Root/lib/lru_cache.c

1/*
2   lru_cache.c
3
4   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
5
6   Copyright (C) 2003-2008, LINBIT Information Technologies GmbH.
7   Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>.
8   Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.
9
10   drbd is free software; you can redistribute it and/or modify
11   it under the terms of the GNU General Public License as published by
12   the Free Software Foundation; either version 2, or (at your option)
13   any later version.
14
15   drbd is distributed in the hope that it will be useful,
16   but WITHOUT ANY WARRANTY; without even the implied warranty of
17   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
18   GNU General Public License for more details.
19
20   You should have received a copy of the GNU General Public License
21   along with drbd; see the file COPYING. If not, write to
22   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
23
24 */
25
26#include <linux/module.h>
27#include <linux/bitops.h>
28#include <linux/slab.h>
29#include <linux/string.h> /* for memset */
30#include <linux/seq_file.h> /* for seq_printf */
31#include <linux/lru_cache.h>
32
33MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, "
34          "Lars Ellenberg <lars@linbit.com>");
35MODULE_DESCRIPTION("lru_cache - Track sets of hot objects");
36MODULE_LICENSE("GPL");
37
38/* this is developers aid only.
39 * it catches concurrent access (lack of locking on the users part) */
40#define PARANOIA_ENTRY() do { \
41    BUG_ON(!lc); \
42    BUG_ON(!lc->nr_elements); \
43    BUG_ON(test_and_set_bit(__LC_PARANOIA, &lc->flags)); \
44} while (0)
45
46#define RETURN(x...) do { \
47    clear_bit(__LC_PARANOIA, &lc->flags); \
48    smp_mb__after_clear_bit(); return x ; } while (0)
49
50/* BUG() if e is not one of the elements tracked by lc */
51#define PARANOIA_LC_ELEMENT(lc, e) do { \
52    struct lru_cache *lc_ = (lc); \
53    struct lc_element *e_ = (e); \
54    unsigned i = e_->lc_index; \
55    BUG_ON(i >= lc_->nr_elements); \
56    BUG_ON(lc_->lc_element[i] != e_); } while (0)
57
58/**
59 * lc_create - prepares to track objects in an active set
60 * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details
61 * @e_count: number of elements allowed to be active simultaneously
62 * @e_size: size of the tracked objects
63 * @e_off: offset to the &struct lc_element member in a tracked object
64 *
65 * Returns a pointer to a newly initialized struct lru_cache on success,
66 * or NULL on (allocation) failure.
67 */
68struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
69        unsigned e_count, size_t e_size, size_t e_off)
70{
71    struct hlist_head *slot = NULL;
72    struct lc_element **element = NULL;
73    struct lru_cache *lc;
74    struct lc_element *e;
75    unsigned cache_obj_size = kmem_cache_size(cache);
76    unsigned i;
77
78    WARN_ON(cache_obj_size < e_size);
79    if (cache_obj_size < e_size)
80        return NULL;
81
82    /* e_count too big; would probably fail the allocation below anyways.
83     * for typical use cases, e_count should be few thousand at most. */
84    if (e_count > LC_MAX_ACTIVE)
85        return NULL;
86
87    slot = kzalloc(e_count * sizeof(struct hlist_head*), GFP_KERNEL);
88    if (!slot)
89        goto out_fail;
90    element = kzalloc(e_count * sizeof(struct lc_element *), GFP_KERNEL);
91    if (!element)
92        goto out_fail;
93
94    lc = kzalloc(sizeof(*lc), GFP_KERNEL);
95    if (!lc)
96        goto out_fail;
97
98    INIT_LIST_HEAD(&lc->in_use);
99    INIT_LIST_HEAD(&lc->lru);
100    INIT_LIST_HEAD(&lc->free);
101
102    lc->name = name;
103    lc->element_size = e_size;
104    lc->element_off = e_off;
105    lc->nr_elements = e_count;
106    lc->new_number = LC_FREE;
107    lc->lc_cache = cache;
108    lc->lc_element = element;
109    lc->lc_slot = slot;
110
111    /* preallocate all objects */
112    for (i = 0; i < e_count; i++) {
113        void *p = kmem_cache_alloc(cache, GFP_KERNEL);
114        if (!p)
115            break;
116        memset(p, 0, lc->element_size);
117        e = p + e_off;
118        e->lc_index = i;
119        e->lc_number = LC_FREE;
120        list_add(&e->list, &lc->free);
121        element[i] = e;
122    }
123    if (i == e_count)
124        return lc;
125
126    /* else: could not allocate all elements, give up */
127    for (i--; i; i--) {
128        void *p = element[i];
129        kmem_cache_free(cache, p - e_off);
130    }
131    kfree(lc);
132out_fail:
133    kfree(element);
134    kfree(slot);
135    return NULL;
136}
137
138void lc_free_by_index(struct lru_cache *lc, unsigned i)
139{
140    void *p = lc->lc_element[i];
141    WARN_ON(!p);
142    if (p) {
143        p -= lc->element_off;
144        kmem_cache_free(lc->lc_cache, p);
145    }
146}
147
148/**
149 * lc_destroy - frees memory allocated by lc_create()
150 * @lc: the lru cache to destroy
151 */
152void lc_destroy(struct lru_cache *lc)
153{
154    unsigned i;
155    if (!lc)
156        return;
157    for (i = 0; i < lc->nr_elements; i++)
158        lc_free_by_index(lc, i);
159    kfree(lc->lc_element);
160    kfree(lc->lc_slot);
161    kfree(lc);
162}
163
164/**
165 * lc_reset - does a full reset for @lc and the hash table slots.
166 * @lc: the lru cache to operate on
167 *
168 * It is roughly the equivalent of re-allocating a fresh lru_cache object,
169 * basically a short cut to lc_destroy(lc); lc = lc_create(...);
170 */
171void lc_reset(struct lru_cache *lc)
172{
173    unsigned i;
174
175    INIT_LIST_HEAD(&lc->in_use);
176    INIT_LIST_HEAD(&lc->lru);
177    INIT_LIST_HEAD(&lc->free);
178    lc->used = 0;
179    lc->hits = 0;
180    lc->misses = 0;
181    lc->starving = 0;
182    lc->dirty = 0;
183    lc->changed = 0;
184    lc->flags = 0;
185    lc->changing_element = NULL;
186    lc->new_number = LC_FREE;
187    memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
188
189    for (i = 0; i < lc->nr_elements; i++) {
190        struct lc_element *e = lc->lc_element[i];
191        void *p = e;
192        p -= lc->element_off;
193        memset(p, 0, lc->element_size);
194        /* re-init it */
195        e->lc_index = i;
196        e->lc_number = LC_FREE;
197        list_add(&e->list, &lc->free);
198    }
199}
200
201/**
202 * lc_seq_printf_stats - print stats about @lc into @seq
203 * @seq: the seq_file to print into
204 * @lc: the lru cache to print statistics of
205 */
206size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
207{
208    /* NOTE:
209     * total calls to lc_get are
210     * (starving + hits + misses)
211     * misses include "dirty" count (update from an other thread in
212     * progress) and "changed", when this in fact lead to an successful
213     * update of the cache.
214     */
215    return seq_printf(seq, "\t%s: used:%u/%u "
216        "hits:%lu misses:%lu starving:%lu dirty:%lu changed:%lu\n",
217        lc->name, lc->used, lc->nr_elements,
218        lc->hits, lc->misses, lc->starving, lc->dirty, lc->changed);
219}
220
221static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
222{
223    return lc->lc_slot + (enr % lc->nr_elements);
224}
225
226
227/**
228 * lc_find - find element by label, if present in the hash table
229 * @lc: The lru_cache object
230 * @enr: element number
231 *
232 * Returns the pointer to an element, if the element with the requested
233 * "label" or element number is present in the hash table,
234 * or NULL if not found. Does not change the refcnt.
235 */
236struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
237{
238    struct hlist_node *n;
239    struct lc_element *e;
240
241    BUG_ON(!lc);
242    BUG_ON(!lc->nr_elements);
243    hlist_for_each_entry(e, n, lc_hash_slot(lc, enr), colision) {
244        if (e->lc_number == enr)
245            return e;
246    }
247    return NULL;
248}
249
250/* returned element will be "recycled" immediately */
251static struct lc_element *lc_evict(struct lru_cache *lc)
252{
253    struct list_head *n;
254    struct lc_element *e;
255
256    if (list_empty(&lc->lru))
257        return NULL;
258
259    n = lc->lru.prev;
260    e = list_entry(n, struct lc_element, list);
261
262    PARANOIA_LC_ELEMENT(lc, e);
263
264    list_del(&e->list);
265    hlist_del(&e->colision);
266    return e;
267}
268
269/**
270 * lc_del - removes an element from the cache
271 * @lc: The lru_cache object
272 * @e: The element to remove
273 *
274 * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list,
275 * sets @e->enr to %LC_FREE.
276 */
277void lc_del(struct lru_cache *lc, struct lc_element *e)
278{
279    PARANOIA_ENTRY();
280    PARANOIA_LC_ELEMENT(lc, e);
281    BUG_ON(e->refcnt);
282
283    e->lc_number = LC_FREE;
284    hlist_del_init(&e->colision);
285    list_move(&e->list, &lc->free);
286    RETURN();
287}
288
289static struct lc_element *lc_get_unused_element(struct lru_cache *lc)
290{
291    struct list_head *n;
292
293    if (list_empty(&lc->free))
294        return lc_evict(lc);
295
296    n = lc->free.next;
297    list_del(n);
298    return list_entry(n, struct lc_element, list);
299}
300
301static int lc_unused_element_available(struct lru_cache *lc)
302{
303    if (!list_empty(&lc->free))
304        return 1; /* something on the free list */
305    if (!list_empty(&lc->lru))
306        return 1; /* something to evict */
307
308    return 0;
309}
310
311
312/**
313 * lc_get - get element by label, maybe change the active set
314 * @lc: the lru cache to operate on
315 * @enr: the label to look up
316 *
317 * Finds an element in the cache, increases its usage count,
318 * "touches" and returns it.
319 *
320 * In case the requested number is not present, it needs to be added to the
321 * cache. Therefore it is possible that an other element becomes evicted from
322 * the cache. In either case, the user is notified so he is able to e.g. keep
323 * a persistent log of the cache changes, and therefore the objects in use.
324 *
325 * Return values:
326 * NULL
327 * The cache was marked %LC_STARVING,
328 * or the requested label was not in the active set
329 * and a changing transaction is still pending (@lc was marked %LC_DIRTY).
330 * Or no unused or free element could be recycled (@lc will be marked as
331 * %LC_STARVING, blocking further lc_get() operations).
332 *
333 * pointer to the element with the REQUESTED element number.
334 * In this case, it can be used right away
335 *
336 * pointer to an UNUSED element with some different element number,
337 * where that different number may also be %LC_FREE.
338 *
339 * In this case, the cache is marked %LC_DIRTY (blocking further changes),
340 * and the returned element pointer is removed from the lru list and
341 * hash collision chains. The user now should do whatever housekeeping
342 * is necessary.
343 * Then he must call lc_changed(lc,element_pointer), to finish
344 * the change.
345 *
346 * NOTE: The user needs to check the lc_number on EACH use, so he recognizes
347 * any cache set change.
348 */
349struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
350{
351    struct lc_element *e;
352
353    PARANOIA_ENTRY();
354    if (lc->flags & LC_STARVING) {
355        ++lc->starving;
356        RETURN(NULL);
357    }
358
359    e = lc_find(lc, enr);
360    if (e) {
361        ++lc->hits;
362        if (e->refcnt++ == 0)
363            lc->used++;
364        list_move(&e->list, &lc->in_use); /* Not evictable... */
365        RETURN(e);
366    }
367
368    ++lc->misses;
369
370    /* In case there is nothing available and we can not kick out
371     * the LRU element, we have to wait ...
372     */
373    if (!lc_unused_element_available(lc)) {
374        __set_bit(__LC_STARVING, &lc->flags);
375        RETURN(NULL);
376    }
377
378    /* it was not present in the active set.
379     * we are going to recycle an unused (or even "free") element.
380     * user may need to commit a transaction to record that change.
381     * we serialize on flags & TF_DIRTY */
382    if (test_and_set_bit(__LC_DIRTY, &lc->flags)) {
383        ++lc->dirty;
384        RETURN(NULL);
385    }
386
387    e = lc_get_unused_element(lc);
388    BUG_ON(!e);
389
390    clear_bit(__LC_STARVING, &lc->flags);
391    BUG_ON(++e->refcnt != 1);
392    lc->used++;
393
394    lc->changing_element = e;
395    lc->new_number = enr;
396
397    RETURN(e);
398}
399
400/* similar to lc_get,
401 * but only gets a new reference on an existing element.
402 * you either get the requested element, or NULL.
403 * will be consolidated into one function.
404 */
405struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
406{
407    struct lc_element *e;
408
409    PARANOIA_ENTRY();
410    if (lc->flags & LC_STARVING) {
411        ++lc->starving;
412        RETURN(NULL);
413    }
414
415    e = lc_find(lc, enr);
416    if (e) {
417        ++lc->hits;
418        if (e->refcnt++ == 0)
419            lc->used++;
420        list_move(&e->list, &lc->in_use); /* Not evictable... */
421    }
422    RETURN(e);
423}
424
425/**
426 * lc_changed - tell @lc that the change has been recorded
427 * @lc: the lru cache to operate on
428 * @e: the element pending label change
429 */
430void lc_changed(struct lru_cache *lc, struct lc_element *e)
431{
432    PARANOIA_ENTRY();
433    BUG_ON(e != lc->changing_element);
434    PARANOIA_LC_ELEMENT(lc, e);
435    ++lc->changed;
436    e->lc_number = lc->new_number;
437    list_add(&e->list, &lc->in_use);
438    hlist_add_head(&e->colision, lc_hash_slot(lc, lc->new_number));
439    lc->changing_element = NULL;
440    lc->new_number = LC_FREE;
441    clear_bit(__LC_DIRTY, &lc->flags);
442    smp_mb__after_clear_bit();
443    RETURN();
444}
445
446
447/**
448 * lc_put - give up refcnt of @e
449 * @lc: the lru cache to operate on
450 * @e: the element to put
451 *
452 * If refcnt reaches zero, the element is moved to the lru list,
453 * and a %LC_STARVING (if set) is cleared.
454 * Returns the new (post-decrement) refcnt.
455 */
456unsigned int lc_put(struct lru_cache *lc, struct lc_element *e)
457{
458    PARANOIA_ENTRY();
459    PARANOIA_LC_ELEMENT(lc, e);
460    BUG_ON(e->refcnt == 0);
461    BUG_ON(e == lc->changing_element);
462    if (--e->refcnt == 0) {
463        /* move it to the front of LRU. */
464        list_move(&e->list, &lc->lru);
465        lc->used--;
466        clear_bit(__LC_STARVING, &lc->flags);
467        smp_mb__after_clear_bit();
468    }
469    RETURN(e->refcnt);
470}
471
472/**
473 * lc_element_by_index
474 * @lc: the lru cache to operate on
475 * @i: the index of the element to return
476 */
477struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i)
478{
479    BUG_ON(i >= lc->nr_elements);
480    BUG_ON(lc->lc_element[i] == NULL);
481    BUG_ON(lc->lc_element[i]->lc_index != i);
482    return lc->lc_element[i];
483}
484
485/**
486 * lc_index_of
487 * @lc: the lru cache to operate on
488 * @e: the element to query for its index position in lc->element
489 */
490unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
491{
492    PARANOIA_LC_ELEMENT(lc, e);
493    return e->lc_index;
494}
495
496/**
497 * lc_set - associate index with label
498 * @lc: the lru cache to operate on
499 * @enr: the label to set
500 * @index: the element index to associate label with.
501 *
502 * Used to initialize the active set to some previously recorded state.
503 */
504void lc_set(struct lru_cache *lc, unsigned int enr, int index)
505{
506    struct lc_element *e;
507
508    if (index < 0 || index >= lc->nr_elements)
509        return;
510
511    e = lc_element_by_index(lc, index);
512    e->lc_number = enr;
513
514    hlist_del_init(&e->colision);
515    hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
516    list_move(&e->list, e->refcnt ? &lc->in_use : &lc->lru);
517}
518
519/**
520 * lc_dump - Dump a complete LRU cache to seq in textual form.
521 * @lc: the lru cache to operate on
522 * @seq: the &struct seq_file pointer to seq_printf into
523 * @utext: user supplied "heading" or other info
524 * @detail: function pointer the user may provide to dump further details
525 * of the object the lc_element is embedded in.
526 */
527void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext,
528         void (*detail) (struct seq_file *, struct lc_element *))
529{
530    unsigned int nr_elements = lc->nr_elements;
531    struct lc_element *e;
532    int i;
533
534    seq_printf(seq, "\tnn: lc_number refcnt %s\n ", utext);
535    for (i = 0; i < nr_elements; i++) {
536        e = lc_element_by_index(lc, i);
537        if (e->lc_number == LC_FREE) {
538            seq_printf(seq, "\t%2d: FREE\n", i);
539        } else {
540            seq_printf(seq, "\t%2d: %4u %4u ", i,
541                   e->lc_number, e->refcnt);
542            detail(seq, e);
543        }
544    }
545}
546
547EXPORT_SYMBOL(lc_create);
548EXPORT_SYMBOL(lc_reset);
549EXPORT_SYMBOL(lc_destroy);
550EXPORT_SYMBOL(lc_set);
551EXPORT_SYMBOL(lc_del);
552EXPORT_SYMBOL(lc_try_get);
553EXPORT_SYMBOL(lc_find);
554EXPORT_SYMBOL(lc_get);
555EXPORT_SYMBOL(lc_put);
556EXPORT_SYMBOL(lc_changed);
557EXPORT_SYMBOL(lc_element_by_index);
558EXPORT_SYMBOL(lc_index_of);
559EXPORT_SYMBOL(lc_seq_printf_stats);
560EXPORT_SYMBOL(lc_seq_dump_details);
561

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