xref: /openbmc/linux/mm/workingset.c (revision 8e694cd2)
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 zone, 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 zone's file LRU lists, a counter for inactive evictions
145  * and activations is maintained (zone->inactive_age).
146  *
147  * On eviction, a snapshot of this counter (along with some bits to
148  * identify the zone) 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 			 ZONES_SHIFT + 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, struct zone *zone, unsigned long eviction)
171 {
172 	eviction >>= bucket_order;
173 	eviction = (eviction << MEM_CGROUP_ID_SHIFT) | memcgid;
174 	eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone);
175 	eviction = (eviction << ZONES_SHIFT) | zone_idx(zone);
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, struct zone **zonep,
182 			  unsigned long *evictionp)
183 {
184 	unsigned long entry = (unsigned long)shadow;
185 	int memcgid, nid, zid;
186 
187 	entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
188 	zid = entry & ((1UL << ZONES_SHIFT) - 1);
189 	entry >>= ZONES_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 	*zonep = NODE_DATA(nid)->node_zones + zid;
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->page_tree in place
206  * of the evicted @page so that a later refault can be detected.
207  */
208 void *workingset_eviction(struct address_space *mapping, struct page *page)
209 {
210 	struct mem_cgroup *memcg = page_memcg(page);
211 	struct zone *zone = page_zone(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_zone_lruvec(zone, memcg);
222 	eviction = atomic_long_inc_return(&lruvec->inactive_age);
223 	return pack_shadow(memcgid, zone, 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 zone it was allocated in.
232  *
233  * Returns %true if the page should be activated, %false otherwise.
234  */
235 bool 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 zone *zone;
244 	int memcgid;
245 
246 	unpack_shadow(shadow, &memcgid, &zone, &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_zone_lruvec(zone, memcg);
271 	refault = atomic_long_read(&lruvec->inactive_age);
272 	active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE);
273 	rcu_read_unlock();
274 
275 	/*
276 	 * The unsigned subtraction here gives an accurate distance
277 	 * across inactive_age overflows in most cases.
278 	 *
279 	 * There is a special case: usually, shadow entries have a
280 	 * short lifetime and are either refaulted or reclaimed along
281 	 * with the inode before they get too old.  But it is not
282 	 * impossible for the inactive_age to lap a shadow entry in
283 	 * the field, which can then can result in a false small
284 	 * refault distance, leading to a false activation should this
285 	 * old entry actually refault again.  However, earlier kernels
286 	 * used to deactivate unconditionally with *every* reclaim
287 	 * invocation for the longest time, so the occasional
288 	 * inappropriate activation leading to pressure on the active
289 	 * list is not a problem.
290 	 */
291 	refault_distance = (refault - eviction) & EVICTION_MASK;
292 
293 	inc_zone_state(zone, WORKINGSET_REFAULT);
294 
295 	if (refault_distance <= active_file) {
296 		inc_zone_state(zone, WORKINGSET_ACTIVATE);
297 		return true;
298 	}
299 	return false;
300 }
301 
302 /**
303  * workingset_activation - note a page activation
304  * @page: page that is being activated
305  */
306 void workingset_activation(struct page *page)
307 {
308 	struct lruvec *lruvec;
309 
310 	lock_page_memcg(page);
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 	if (!mem_cgroup_disabled() && !page_memcg(page))
319 		goto out;
320 	lruvec = mem_cgroup_zone_lruvec(page_zone(page), page_memcg(page));
321 	atomic_long_inc(&lruvec->inactive_age);
322 out:
323 	unlock_page_memcg(page);
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 struct list_lru workingset_shadow_nodes;
339 
340 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
341 					struct shrink_control *sc)
342 {
343 	unsigned long shadow_nodes;
344 	unsigned long max_nodes;
345 	unsigned long pages;
346 
347 	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
348 	local_irq_disable();
349 	shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
350 	local_irq_enable();
351 
352 	if (memcg_kmem_enabled())
353 		pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
354 						     LRU_ALL_FILE);
355 	else
356 		pages = node_page_state(sc->nid, NR_ACTIVE_FILE) +
357 			node_page_state(sc->nid, NR_INACTIVE_FILE);
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_zone_state(page_zone(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);
510