lruvec there might 166 * not be enough left to represent every single actionable refault. In 167 * that case, we have to sacrifice granularity for distance, and group 168 * evictions into coarser buckets by shaving off lower timestamp bits. 169 */ 170 static unsigned int bucket_order __read_mostly; 171 172 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction) 173 { 174 eviction >>= bucket_order; 175 eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid; 176 eviction = (eviction << NODES_SHIFT) | pgdat->node_id; 177 eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT); 178 179 return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY); 180 } 181 182 static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat, 183 unsigned long *evictionp) 184 { 185 unsigned long entry = (unsigned long)shadow; 186 int memcgid, nid; 187 188 entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT; 189 nid = entry & ((1UL << NODES_SHIFT) - 1); 190 entry >>= NODES_SHIFT; 191 memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1); 192 entry >>= MEM_CGROUP_ID_SHIFT; 193 194 *memcgidp = memcgid; 195 *pgdat = NODE_DATA(nid); 196 *evictionp = entry << bucket_order; 197 } 198 199 /** 200 * workingset_eviction - note the eviction of a page from memory 201 * @mapping: address space the page was backing 202 * @page: the page being evicted 203 * 204 * Returns a shadow entry to be stored in @mapping->page_tree in place 205 * of the evicted @page so that a later refault can be detected. 206 */ 207 void *workingset_eviction(struct address_space *mapping, struct page *page) 208 { 209 struct mem_cgroup *memcg = page_memcg(page); 210 struct pglist_data *pgdat = page_pgdat(page); 211 int memcgid = mem_cgroup_id(memcg); 212 unsigned long eviction; 213 struct lruvec *lruvec; 214 215 /* Page is fully exclusive and pins page->mem_cgroup */ 216 VM_BUG_ON_PAGE(PageLRU(page), page); 217 VM_BUG_ON_PAGE(page_count(page), page); 218 VM_BUG_ON_PAGE(!PageLocked(page), page); 219 220 lruvec = mem_cgroup_lruvec(pgdat, memcg); 221 eviction = atomic_long_inc_return(&lruvec->inactive_age); 222 return pack_shadow(memcgid, pgdat, eviction); 223 } 224 225 /** 226 * workingset_refault - evaluate the refault of a previously evicted page 227 * @shadow: shadow entry of the evicted page 228 * 229 * Calculates and evaluates the refault distance of the previously 230 * evicted page in the context of the node it was allocated in. 231 * 232 * Returns %true if the page should be activated, %false otherwise. 233 */ 234 bool workingset_refault(void *shadow) 235 { 236 unsigned long refault_distance; 237 unsigned long active_file; 238 struct mem_cgroup *memcg; 239 unsigned long eviction; 240 struct lruvec *lruvec; 241 unsigned long refault; 242 struct pglist_data *pgdat; 243 int memcgid; 244 245 unpack_shadow(shadow, &memcgid, &pgdat, &eviction); 246 247 rcu_read_lock(); 248 /* 249 * Look up the memcg associated with the stored ID. It might 250 * have been deleted since the page's eviction. 251 * 252 * Note that in rare events the ID could have been recycled 253 * for a new cgroup that refaults a shared page. This is 254 * impossible to tell from the available data. However, this 255 * should be a rare and limited disturbance, and activations 256 * are always speculative anyway. Ultimately, it's the aging 257 * algorithm's job to shake out the minimum access frequency 258 * for the active cache. 259 * 260 * XXX: On !CONFIG_MEMCG, this will always return NULL; it 261 * would be better if the root_mem_cgroup existed in all 262 * configurations instead. 263 */ 264 memcg = mem_cgroup_from_id(memcgid); 265 if (!mem_cgroup_disabled() && !memcg) { 266 rcu_read_unlock(); 267 return false; 268 } 269 lruvec = mem_cgroup_lruvec(pgdat, memcg); 270 refault = atomic_long_read(&lruvec->inactive_age); 271 active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES); 272 rcu_read_unlock(); 273 274 /* 275 * The unsigned subtraction here gives an accurate distance 276 * across inactive_age overflows in most cases. 277 * 278 * There is a special case: usually, shadow entries have a 279 * short lifetime and are either refaulted or reclaimed along 280 * with the inode before they get too old. But it is not 281 * impossible for the inactive_age to lap a shadow entry in 282 * the field, which can then can result in a false small 283 * refault distance, leading to a false activation should this 284 * old entry actually refault again. However, earlier kernels 285 * used to deactivate unconditionally with *every* reclaim 286 * invocation for the longest time, so the occasional 287 * inappropriate activation leading to pressure on the active 288 * list is not a problem. 289 */ 290 refault_distance = (refault - eviction) & EVICTION_MASK; 291 292 inc_node_state(pgdat, WORKINGSET_REFAULT); 293 294 if (refault_distance <= active_file) { 295 inc_node_state(pgdat, WORKINGSET_ACTIVATE); 296 return true; 297 } 298 return false; 299 } 300 301 /** 302 * workingset_activation - note a page activation 303 * @page: page that is being activated 304 */ 305 void workingset_activation(struct page *page) 306 { 307 struct mem_cgroup *memcg; 308 struct lruvec *lruvec; 309 310 rcu_read_lock(); 311 /* 312 * Filter non-memcg pages here, e.g. unmap can call 313 * mark_page_accessed() on VDSO pages. 314 * 315 * XXX: See workingset_refault() - this should return 316 * root_mem_cgroup even for !CONFIG_MEMCG. 317 */ 318 memcg = page_memcg_rcu(page); 319 if (!mem_cgroup_disabled() && !memcg) 320 goto out; 321 lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg); 322 atomic_long_inc(&lruvec->inactive_age); 323 out: 324 rcu_read_unlock(); 325 } 326 327 /* 328 * Shadow entries reflect the share of the working set that does not 329 * fit into memory, so their number depends on the access pattern of 330 * the workload. In most cases, they will refault or get reclaimed 331 * along with the inode, but a (malicious) workload that streams 332 * through files with a total size several times that of available 333 * memory, while preventing the inodes from being reclaimed, can 334 * create excessive amounts of shadow nodes. To keep a lid on this, 335 * track shadow nodes and reclaim them when they grow way past the 336 * point where they would still be useful. 337 */ 338 339 static struct list_lru shadow_nodes; 340 341 void workingset_update_node(struct radix_tree_node *node, void *private) 342 { 343 struct address_space *mapping = private; 344 345 /* Only regular page cache has shadow entries */ 346 if (dax_mapping(mapping) || shmem_mapping(mapping)) 347 return; 348 349 /* 350 * Track non-empty nodes that contain only shadow entries; 351 * unlink those that contain pages or are being freed. 352 * 353 * Avoid acquiring the list_lru lock when the nodes are 354 * already where they should be. The list_empty() test is safe 355 * as node->private_list is protected by &mapping->tree_lock. 356 */ 357 if (node->count && node->count == node->exceptional) { 358 if (list_empty(&node->private_list)) { 359 node->private_data = mapping; 360 list_lru_add(&shadow_nodes, &node->private_list); 361 } 362 } else { 363 if (!list_empty(&node->private_list)) 364 list_lru_del(&shadow_nodes, &node->private_list); 365 } 366 } 367 368 static unsigned long count_shadow_nodes(struct shrinker *shrinker, 369 struct shrink_control *sc) 370 { 371 unsigned long max_nodes; 372 unsigned long nodes; 373 unsigned long cache; 374 375 /* list_lru lock nests inside IRQ-safe mapping->tree_lock */ 376 local_irq_disable(); 377 nodes = list_lru_shrink_count(&shadow_nodes, sc); 378 local_irq_enable(); 379 380 /* 381 * Approximate a reasonable limit for the radix tree nodes 382 * containing shadow entries. We don't need to keep more 383 * shadow entries than possible pages on the active list, 384 * since refault distances bigger than that are dismissed. 385 * 386 * The size of the active list converges toward 100% of 387 * overall page cache as memory grows, with only a tiny 388 * inactive list. Assume the total cache size for that. 389 * 390 * Nodes might be sparsely populated, with only one shadow 391 * entry in the extreme case. Obviously, we cannot keep one 392 * node for every eligible shadow entry, so compromise on a 393 * worst-case density of 1/8th. Below that, not all eligible 394 * refaults can be detected anymore. 395 * 396 * On 64-bit with 7 radix_tree_nodes per page and 64 slots 397 * each, this will reclaim shadow entries when they consume 398 * ~1.8% of available memory: 399 * 400 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE 401 */ 402 if (sc->memcg) { 403 cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid, 404 LRU_ALL_FILE); 405 } else { 406 cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) + 407 node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE); 408 } 409 max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3); 410 411 if (nodes <= max_nodes) 412 return 0; 413 return nodes - max_nodes; 414 } 415 416 static enum lru_status shadow_lru_isolate(struct list_head *item, 417 struct list_lru_one *lru, 418 spinlock_t *lru_lock, 419 void *arg) 420 { 421 struct address_space *mapping; 422 struct radix_tree_node *node; 423 unsigned int i; 424 int ret; 425 426 /* 427 * Page cache insertions and deletions synchroneously maintain 428 * the shadow node LRU under the mapping->tree_lock and the 429 * lru_lock. Because the page cache tree is emptied before 430 * the inode can be destroyed, holding the lru_lock pins any 431 * address_space that has radix tree nodes on the LRU. 432 * 433 * We can then safely transition to the mapping->tree_lock to 434 * pin only the address_space of the particular node we want 435 * to reclaim, take the node off-LRU, and drop the lru_lock. 436 */ 437 438 node = container_of(item, struct radix_tree_node, private_list); 439 mapping = node->private_data; 440 441 /* Coming from the list, invert the lock order */ 442 if (!spin_trylock(&mapping->tree_lock)) { 443 spin_unlock(lru_lock); 444 ret = LRU_RETRY; 445 goto out; 446 } 447 448 list_lru_isolate(lru, item); 449 spin_unlock(lru_lock); 450 451 /* 452 * The nodes should only contain one or more shadow entries, 453 * no pages, so we expect to be able to remove them all and 454 * delete and free the empty node afterwards. 455 */ 456 if (WARN_ON_ONCE(!node->exceptional)) 457 goto out_invalid; 458 if (WARN_ON_ONCE(node->count != node->exceptional)) 459 goto out_invalid; 460 for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) { 461 if (node->slots[i]) { 462 if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i]))) 463 goto out_invalid; 464 if (WARN_ON_ONCE(!node->exceptional)) 465 goto out_invalid; 466 if (WARN_ON_ONCE(!mapping->nrexceptional)) 467 goto out_invalid; 468 node->slots[i] = NULL; 469 node->exceptional--; 470 node->count--; 471 mapping->nrexceptional--; 472 } 473 } 474 if (WARN_ON_ONCE(node->exceptional)) 475 goto out_invalid; 476 inc_node_state(page_pgdat(virt_to_page(node)), WORKINGSET_NODERECLAIM); 477 __radix_tree_delete_node(&mapping->page_tree, node, 478 workingset_update_node, mapping); 479 480 out_invalid: 481 spin_unlock(&mapping->tree_lock); 482 ret = LRU_REMOVED_RETRY; 483 out: 484 local_irq_enable(); 485 cond_resched(); 486 local_irq_disable(); 487 spin_lock(lru_lock); 488 return ret; 489 } 490 491 static unsigned long scan_shadow_nodes(struct shrinker *shrinker, 492 struct shrink_control *sc) 493 { 494 unsigned long ret; 495 496 /* list_lru lock nests inside IRQ-safe mapping->tree_lock */ 497 local_irq_disable(); 498 ret = list_lru_shrink_walk(&shadow_nodes, sc, shadow_lru_isolate, NULL); 499 local_irq_enable(); 500 return ret; 501 } 502 503 static struct shrinker workingset_shadow_shrinker = { 504 .count_objects = count_shadow_nodes, 505 .scan_objects = scan_shadow_nodes, 506 .seeks = DEFAULT_SEEKS, 507 .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, 508 }; 509 510 /* 511 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe 512 * mapping->tree_lock. 513 */ 514 static struct lock_class_key shadow_nodes_key; 515 516 static int __init workingset_init(void) 517 { 518 unsigned int timestamp_bits; 519 unsigned int max_order; 520 int ret; 521 522 BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT); 523 /* 524 * Calculate the eviction bucket size to cover the longest 525 * actionable refault distance, which is currently half of 526 * memory (totalram_pages/2). However, memory hotplug may add 527 * some more pages at runtime, so keep working with up to 528 * double the initial memory by using totalram_pages as-is. 529 */ 530 timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT; 531 max_order = fls_long(totalram_pages - 1); 532 if (max_order > timestamp_bits) 533 bucket_order = max_order - timestamp_bits; 534 pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n", 535 timestamp_bits, max_order, bucket_order); 536 537 ret = list_lru_init_key(&shadow_nodes, &shadow_nodes_key); 538 if (ret) 539 goto err; 540 ret = register_shrinker(&workingset_shadow_shrinker); 541 if (ret) 542 goto err_list_lru; 543 return 0; 544 err_list_lru: 545 list_lru_destroy(&shadow_nodes); 546 err: 547 return ret; 548 } 549 module_init(workingset_init); 550