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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 | #ifndef LRU_CACHE_H | |
27 | #define LRU_CACHE_H | |
28 | ||
29 | #include <linux/list.h> | |
30 | #include <linux/slab.h> | |
31 | #include <linux/bitops.h> | |
32 | #include <linux/string.h> /* for memset */ | |
33 | #include <linux/seq_file.h> | |
34 | ||
35 | /* | |
36 | This header file (and its .c file; kernel-doc of functions see there) | |
37 | define a helper framework to easily keep track of index:label associations, | |
38 | and changes to an "active set" of objects, as well as pending transactions, | |
39 | to persistently record those changes. | |
40 | ||
41 | We use an LRU policy if it is necessary to "cool down" a region currently in | |
42 | the active set before we can "heat" a previously unused region. | |
43 | ||
44 | Because of this later property, it is called "lru_cache". | |
45 | As it actually Tracks Objects in an Active SeT, we could also call it | |
46 | toast (incidentally that is what may happen to the data on the | |
47 | backend storage uppon next resync, if we don't get it right). | |
48 | ||
49 | What for? | |
50 | ||
51 | We replicate IO (more or less synchronously) to local and remote disk. | |
52 | ||
53 | For crash recovery after replication node failure, | |
54 | we need to resync all regions that have been target of in-flight WRITE IO | |
55 | (in use, or "hot", regions), as we don't know wether or not those WRITEs have | |
56 | made it to stable storage. | |
57 | ||
58 | To avoid a "full resync", we need to persistently track these regions. | |
59 | ||
60 | This is known as "write intent log", and can be implemented as on-disk | |
61 | (coarse or fine grained) bitmap, or other meta data. | |
62 | ||
63 | To avoid the overhead of frequent extra writes to this meta data area, | |
64 | usually the condition is softened to regions that _may_ have been target of | |
65 | in-flight WRITE IO, e.g. by only lazily clearing the on-disk write-intent | |
66 | bitmap, trading frequency of meta data transactions against amount of | |
3ad2f3fb | 67 | (possibly unnecessary) resync traffic. |
b411b363 PR |
68 | |
69 | If we set a hard limit on the area that may be "hot" at any given time, we | |
70 | limit the amount of resync traffic needed for crash recovery. | |
71 | ||
72 | For recovery after replication link failure, | |
73 | we need to resync all blocks that have been changed on the other replica | |
74 | in the mean time, or, if both replica have been changed independently [*], | |
75 | all blocks that have been changed on either replica in the mean time. | |
76 | [*] usually as a result of a cluster split-brain and insufficient protection. | |
77 | but there are valid use cases to do this on purpose. | |
78 | ||
79 | Tracking those blocks can be implemented as "dirty bitmap". | |
80 | Having it fine-grained reduces the amount of resync traffic. | |
81 | It should also be persistent, to allow for reboots (or crashes) | |
82 | while the replication link is down. | |
83 | ||
84 | There are various possible implementations for persistently storing | |
85 | write intent log information, three of which are mentioned here. | |
86 | ||
87 | "Chunk dirtying" | |
88 | The on-disk "dirty bitmap" may be re-used as "write-intent" bitmap as well. | |
89 | To reduce the frequency of bitmap updates for write-intent log purposes, | |
90 | one could dirty "chunks" (of some size) at a time of the (fine grained) | |
91 | on-disk bitmap, while keeping the in-memory "dirty" bitmap as clean as | |
92 | possible, flushing it to disk again when a previously "hot" (and on-disk | |
93 | dirtied as full chunk) area "cools down" again (no IO in flight anymore, | |
94 | and none expected in the near future either). | |
95 | ||
96 | "Explicit (coarse) write intent bitmap" | |
97 | An other implementation could chose a (probably coarse) explicit bitmap, | |
98 | for write-intent log purposes, additionally to the fine grained dirty bitmap. | |
99 | ||
100 | "Activity log" | |
101 | Yet an other implementation may keep track of the hot regions, by starting | |
102 | with an empty set, and writing down a journal of region numbers that have | |
103 | become "hot", or have "cooled down" again. | |
104 | ||
105 | To be able to use a ring buffer for this journal of changes to the active | |
106 | set, we not only record the actual changes to that set, but also record the | |
107 | not changing members of the set in a round robin fashion. To do so, we use a | |
108 | fixed (but configurable) number of slots which we can identify by index, and | |
109 | associate region numbers (labels) with these indices. | |
110 | For each transaction recording a change to the active set, we record the | |
111 | change itself (index: -old_label, +new_label), and which index is associated | |
112 | with which label (index: current_label) within a certain sliding window that | |
113 | is moved further over the available indices with each such transaction. | |
114 | ||
115 | Thus, for crash recovery, if the ringbuffer is sufficiently large, we can | |
116 | accurately reconstruct the active set. | |
117 | ||
118 | Sufficiently large depends only on maximum number of active objects, and the | |
119 | size of the sliding window recording "index: current_label" associations within | |
120 | each transaction. | |
121 | ||
122 | This is what we call the "activity log". | |
123 | ||
124 | Currently we need one activity log transaction per single label change, which | |
125 | does not give much benefit over the "dirty chunks of bitmap" approach, other | |
126 | than potentially less seeks. | |
127 | ||
128 | We plan to change the transaction format to support multiple changes per | |
129 | transaction, which then would reduce several (disjoint, "random") updates to | |
130 | the bitmap into one transaction to the activity log ring buffer. | |
131 | */ | |
132 | ||
133 | /* this defines an element in a tracked set | |
134 | * .colision is for hash table lookup. | |
135 | * When we process a new IO request, we know its sector, thus can deduce the | |
136 | * region number (label) easily. To do the label -> object lookup without a | |
137 | * full list walk, we use a simple hash table. | |
138 | * | |
139 | * .list is on one of three lists: | |
140 | * in_use: currently in use (refcnt > 0, lc_number != LC_FREE) | |
141 | * lru: unused but ready to be reused or recycled | |
600942e0 | 142 | * (lc_refcnt == 0, lc_number != LC_FREE), |
b411b363 | 143 | * free: unused but ready to be recycled |
600942e0 | 144 | * (lc_refcnt == 0, lc_number == LC_FREE), |
b411b363 PR |
145 | * |
146 | * an element is said to be "in the active set", | |
147 | * if either on "in_use" or "lru", i.e. lc_number != LC_FREE. | |
148 | * | |
149 | * DRBD currently (May 2009) only uses 61 elements on the resync lru_cache | |
150 | * (total memory usage 2 pages), and up to 3833 elements on the act_log | |
25985edc | 151 | * lru_cache, totalling ~215 kB for 64bit architecture, ~53 pages. |
b411b363 PR |
152 | * |
153 | * We usually do not actually free these objects again, but only "recycle" | |
154 | * them, as the change "index: -old_label, +LC_FREE" would need a transaction | |
155 | * as well. Which also means that using a kmem_cache to allocate the objects | |
156 | * from wastes some resources. | |
157 | * But it avoids high order page allocations in kmalloc. | |
158 | */ | |
159 | struct lc_element { | |
160 | struct hlist_node colision; | |
161 | struct list_head list; /* LRU list or free list */ | |
162 | unsigned refcnt; | |
600942e0 LE |
163 | /* back "pointer" into lc_cache->element[index], |
164 | * for paranoia, and for "lc_element_to_index" */ | |
b411b363 PR |
165 | unsigned lc_index; |
166 | /* if we want to track a larger set of objects, | |
167 | * it needs to become arch independend u64 */ | |
168 | unsigned lc_number; | |
169 | ||
170 | /* special label when on free list */ | |
171 | #define LC_FREE (~0U) | |
172 | }; | |
173 | ||
174 | struct lru_cache { | |
175 | /* the least recently used item is kept at lru->prev */ | |
176 | struct list_head lru; | |
177 | struct list_head free; | |
178 | struct list_head in_use; | |
179 | ||
180 | /* the pre-created kmem cache to allocate the objects from */ | |
181 | struct kmem_cache *lc_cache; | |
182 | ||
183 | /* size of tracked objects, used to memset(,0,) them in lc_reset */ | |
184 | size_t element_size; | |
185 | /* offset of struct lc_element member in the tracked object */ | |
186 | size_t element_off; | |
187 | ||
188 | /* number of elements (indices) */ | |
189 | unsigned int nr_elements; | |
190 | /* Arbitrary limit on maximum tracked objects. Practical limit is much | |
191 | * lower due to allocation failures, probably. For typical use cases, | |
192 | * nr_elements should be a few thousand at most. | |
600942e0 LE |
193 | * This also limits the maximum value of lc_element.lc_index, allowing the |
194 | * 8 high bits of .lc_index to be overloaded with flags in the future. */ | |
b411b363 PR |
195 | #define LC_MAX_ACTIVE (1<<24) |
196 | ||
197 | /* statistics */ | |
198 | unsigned used; /* number of lelements currently on in_use list */ | |
199 | unsigned long hits, misses, starving, dirty, changed; | |
200 | ||
201 | /* see below: flag-bits for lru_cache */ | |
202 | unsigned long flags; | |
203 | ||
204 | /* when changing the label of an index element */ | |
205 | unsigned int new_number; | |
206 | ||
207 | /* for paranoia when changing the label of an index element */ | |
208 | struct lc_element *changing_element; | |
209 | ||
210 | void *lc_private; | |
211 | const char *name; | |
212 | ||
213 | /* nr_elements there */ | |
214 | struct hlist_head *lc_slot; | |
215 | struct lc_element **lc_element; | |
216 | }; | |
217 | ||
218 | ||
219 | /* flag-bits for lru_cache */ | |
220 | enum { | |
221 | /* debugging aid, to catch concurrent access early. | |
222 | * user needs to guarantee exclusive access by proper locking! */ | |
223 | __LC_PARANOIA, | |
224 | /* if we need to change the set, but currently there is a changing | |
225 | * transaction pending, we are "dirty", and must deferr further | |
226 | * changing requests */ | |
227 | __LC_DIRTY, | |
228 | /* if we need to change the set, but currently there is no free nor | |
229 | * unused element available, we are "starving", and must not give out | |
230 | * further references, to guarantee that eventually some refcnt will | |
231 | * drop to zero and we will be able to make progress again, changing | |
232 | * the set, writing the transaction. | |
233 | * if the statistics say we are frequently starving, | |
234 | * nr_elements is too small. */ | |
235 | __LC_STARVING, | |
236 | }; | |
237 | #define LC_PARANOIA (1<<__LC_PARANOIA) | |
238 | #define LC_DIRTY (1<<__LC_DIRTY) | |
239 | #define LC_STARVING (1<<__LC_STARVING) | |
240 | ||
241 | extern struct lru_cache *lc_create(const char *name, struct kmem_cache *cache, | |
242 | unsigned e_count, size_t e_size, size_t e_off); | |
243 | extern void lc_reset(struct lru_cache *lc); | |
244 | extern void lc_destroy(struct lru_cache *lc); | |
245 | extern void lc_set(struct lru_cache *lc, unsigned int enr, int index); | |
246 | extern void lc_del(struct lru_cache *lc, struct lc_element *element); | |
247 | ||
248 | extern struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr); | |
249 | extern struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr); | |
250 | extern struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr); | |
251 | extern unsigned int lc_put(struct lru_cache *lc, struct lc_element *e); | |
252 | extern void lc_changed(struct lru_cache *lc, struct lc_element *e); | |
253 | ||
254 | struct seq_file; | |
255 | extern size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc); | |
256 | ||
257 | extern void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext, | |
258 | void (*detail) (struct seq_file *, struct lc_element *)); | |
259 | ||
260 | /** | |
261 | * lc_try_lock - can be used to stop lc_get() from changing the tracked set | |
262 | * @lc: the lru cache to operate on | |
263 | * | |
264 | * Note that the reference counts and order on the active and lru lists may | |
46cd09a7 | 265 | * still change. Returns true if we acquired the lock. |
b411b363 PR |
266 | */ |
267 | static inline int lc_try_lock(struct lru_cache *lc) | |
268 | { | |
269 | return !test_and_set_bit(__LC_DIRTY, &lc->flags); | |
270 | } | |
271 | ||
272 | /** | |
273 | * lc_unlock - unlock @lc, allow lc_get() to change the set again | |
274 | * @lc: the lru cache to operate on | |
275 | */ | |
276 | static inline void lc_unlock(struct lru_cache *lc) | |
277 | { | |
4738fa16 | 278 | clear_bit_unlock(__LC_DIRTY, &lc->flags); |
b411b363 PR |
279 | } |
280 | ||
281 | static inline int lc_is_used(struct lru_cache *lc, unsigned int enr) | |
282 | { | |
283 | struct lc_element *e = lc_find(lc, enr); | |
284 | return e && e->refcnt; | |
285 | } | |
286 | ||
287 | #define lc_entry(ptr, type, member) \ | |
288 | container_of(ptr, type, member) | |
289 | ||
290 | extern struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i); | |
291 | extern unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e); | |
292 | ||
293 | #endif |