Root/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/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
155static 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
164static 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 */
213void *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 */
231bool 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 */
250void 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
267struct list_lru workingset_shadow_nodes;
268
269static 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_count_node(&workingset_shadow_nodes, sc->nid);
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
304static enum lru_status shadow_lru_isolate(struct list_head *item,
305                      spinlock_t *lru_lock,
306                      void *arg)
307{
308    struct address_space *mapping;
309    struct radix_tree_node *node;
310    unsigned int i;
311    int ret;
312
313    /*
314     * Page cache insertions and deletions synchroneously maintain
315     * the shadow node LRU under the mapping->tree_lock and the
316     * lru_lock. Because the page cache tree is emptied before
317     * the inode can be destroyed, holding the lru_lock pins any
318     * address_space that has radix tree nodes on the LRU.
319     *
320     * We can then safely transition to the mapping->tree_lock to
321     * pin only the address_space of the particular node we want
322     * to reclaim, take the node off-LRU, and drop the lru_lock.
323     */
324
325    node = container_of(item, struct radix_tree_node, private_list);
326    mapping = node->private_data;
327
328    /* Coming from the list, invert the lock order */
329    if (!spin_trylock(&mapping->tree_lock)) {
330        spin_unlock(lru_lock);
331        ret = LRU_RETRY;
332        goto out;
333    }
334
335    list_del_init(item);
336    spin_unlock(lru_lock);
337
338    /*
339     * The nodes should only contain one or more shadow entries,
340     * no pages, so we expect to be able to remove them all and
341     * delete and free the empty node afterwards.
342     */
343
344    BUG_ON(!node->count);
345    BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
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            BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
352            node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
353            BUG_ON(!mapping->nrshadows);
354            mapping->nrshadows--;
355        }
356    }
357    BUG_ON(node->count);
358    inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
359    if (!__radix_tree_delete_node(&mapping->page_tree, node))
360        BUG();
361
362    spin_unlock(&mapping->tree_lock);
363    ret = LRU_REMOVED_RETRY;
364out:
365    local_irq_enable();
366    cond_resched();
367    local_irq_disable();
368    spin_lock(lru_lock);
369    return ret;
370}
371
372static unsigned long scan_shadow_nodes(struct shrinker *shrinker,
373                       struct shrink_control *sc)
374{
375    unsigned long ret;
376
377    /* list_lru lock nests inside IRQ-safe mapping->tree_lock */
378    local_irq_disable();
379    ret = list_lru_walk_node(&workingset_shadow_nodes, sc->nid,
380                  shadow_lru_isolate, NULL, &sc->nr_to_scan);
381    local_irq_enable();
382    return ret;
383}
384
385static struct shrinker workingset_shadow_shrinker = {
386    .count_objects = count_shadow_nodes,
387    .scan_objects = scan_shadow_nodes,
388    .seeks = DEFAULT_SEEKS,
389    .flags = SHRINKER_NUMA_AWARE,
390};
391
392/*
393 * Our list_lru->lock is IRQ-safe as it nests inside the IRQ-safe
394 * mapping->tree_lock.
395 */
396static struct lock_class_key shadow_nodes_key;
397
398static int __init workingset_init(void)
399{
400    int ret;
401
402    ret = list_lru_init_key(&workingset_shadow_nodes, &shadow_nodes_key);
403    if (ret)
404        goto err;
405    ret = register_shrinker(&workingset_shadow_shrinker);
406    if (ret)
407        goto err_list_lru;
408    return 0;
409err_list_lru:
410    list_lru_destroy(&workingset_shadow_nodes);
411err:
412    return ret;
413}
414module_init(workingset_init);
415

Archive Download this file



interactive