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

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