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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 
pack_shadow(unsigned long eviction,struct zone * zone)155 static void *pack_shadow(unsigned long eviction, struct zone *zone)
156 {
157 	eviction = (eviction << NODES_SHIFT) | zone_to_nid(zone);
158 	eviction = (eviction << ZONES_SHIFT) | zone_idx(zone);
159 	eviction = (eviction << RADIX_TREE_EXCEPTIONAL_SHIFT);
160 
161 	return (void *)(eviction | RADIX_TREE_EXCEPTIONAL_ENTRY);
162 }
163 
unpack_shadow(void * shadow,struct zone ** zone,unsigned long * distance)164 static void unpack_shadow(void *shadow,
165 			  struct zone **zone,
166 			  unsigned long *distance)
167 {
168 	unsigned long entry = (unsigned long)shadow;
169 	unsigned long eviction;
170 	unsigned long refault;
171 	unsigned long mask;
172 	int zid, nid;
173 
174 	entry >>= RADIX_TREE_EXCEPTIONAL_SHIFT;
175 	zid = entry & ((1UL << ZONES_SHIFT) - 1);
176 	entry >>= ZONES_SHIFT;
177 	nid = entry & ((1UL << NODES_SHIFT) - 1);
178 	entry >>= NODES_SHIFT;
179 	eviction = entry;
180 
181 	*zone = NODE_DATA(nid)->node_zones + zid;
182 
183 	refault = atomic_long_read(&(*zone)->inactive_age);
184 	mask = ~0UL >> (NODES_SHIFT + ZONES_SHIFT +
185 			RADIX_TREE_EXCEPTIONAL_SHIFT);
186 	/*
187 	 * The unsigned subtraction here gives an accurate distance
188 	 * across inactive_age overflows in most cases.
189 	 *
190 	 * There is a special case: usually, shadow entries have a
191 	 * short lifetime and are either refaulted or reclaimed along
192 	 * with the inode before they get too old.  But it is not
193 	 * impossible for the inactive_age to lap a shadow entry in
194 	 * the field, which can then can result in a false small
195 	 * refault distance, leading to a false activation should this
196 	 * old entry actually refault again.  However, earlier kernels
197 	 * used to deactivate unconditionally with *every* reclaim
198 	 * invocation for the longest time, so the occasional
199 	 * inappropriate activation leading to pressure on the active
200 	 * list is not a problem.
201 	 */
202 	*distance = (refault - eviction) & mask;
203 }
204 
205 /**
206  * workingset_eviction - note the eviction of a page from memory
207  * @mapping: address space the page was backing
208  * @page: the page being evicted
209  *
210  * Returns a shadow entry to be stored in @mapping->page_tree in place
211  * of the evicted @page so that a later refault can be detected.
212  */
workingset_eviction(struct address_space * mapping,struct page * page)213 void *workingset_eviction(struct address_space *mapping, struct page *page)
214 {
215 	struct zone *zone = page_zone(page);
216 	unsigned long eviction;
217 
218 	eviction = atomic_long_inc_return(&zone->inactive_age);
219 	return pack_shadow(eviction, zone);
220 }
221 
222 /**
223  * workingset_refault - evaluate the refault of a previously evicted page
224  * @shadow: shadow entry of the evicted page
225  *
226  * Calculates and evaluates the refault distance of the previously
227  * evicted page in the context of the zone it was allocated in.
228  *
229  * Returns %true if the page should be activated, %false otherwise.
230  */
workingset_refault(void * shadow)231 bool workingset_refault(void *shadow)
232 {
233 	unsigned long refault_distance;
234 	struct zone *zone;
235 
236 	unpack_shadow(shadow, &zone, &refault_distance);
237 	inc_zone_state(zone, WORKINGSET_REFAULT);
238 
239 	if (refault_distance <= zone_page_state(zone, NR_ACTIVE_FILE)) {
240 		inc_zone_state(zone, WORKINGSET_ACTIVATE);
241 		return true;
242 	}
243 	return false;
244 }
245 
246 /**
247  * workingset_activation - note a page activation
248  * @page: page that is being activated
249  */
workingset_activation(struct page * page)250 void workingset_activation(struct page *page)
251 {
252 	atomic_long_inc(&page_zone(page)->inactive_age);
253 }
254 
255 /*
256  * Shadow entries reflect the share of the working set that does not
257  * fit into memory, so their number depends on the access pattern of
258  * the workload.  In most cases, they will refault or get reclaimed
259  * along with the inode, but a (malicious) workload that streams
260  * through files with a total size several times that of available
261  * memory, while preventing the inodes from being reclaimed, can
262  * create excessive amounts of shadow nodes.  To keep a lid on this,
263  * track shadow nodes and reclaim them when they grow way past the
264  * point where they would still be useful.
265  */
266 
267 struct list_lru workingset_shadow_nodes;
268 
count_shadow_nodes(struct shrinker * shrinker,struct shrink_control * sc)269 static unsigned long count_shadow_nodes(struct shrinker *shrinker,
270 					struct shrink_control *sc)
271 {
272 	unsigned long shadow_nodes;
273 	unsigned long max_nodes;
274 	unsigned long pages;
275 
276 	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
277 	local_irq_disable();
278 	shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
279 	local_irq_enable();
280 
281 	pages = node_present_pages(sc->nid);
282 	/*
283 	 * Active cache pages are limited to 50% of memory, and shadow
284 	 * entries that represent a refault distance bigger than that
285 	 * do not have any effect.  Limit the number of shadow nodes
286 	 * such that shadow entries do not exceed the number of active
287 	 * cache pages, assuming a worst-case node population density
288 	 * of 1/8th on average.
289 	 *
290 	 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
291 	 * each, this will reclaim shadow entries when they consume
292 	 * ~2% of available memory:
293 	 *
294 	 * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
295 	 */
296 	max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
297 
298 	if (shadow_nodes <= max_nodes)
299 		return 0;
300 
301 	return shadow_nodes - max_nodes;
302 }
303 
shadow_lru_isolate(struct list_head * item,struct list_lru_one * lru,spinlock_t * lru_lock,void * arg)304 static enum lru_status shadow_lru_isolate(struct list_head *item,
305 					  struct list_lru_one *lru,
306 					  spinlock_t *lru_lock,
307 					  void *arg)
308 {
309 	struct address_space *mapping;
310 	struct radix_tree_node *node;
311 	unsigned int i;
312 	int ret;
313 
314 	/*
315 	 * Page cache insertions and deletions synchroneously maintain
316 	 * the shadow node LRU under the mapping->tree_lock and the
317 	 * lru_lock.  Because the page cache tree is emptied before
318 	 * the inode can be destroyed, holding the lru_lock pins any
319 	 * address_space that has radix tree nodes on the LRU.
320 	 *
321 	 * We can then safely transition to the mapping->tree_lock to
322 	 * pin only the address_space of the particular node we want
323 	 * to reclaim, take the node off-LRU, and drop the lru_lock.
324 	 */
325 
326 	node = container_of(item, struct radix_tree_node, private_list);
327 	mapping = node->private_data;
328 
329 	/* Coming from the list, invert the lock order */
330 	if (!spin_trylock(&mapping->tree_lock)) {
331 		spin_unlock(lru_lock);
332 		ret = LRU_RETRY;
333 		goto out;
334 	}
335 
336 	list_lru_isolate(lru, item);
337 	spin_unlock(lru_lock);
338 
339 	/*
340 	 * The nodes should only contain one or more shadow entries,
341 	 * no pages, so we expect to be able to remove them all and
342 	 * delete and free the empty node afterwards.
343 	 */
344 	BUG_ON(!workingset_node_shadows(node));
345 	BUG_ON(workingset_node_pages(node));
346 
347 	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
348 		if (node->slots[i]) {
349 			BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
350 			node->slots[i] = NULL;
351 			workingset_node_shadows_dec(node);
352 			BUG_ON(!mapping->nrshadows);
353 			mapping->nrshadows--;
354 		}
355 	}
356 	BUG_ON(workingset_node_shadows(node));
357 	inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
358 	if (!__radix_tree_delete_node(&mapping->page_tree, node))
359 		BUG();
360 
361 	spin_unlock(&mapping->tree_lock);
362 	ret = LRU_REMOVED_RETRY;
363 out:
364 	local_irq_enable();
365 	cond_resched();
366 	local_irq_disable();
367 	spin_lock(lru_lock);
368 	return ret;
369 }
370 
scan_shadow_nodes(struct shrinker * shrinker,struct shrink_control * sc)371 static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
372 				       struct shrink_control *sc)
373 {
374 	unsigned long ret;
375 
376 	/* list_lru lock nests inside IRQ-safe mapping->tree_lock */
377 	local_irq_disable();
378 	ret =  list_lru_shrink_walk(&workingset_shadow_nodes, sc,
379 				    shadow_lru_isolate, NULL);
380 	local_irq_enable();
381 	return ret;
382 }
383 
384 static struct shrinker workingset_shadow_shrinker = {
385 	.count_objects = count_shadow_nodes,
386 	.scan_objects = scan_shadow_nodes,
387 	.seeks = DEFAULT_SEEKS,
388 	.flags = SHRINKER_NUMA_AWARE,
389 };
390 
391 /*
392  * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
393  * mapping->tree_lock.
394  */
395 static struct lock_class_key shadow_nodes_key;
396 
workingset_init(void)397 static int __init workingset_init(void)
398 {
399 	int ret;
400 
401 	ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
402 	if (ret)
403 		goto err;
404 	ret = register_shrinker(&workingset_shadow_shrinker);
405 	if (ret)
406 		goto err_list_lru;
407 	return 0;
408 err_list_lru:
409 	list_lru_destroy(&workingset_shadow_nodes);
410 err:
411 	return ret;
412 }
413 module_init(workingset_init);
414