1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Workingset detection 4 * 5 * Copyright (C) 2013 Red Hat, Inc., Johannes Weiner 6 */ 7 8 #include <linux/memcontrol.h> 9 #include <linux/writeback.h> 10 #include <linux/shmem_fs.h> 11 #include <linux/pagemap.h> 12 #include <linux/atomic.h> 13 #include <linux/module.h> 14 #include <linux/swap.h> 15 #include <linux/dax.h> 16 #include <linux/fs.h> 17 #include <linux/mm.h> 18 19 /* 20 * Double CLOCK lists 21 * 22 * Per node, two clock lists are maintained for file pages: the 23 * inactive and the active list. Freshly faulted pages start out at 24 * the head of the inactive list and page reclaim scans pages from the 25 * tail. Pages that are accessed multiple times on the inactive list 26 * are promoted to the active list, to protect them from reclaim, 27 * whereas active pages are demoted to the inactive list when the 28 * active list grows too big. 29 * 30 * fault ------------------------+ 31 * | 32 * +--------------+ | +-------------+ 33 * reclaim <- | inactive | <-+-- demotion | active | <--+ 34 * +--------------+ +-------------+ | 35 * | | 36 * +-------------- promotion ------------------+ 37 * 38 * 39 * Access frequency and refault distance 40 * 41 * A workload is thrashing when its pages are frequently used but they 42 * are evicted from the inactive list every time before another access 43 * would have promoted them to the active list. 44 * 45 * In cases where the average access distance between thrashing pages 46 * is bigger than the size of memory there is nothing that can be 47 * done - the thrashing set could never fit into memory under any 48 * circumstance. 49 * 50 * However, the average access distance could be bigger than the 51 * inactive list, yet smaller than the size of memory. In this case, 52 * the set could fit into memory if it weren't for the currently 53 * active pages - which may be used more, hopefully less frequently: 54 * 55 * +-memory available to cache-+ 56 * | | 57 * +-inactive------+-active----+ 58 * a b | c d e f g h i | J K L M N | 59 * +---------------+-----------+ 60 * 61 * It is prohibitively expensive to accurately track access frequency 62 * of pages. But a reasonable approximation can be made to measure 63 * thrashing on the inactive list, after which refaulting pages can be 64 * activated optimistically to compete with the existing active pages. 65 * 66 * Approximating inactive page access frequency - Observations: 67 * 68 * 1. When a page is accessed for the first time, it is added to the 69 * head of the inactive list, slides every existing inactive page 70 * towards the tail by one slot, and pushes the current tail page 71 * out of memory. 72 * 73 * 2. When a page is accessed for the second time, it is promoted to 74 * the active list, shrinking the inactive list by one slot. This 75 * also slides all inactive pages that were faulted into the cache 76 * more recently than the activated page towards the tail of the 77 * inactive list. 78 * 79 * Thus: 80 * 81 * 1. The sum of evictions and activations between any two points in 82 * time indicate the minimum number of inactive pages accessed in 83 * between. 84 * 85 * 2. Moving one inactive page N page slots towards the tail of the 86 * list requires at least N inactive page accesses. 87 * 88 * Combining these: 89 * 90 * 1. When a page is finally evicted from memory, the number of 91 * inactive pages accessed while the page was in cache is at least 92 * the number of page slots on the inactive list. 93 * 94 * 2. In addition, measuring the sum of evictions and activations (E) 95 * at the time of a page's eviction, and comparing it to another 96 * reading (R) at the time the page faults back into memory tells 97 * the minimum number of accesses while the page was not cached. 98 * This is called the refault distance. 99 * 100 * Because the first access of the page was the fault and the second 101 * access the refault, we combine the in-cache distance with the 102 * out-of-cache distance to get the complete minimum access distance 103 * of this page: 104 * 105 * NR_inactive + (R - E) 106 * 107 * And knowing the minimum access distance of a page, we can easily 108 * tell if the page would be able to stay in cache assuming all page 109 * slots in the cache were available: 110 * 111 * NR_inactive + (R - E) <= NR_inactive + NR_active 112 * 113 * which can be further simplified to 114 * 115 * (R - E) <= NR_active 116 * 117 * Put into words, the refault distance (out-of-cache) can be seen as 118 * a deficit in inactive list space (in-cache). If the inactive list 119 * had (R - E) more page slots, the page would not have been evicted 120 * in between accesses, but activated instead. And on a full system, 121 * the only thing eating into inactive list space is active pages. 122 * 123 * 124 * Refaulting inactive pages 125 * 126 * All that is known about the active list is that the pages have been 127 * accessed more than once in the past. This means that at any given 128 * time there is actually a good chance that pages on the active list 129 * are no longer in active use. 130 * 131 * So when a refault distance of (R - E) is observed and there are at 132 * least (R - E) active pages, the refaulting page is activated 133 * optimistically in the hope that (R - E) active pages are actually 134 * used less frequently than the refaulting page - or even not used at 135 * all anymore. 136 * 137 * That means if inactive cache is refaulting with a suitable refault 138 * distance, we assume the cache workingset is transitioning and put 139 * pressure on the current active list. 140 * 141 * If this is wrong and demotion kicks in, the pages which are truly 142 * used more frequently will be reactivated while the less frequently 143 * used once will be evicted from memory. 144 * 145 * But if this is right, the stale pages will be pushed out of memory 146 * and the used pages get to stay in cache. 147 * 148 * Refaulting active pages 149 * 150 * If on the other hand the refaulting pages have recently been 151 * deactivated, it means that the active list is no longer protecting 152 * actively used cache from reclaim. The cache is NOT transitioning to 153 * a different workingset; the existing workingset is thrashing in the 154 * space allocated to the page cache. 155 * 156 * 157 * Implementation 158 * 159 * For each node's file LRU lists, a counter for inactive evictions 160 * and activations is maintained (node->inactive_age). 161 * 162 * On eviction, a snapshot of this counter (along with some bits to 163 * identify the node) is stored in the now empty page cache 164 * slot of the evicted page. This is called a shadow entry. 165 * 166 * On cache misses for which there are shadow entries, an eligible 167 * refault distance will immediately activate the refaulting page. 168 */ 169 170 #define EVICTION_SHIFT ((BITS_PER_LONG - BITS_PER_XA_VALUE) + \ 171 1 + NODES_SHIFT + MEM_CGROUP_ID_SHIFT) 172 #define EVICTION_MASK (~0UL >> EVICTION_SHIFT) 173 174 /* 175 * Eviction timestamps need to be able to cover the full range of 176 * actionable refaults. However, bits are tight in the xarray 177 * entry, and after storing the identifier for the lruvec there might 178 * not be enough left to represent every single actionable refault. In 179 * that case, we have to sacrifice granularity for distance, and group 180 * evictions into coarser buckets by shaving off lower timestamp bits. 181 */ 182 static unsigned int bucket_order __read_mostly; 183 184 static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction, 185 bool workingset) 186 { 187 eviction >>= bucket_order; 188 eviction &= EVICTION_MASK; 189 eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid; 190 eviction = (eviction << NODES_SHIFT) | pgdat->node_id; 191 eviction = (eviction << 1) | workingset; 192 193 return xa_mk_value(eviction); 194 } 195 196 static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat, 197 unsigned long *evictionp, bool *workingsetp) 198 { 199 unsigned long entry = xa_to_value(shadow); 200 int memcgid, nid; 201 bool workingset; 202 203 workingset = entry & 1; 204 entry >>= 1; 205 nid = entry & ((1UL << NODES_SHIFT) - 1); 206 entry >>= NODES_SHIFT; 207 memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1); 208 entry >>= MEM_CGROUP_ID_SHIFT; 209 210 *memcgidp = memcgid; 211 *pgdat = NODE_DATA(nid); 212 *evictionp = entry << bucket_order; 213 *workingsetp = workingset; 214 } 215 216 /** 217 * workingset_eviction - note the eviction of a page from memory 218 * @page: the page being evicted 219 * 220 * Returns a shadow entry to be stored in @page->mapping->i_pages in place 221 * of the evicted @page so that a later refault can be detected. 222 */ 223 void *workingset_eviction(struct page *page) 224 { 225 struct pglist_data *pgdat = page_pgdat(page); 226 struct mem_cgroup *memcg = page_memcg(page); 227 int memcgid = mem_cgroup_id(memcg); 228 unsigned long eviction; 229 struct lruvec *lruvec; 230 231 /* Page is fully exclusive and pins page->mem_cgroup */ 232 VM_BUG_ON_PAGE(PageLRU(page), page); 233 VM_BUG_ON_PAGE(page_count(page), page); 234 VM_BUG_ON_PAGE(!PageLocked(page), page); 235 236 lruvec = mem_cgroup_lruvec(pgdat, memcg); 237 eviction = atomic_long_inc_return(&lruvec->inactive_age); 238 return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page)); 239 } 240 241 /** 242 * workingset_refault - evaluate the refault of a previously evicted page 243 * @page: the freshly allocated replacement page 244 * @shadow: shadow entry of the evicted page 245 * 246 * Calculates and evaluates the refault distance of the previously 247 * evicted page in the context of the node it was allocated in. 248 */ 249 void workingset_refault(struct page *page, void *shadow) 250 { 251 unsigned long refault_distance; 252 struct pglist_data *pgdat; 253 unsigned long active_file; 254 struct mem_cgroup *memcg; 255 unsigned long eviction; 256 struct lruvec *lruvec; 257 unsigned long refault; 258 bool workingset; 259 int memcgid; 260 261 unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset); 262 263 rcu_read_lock(); 264 /* 265 * Look up the memcg associated with the stored ID. It might 266 * have been deleted since the page's eviction. 267 * 268 * Note that in rare events the ID could have been recycled 269 * for a new cgroup that refaults a shared page. This is 270 * impossible to tell from the available data. However, this 271 * should be a rare and limited disturbance, and activations 272 * are always speculative anyway. Ultimately, it's the aging 273 * algorithm's job to shake out the minimum access frequency 274 * for the active cache. 275 * 276 * XXX: On !CONFIG_MEMCG, this will always return NULL; it 277 * would be better if the root_mem_cgroup existed in all 278 * configurations instead. 279 */ 280 memcg = mem_cgroup_from_id(memcgid); 281 if (!mem_cgroup_disabled() && !memcg) 282 goto out; 283 lruvec = mem_cgroup_lruvec(pgdat, memcg); 284 refault = atomic_long_read(&lruvec->inactive_age); 285 active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES); 286 287 /* 288 * Calculate the refault distance 289 * 290 * The unsigned subtraction here gives an accurate distance 291 * across inactive_age overflows in most cases. There is a 292 * special case: usually, shadow entries have a short lifetime 293 * and are either refaulted or reclaimed along with the inode 294 * before they get too old. But it is not impossible for the 295 * inactive_age to lap a shadow entry in the field, which can 296 * then result in a false small refault distance, leading to a 297 * false activation should this old entry actually refault 298 * again. However, earlier kernels used to deactivate 299 * unconditionally with *every* reclaim invocation for the 300 * longest time, so the occasional inappropriate activation 301 * leading to pressure on the active list is not a problem. 302 */ 303 refault_distance = (refault - eviction) & EVICTION_MASK; 304 305 inc_lruvec_state(lruvec, WORKINGSET_REFAULT); 306 307 /* 308 * Compare the distance to the existing workingset size. We 309 * don't act on pages that couldn't stay resident even if all 310 * the memory was available to the page cache. 311 */ 312 if (refault_distance > active_file) 313 goto out; 314 315 SetPageActive(page); 316 atomic_long_inc(&lruvec->inactive_age); 317 inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE); 318 319 /* Page was active prior to eviction */ 320 if (workingset) { 321 SetPageWorkingset(page); 322 inc_lruvec_state(lruvec, WORKINGSET_RESTORE); 323 } 324 out: 325 rcu_read_unlock(); 326 } 327 328 /** 329 * workingset_activation - note a page activation 330 * @page: page that is being activated 331 */ 332 void workingset_activation(struct page *page) 333 { 334 struct mem_cgroup *memcg; 335 struct lruvec *lruvec; 336 337 rcu_read_lock(); 338 /* 339 * Filter non-memcg pages here, e.g. unmap can call 340 * mark_page_accessed() on VDSO pages. 341 * 342 * XXX: See workingset_refault() - this should return 343 * root_mem_cgroup even for !CONFIG_MEMCG. 344 */ 345 memcg = page_memcg_rcu(page); 346 if (!mem_cgroup_disabled() && !memcg) 347 goto out; 348 lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg); 349 atomic_long_inc(&lruvec->inactive_age); 350 out: 351 rcu_read_unlock(); 352 } 353 354 /* 355 * Shadow entries reflect the share of the working set that does not 356 * fit into memory, so their number depends on the access pattern of 357 * the workload. In most cases, they will refault or get reclaimed 358 * along with the inode, but a (malicious) workload that streams 359 * through files with a total size several times that of available 360 * memory, while preventing the inodes from being reclaimed, can 361 * create excessive amounts of shadow nodes. To keep a lid on this, 362 * track shadow nodes and reclaim them when they grow way past the 363 * point where they would still be useful. 364 */ 365 366 static struct list_lru shadow_nodes; 367 368 void workingset_update_node(struct xa_node *node) 369 { 370 /* 371 * Track non-empty nodes that contain only shadow entries; 372 * unlink those that contain pages or are being freed. 373 * 374 * Avoid acquiring the list_lru lock when the nodes are 375 * already where they should be. The list_empty() test is safe 376 * as node->private_list is protected by the i_pages lock. 377 */ 378 VM_WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */ 379 380 if (node->count && node->count == node->nr_values) { 381 if (list_empty(&node->private_list)) { 382 list_lru_add(&shadow_nodes, &node->private_list); 383 __inc_lruvec_slab_state(node, WORKINGSET_NODES); 384 } 385 } else { 386 if (!list_empty(&node->private_list)) { 387 list_lru_del(&shadow_nodes, &node->private_list); 388 __dec_lruvec_slab_state(node, WORKINGSET_NODES); 389 } 390 } 391 } 392 393 static unsigned long count_shadow_nodes(struct shrinker *shrinker, 394 struct shrink_control *sc) 395 { 396 unsigned long max_nodes; 397 unsigned long nodes; 398 unsigned long pages; 399 400 nodes = list_lru_shrink_count(&shadow_nodes, sc); 401 402 /* 403 * Approximate a reasonable limit for the nodes 404 * containing shadow entries. We don't need to keep more 405 * shadow entries than possible pages on the active list, 406 * since refault distances bigger than that are dismissed. 407 * 408 * The size of the active list converges toward 100% of 409 * overall page cache as memory grows, with only a tiny 410 * inactive list. Assume the total cache size for that. 411 * 412 * Nodes might be sparsely populated, with only one shadow 413 * entry in the extreme case. Obviously, we cannot keep one 414 * node for every eligible shadow entry, so compromise on a 415 * worst-case density of 1/8th. Below that, not all eligible 416 * refaults can be detected anymore. 417 * 418 * On 64-bit with 7 xa_nodes per page and 64 slots 419 * each, this will reclaim shadow entries when they consume 420 * ~1.8% of available memory: 421 * 422 * PAGE_SIZE / xa_nodes / node_entries * 8 / PAGE_SIZE 423 */ 424 #ifdef CONFIG_MEMCG 425 if (sc->memcg) { 426 struct lruvec *lruvec; 427 int i; 428 429 lruvec = mem_cgroup_lruvec(NODE_DATA(sc->nid), sc->memcg); 430 for (pages = 0, i = 0; i < NR_LRU_LISTS; i++) 431 pages += lruvec_page_state_local(lruvec, 432 NR_LRU_BASE + i); 433 pages += lruvec_page_state_local(lruvec, NR_SLAB_RECLAIMABLE); 434 pages += lruvec_page_state_local(lruvec, NR_SLAB_UNRECLAIMABLE); 435 } else 436 #endif 437 pages = node_present_pages(sc->nid); 438 439 max_nodes = pages >> (XA_CHUNK_SHIFT - 3); 440 441 if (!nodes) 442 return SHRINK_EMPTY; 443 444 if (nodes <= max_nodes) 445 return 0; 446 return nodes - max_nodes; 447 } 448 449 static enum lru_status shadow_lru_isolate(struct list_head *item, 450 struct list_lru_one *lru, 451 spinlock_t *lru_lock, 452 void *arg) __must_hold(lru_lock) 453 { 454 struct xa_node *node = container_of(item, struct xa_node, private_list); 455 XA_STATE(xas, node->array, 0); 456 struct address_space *mapping; 457 int ret; 458 459 /* 460 * Page cache insertions and deletions synchroneously maintain 461 * the shadow node LRU under the i_pages lock and the 462 * lru_lock. Because the page cache tree is emptied before 463 * the inode can be destroyed, holding the lru_lock pins any 464 * address_space that has nodes on the LRU. 465 * 466 * We can then safely transition to the i_pages lock to 467 * pin only the address_space of the particular node we want 468 * to reclaim, take the node off-LRU, and drop the lru_lock. 469 */ 470 471 mapping = container_of(node->array, struct address_space, i_pages); 472 473 /* Coming from the list, invert the lock order */ 474 if (!xa_trylock(&mapping->i_pages)) { 475 spin_unlock_irq(lru_lock); 476 ret = LRU_RETRY; 477 goto out; 478 } 479 480 list_lru_isolate(lru, item); 481 __dec_lruvec_slab_state(node, WORKINGSET_NODES); 482 483 spin_unlock(lru_lock); 484 485 /* 486 * The nodes should only contain one or more shadow entries, 487 * no pages, so we expect to be able to remove them all and 488 * delete and free the empty node afterwards. 489 */ 490 if (WARN_ON_ONCE(!node->nr_values)) 491 goto out_invalid; 492 if (WARN_ON_ONCE(node->count != node->nr_values)) 493 goto out_invalid; 494 mapping->nrexceptional -= node->nr_values; 495 xas.xa_node = xa_parent_locked(&mapping->i_pages, node); 496 xas.xa_offset = node->offset; 497 xas.xa_shift = node->shift + XA_CHUNK_SHIFT; 498 xas_set_update(&xas, workingset_update_node); 499 /* 500 * We could store a shadow entry here which was the minimum of the 501 * shadow entries we were tracking ... 502 */ 503 xas_store(&xas, NULL); 504 __inc_lruvec_slab_state(node, WORKINGSET_NODERECLAIM); 505 506 out_invalid: 507 xa_unlock_irq(&mapping->i_pages); 508 ret = LRU_REMOVED_RETRY; 509 out: 510 cond_resched(); 511 spin_lock_irq(lru_lock); 512 return ret; 513 } 514 515 static unsigned long scan_shadow_nodes(struct shrinker *shrinker, 516 struct shrink_control *sc) 517 { 518 /* list_lru lock nests inside the IRQ-safe i_pages lock */ 519 return list_lru_shrink_walk_irq(&shadow_nodes, sc, shadow_lru_isolate, 520 NULL); 521 } 522 523 static struct shrinker workingset_shadow_shrinker = { 524 .count_objects = count_shadow_nodes, 525 .scan_objects = scan_shadow_nodes, 526 .seeks = 0, /* ->count reports only fully expendable nodes */ 527 .flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, 528 }; 529 530 /* 531 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe 532 * i_pages lock. 533 */ 534 static struct lock_class_key shadow_nodes_key; 535 536 static int __init workingset_init(void) 537 { 538 unsigned int timestamp_bits; 539 unsigned int max_order; 540 int ret; 541 542 BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT); 543 /* 544 * Calculate the eviction bucket size to cover the longest 545 * actionable refault distance, which is currently half of 546 * memory (totalram_pages/2). However, memory hotplug may add 547 * some more pages at runtime, so keep working with up to 548 * double the initial memory by using totalram_pages as-is. 549 */ 550 timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT; 551 max_order = fls_long(totalram_pages() - 1); 552 if (max_order > timestamp_bits) 553 bucket_order = max_order - timestamp_bits; 554 pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n", 555 timestamp_bits, max_order, bucket_order); 556 557 ret = prealloc_shrinker(&workingset_shadow_shrinker); 558 if (ret) 559 goto err; 560 ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key, 561 &workingset_shadow_shrinker); 562 if (ret) 563 goto err_list_lru; 564 register_shrinker_prepared(&workingset_shadow_shrinker); 565 return 0; 566 err_list_lru: 567 free_prealloced_shrinker(&workingset_shadow_shrinker); 568 err: 569 return ret; 570 } 571 module_init(workingset_init);