Linux Audio

Check our new training course

Loading...
Note: File does not exist in v3.1.
  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 *		Activating refaulting 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 * If this is wrong and demotion kicks in, the pages which are truly
138 * used more frequently will be reactivated while the less frequently
139 * used once will be evicted from memory.
140 *
141 * But if this is right, the stale pages will be pushed out of memory
142 * and the used pages get to stay in cache.
143 *
144 *
145 *		Implementation
146 *
147 * For each node's file LRU lists, a counter for inactive evictions
148 * and activations is maintained (node->inactive_age).
149 *
150 * On eviction, a snapshot of this counter (along with some bits to
151 * identify the node) is stored in the now empty page cache radix tree
152 * slot of the evicted page.  This is called a shadow entry.
153 *
154 * On cache misses for which there are shadow entries, an eligible
155 * refault distance will immediately activate the refaulting page.
156 */
157
158#define EVICTION_SHIFT	(RADIX_TREE_EXCEPTIONAL_ENTRY + \
159			 NODES_SHIFT +	\
160			 MEM_CGROUP_ID_SHIFT)
161#define EVICTION_MASK	(~0UL >> EVICTION_SHIFT)
162
163/*
164 * Eviction timestamps need to be able to cover the full range of
165 * actionable refaults. However, bits are tight in the radix tree
166 * entry, and after storing the identifier for the lruvec there might
167 * not be enough left to represent every single actionable refault. In
168 * that case, we have to sacrifice granularity for distance, and group
169 * evictions into coarser buckets by shaving off lower timestamp bits.
170 */
171static unsigned int bucket_order __read_mostly;
172
173static void *pack_shadow(int memcgid, pg_data_t *pgdat, unsigned long eviction)
174{
175	eviction >>= bucket_order;
176	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
177	eviction = (eviction << NODES_SHIFT) | pgdat->node_id;
178	eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
179
180	return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
181}
182
183static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
184			  unsigned long *evictionp)
185{
186	unsigned long entry = (unsigned long)shadow;
187	int memcgid, nid;
188
189	entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
190	nid = entry & ((1UL << NODES_SHIFT) - 1);
191	entry >>= NODES_SHIFT;
192	memcgid = entry & ((1UL << MEM_CGROUP_ID_SHIFT) - 1);
193	entry >>= MEM_CGROUP_ID_SHIFT;
194
195	*memcgidp = memcgid;
196	*pgdat = NODE_DATA(nid);
197	*evictionp = entry << bucket_order;
198}
199
200/**
201 * workingset_eviction - note the eviction of a page from memory
202 * @mapping: address space the page was backing
203 * @page: the page being evicted
204 *
205 * Returns a shadow entry to be stored in @mapping->i_pages in place
206 * of the evicted @page so that a later refault can be detected.
207 */
208void *workingset_eviction(struct address_space *mapping, struct page *page)
209{
210	struct mem_cgroup *memcg = page_memcg(page);
211	struct pglist_data *pgdat = page_pgdat(page);
212	int memcgid = mem_cgroup_id(memcg);
213	unsigned long eviction;
214	struct lruvec *lruvec;
215
216	/* Page is fully exclusive and pins page->mem_cgroup */
217	VM_BUG_ON_PAGE(PageLRU(page), page);
218	VM_BUG_ON_PAGE(page_count(page), page);
219	VM_BUG_ON_PAGE(!PageLocked(page), page);
220
221	lruvec = mem_cgroup_lruvec(pgdat, memcg);
222	eviction = atomic_long_inc_return(&lruvec->inactive_age);
223	return pack_shadow(memcgid, pgdat, eviction);
224}
225
226/**
227 * workingset_refault - evaluate the refault of a previously evicted page
228 * @shadow: shadow entry of the evicted page
229 *
230 * Calculates and evaluates the refault distance of the previously
231 * evicted page in the context of the node it was allocated in.
232 *
233 * Returns %true if the page should be activated, %false otherwise.
234 */
235bool workingset_refault(void *shadow)
236{
237	unsigned long refault_distance;
238	unsigned long active_file;
239	struct mem_cgroup *memcg;
240	unsigned long eviction;
241	struct lruvec *lruvec;
242	unsigned long refault;
243	struct pglist_data *pgdat;
244	int memcgid;
245
246	unpack_shadow(shadow, &memcgid, &pgdat, &eviction);
247
248	rcu_read_lock();
249	/*
250	 * Look up the memcg associated with the stored ID. It might
251	 * have been deleted since the page's eviction.
252	 *
253	 * Note that in rare events the ID could have been recycled
254	 * for a new cgroup that refaults a shared page. This is
255	 * impossible to tell from the available data. However, this
256	 * should be a rare and limited disturbance, and activations
257	 * are always speculative anyway. Ultimately, it's the aging
258	 * algorithm's job to shake out the minimum access frequency
259	 * for the active cache.
260	 *
261	 * XXX: On !CONFIG_MEMCG, this will always return NULL; it
262	 * would be better if the root_mem_cgroup existed in all
263	 * configurations instead.
264	 */
265	memcg = mem_cgroup_from_id(memcgid);
266	if (!mem_cgroup_disabled() && !memcg) {
267		rcu_read_unlock();
268		return false;
269	}
270	lruvec = mem_cgroup_lruvec(pgdat, memcg);
271	refault = atomic_long_read(&lruvec->inactive_age);
272	active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
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_lruvec_state(lruvec, WORKINGSET_REFAULT);
293
294	if (refault_distance <= active_file) {
295		inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
296		rcu_read_unlock();
297		return true;
298	}
299	rcu_read_unlock();
300	return false;
301}
302
303/**
304 * workingset_activation - note a page activation
305 * @page: page that is being activated
306 */
307void workingset_activation(struct page *page)
308{
309	struct mem_cgroup *memcg;
310	struct lruvec *lruvec;
311
312	rcu_read_lock();
313	/*
314	 * Filter non-memcg pages here, e.g. unmap can call
315	 * mark_page_accessed() on VDSO pages.
316	 *
317	 * XXX: See workingset_refault() - this should return
318	 * root_mem_cgroup even for !CONFIG_MEMCG.
319	 */
320	memcg = page_memcg_rcu(page);
321	if (!mem_cgroup_disabled() && !memcg)
322		goto out;
323	lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
324	atomic_long_inc(&lruvec->inactive_age);
325out:
326	rcu_read_unlock();
327}
328
329/*
330 * Shadow entries reflect the share of the working set that does not
331 * fit into memory, so their number depends on the access pattern of
332 * the workload.  In most cases, they will refault or get reclaimed
333 * along with the inode, but a (malicious) workload that streams
334 * through files with a total size several times that of available
335 * memory, while preventing the inodes from being reclaimed, can
336 * create excessive amounts of shadow nodes.  To keep a lid on this,
337 * track shadow nodes and reclaim them when they grow way past the
338 * point where they would still be useful.
339 */
340
341static struct list_lru shadow_nodes;
342
343void workingset_update_node(struct radix_tree_node *node)
344{
345	/*
346	 * Track non-empty nodes that contain only shadow entries;
347	 * unlink those that contain pages or are being freed.
348	 *
349	 * Avoid acquiring the list_lru lock when the nodes are
350	 * already where they should be. The list_empty() test is safe
351	 * as node->private_list is protected by the i_pages lock.
352	 */
353	if (node->count && node->count == node->exceptional) {
354		if (list_empty(&node->private_list))
355			list_lru_add(&shadow_nodes, &node->private_list);
356	} else {
357		if (!list_empty(&node->private_list))
358			list_lru_del(&shadow_nodes, &node->private_list);
359	}
360}
361
362static unsigned long count_shadow_nodes(struct shrinker *shrinker,
363					struct shrink_control *sc)
364{
365	unsigned long max_nodes;
366	unsigned long nodes;
367	unsigned long cache;
368
369	/* list_lru lock nests inside the IRQ-safe i_pages lock */
370	local_irq_disable();
371	nodes = list_lru_shrink_count(&shadow_nodes, sc);
372	local_irq_enable();
373
374	/*
375	 * Approximate a reasonable limit for the radix tree nodes
376	 * containing shadow entries. We don't need to keep more
377	 * shadow entries than possible pages on the active list,
378	 * since refault distances bigger than that are dismissed.
379	 *
380	 * The size of the active list converges toward 100% of
381	 * overall page cache as memory grows, with only a tiny
382	 * inactive list. Assume the total cache size for that.
383	 *
384	 * Nodes might be sparsely populated, with only one shadow
385	 * entry in the extreme case. Obviously, we cannot keep one
386	 * node for every eligible shadow entry, so compromise on a
387	 * worst-case density of 1/8th. Below that, not all eligible
388	 * refaults can be detected anymore.
389	 *
390	 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
391	 * each, this will reclaim shadow entries when they consume
392	 * ~1.8% of available memory:
393	 *
394	 * PAGE_SIZE / radix_tree_nodes / node_entries * 8 / PAGE_SIZE
395	 */
396	if (sc->memcg) {
397		cache = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
398						     LRU_ALL_FILE);
399	} else {
400		cache = node_page_state(NODE_DATA(sc->nid), NR_ACTIVE_FILE) +
401			node_page_state(NODE_DATA(sc->nid), NR_INACTIVE_FILE);
402	}
403	max_nodes = cache >> (RADIX_TREE_MAP_SHIFT - 3);
404
405	if (nodes <= max_nodes)
406		return 0;
407	return nodes - max_nodes;
408}
409
410static enum lru_status shadow_lru_isolate(struct list_head *item,
411					  struct list_lru_one *lru,
412					  spinlock_t *lru_lock,
413					  void *arg)
414{
415	struct address_space *mapping;
416	struct radix_tree_node *node;
417	unsigned int i;
418	int ret;
419
420	/*
421	 * Page cache insertions and deletions synchroneously maintain
422	 * the shadow node LRU under the i_pages lock and the
423	 * lru_lock.  Because the page cache tree is emptied before
424	 * the inode can be destroyed, holding the lru_lock pins any
425	 * address_space that has radix tree nodes on the LRU.
426	 *
427	 * We can then safely transition to the i_pages lock to
428	 * pin only the address_space of the particular node we want
429	 * to reclaim, take the node off-LRU, and drop the lru_lock.
430	 */
431
432	node = container_of(item, struct radix_tree_node, private_list);
433	mapping = container_of(node->root, struct address_space, i_pages);
434
435	/* Coming from the list, invert the lock order */
436	if (!xa_trylock(&mapping->i_pages)) {
437		spin_unlock(lru_lock);
438		ret = LRU_RETRY;
439		goto out;
440	}
441
442	list_lru_isolate(lru, item);
443	spin_unlock(lru_lock);
444
445	/*
446	 * The nodes should only contain one or more shadow entries,
447	 * no pages, so we expect to be able to remove them all and
448	 * delete and free the empty node afterwards.
449	 */
450	if (WARN_ON_ONCE(!node->exceptional))
451		goto out_invalid;
452	if (WARN_ON_ONCE(node->count != node->exceptional))
453		goto out_invalid;
454	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
455		if (node->slots[i]) {
456			if (WARN_ON_ONCE(!radix_tree_exceptional_entry(node->slots[i])))
457				goto out_invalid;
458			if (WARN_ON_ONCE(!node->exceptional))
459				goto out_invalid;
460			if (WARN_ON_ONCE(!mapping->nrexceptional))
461				goto out_invalid;
462			node->slots[i] = NULL;
463			node->exceptional--;
464			node->count--;
465			mapping->nrexceptional--;
466		}
467	}
468	if (WARN_ON_ONCE(node->exceptional))
469		goto out_invalid;
470	inc_lruvec_page_state(virt_to_page(node), WORKINGSET_NODERECLAIM);
471	__radix_tree_delete_node(&mapping->i_pages, node,
472				 workingset_lookup_update(mapping));
473
474out_invalid:
475	xa_unlock(&mapping->i_pages);
476	ret = LRU_REMOVED_RETRY;
477out:
478	local_irq_enable();
479	cond_resched();
480	local_irq_disable();
481	spin_lock(lru_lock);
482	return ret;
483}
484
485static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
486				       struct shrink_control *sc)
487{
488	unsigned long ret;
489
490	/* list_lru lock nests inside the IRQ-safe i_pages lock */
491	local_irq_disable();
492	ret = list_lru_shrink_walk(&shadow_nodes, sc, shadow_lru_isolate, NULL);
493	local_irq_enable();
494	return ret;
495}
496
497static struct shrinker workingset_shadow_shrinker = {
498	.count_objects = count_shadow_nodes,
499	.scan_objects = scan_shadow_nodes,
500	.seeks = DEFAULT_SEEKS,
501	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
502};
503
504/*
505 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
506 * i_pages lock.
507 */
508static struct lock_class_key shadow_nodes_key;
509
510static int __init workingset_init(void)
511{
512	unsigned int timestamp_bits;
513	unsigned int max_order;
514	int ret;
515
516	BUILD_BUG_ON(BITS_PER_LONG < EVICTION_SHIFT);
517	/*
518	 * Calculate the eviction bucket size to cover the longest
519	 * actionable refault distance, which is currently half of
520	 * memory (totalram_pages/2). However, memory hotplug may add
521	 * some more pages at runtime, so keep working with up to
522	 * double the initial memory by using totalram_pages as-is.
523	 */
524	timestamp_bits = BITS_PER_LONG - EVICTION_SHIFT;
525	max_order = fls_long(totalram_pages - 1);
526	if (max_order > timestamp_bits)
527		bucket_order = max_order - timestamp_bits;
528	pr_info("workingset: timestamp_bits=%d max_order=%d bucket_order=%u\n",
529	       timestamp_bits, max_order, bucket_order);
530
531	ret = __list_lru_init(&shadow_nodes, true, &shadow_nodes_key);
532	if (ret)
533		goto err;
534	ret = register_shrinker(&workingset_shadow_shrinker);
535	if (ret)
536		goto err_list_lru;
537	return 0;
538err_list_lru:
539	list_lru_destroy(&shadow_nodes);
540err:
541	return ret;
542}
543module_init(workingset_init);