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Linux/include/linux/lru_cache.h

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  1 /* SPDX-License-Identifier: GPL-2.0-or-later */
  2 /*
  3    lru_cache.c
  4 
  5    This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
  6 
  7    Copyright (C) 2003-2008, LINBIT Information Technologies GmbH.
  8    Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>.
  9    Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.
 10 
 11 
 12  */
 13 
 14 #ifndef LRU_CACHE_H
 15 #define LRU_CACHE_H
 16 
 17 #include <linux/list.h>
 18 #include <linux/slab.h>
 19 #include <linux/bitops.h>
 20 #include <linux/string.h> /* for memset */
 21 #include <linux/seq_file.h>
 22 
 23 /*
 24 This header file (and its .c file; kernel-doc of functions see there)
 25   define a helper framework to easily keep track of index:label associations,
 26   and changes to an "active set" of objects, as well as pending transactions,
 27   to persistently record those changes.
 28 
 29   We use an LRU policy if it is necessary to "cool down" a region currently in
 30   the active set before we can "heat" a previously unused region.
 31 
 32   Because of this later property, it is called "lru_cache".
 33   As it actually Tracks Objects in an Active SeT, we could also call it
 34   toast (incidentally that is what may happen to the data on the
 35   backend storage uppon next resync, if we don't get it right).
 36 
 37 What for?
 38 
 39 We replicate IO (more or less synchronously) to local and remote disk.
 40 
 41 For crash recovery after replication node failure,
 42   we need to resync all regions that have been target of in-flight WRITE IO
 43   (in use, or "hot", regions), as we don't know whether or not those WRITEs
 44   have made it to stable storage.
 45 
 46   To avoid a "full resync", we need to persistently track these regions.
 47 
 48   This is known as "write intent log", and can be implemented as on-disk
 49   (coarse or fine grained) bitmap, or other meta data.
 50 
 51   To avoid the overhead of frequent extra writes to this meta data area,
 52   usually the condition is softened to regions that _may_ have been target of
 53   in-flight WRITE IO, e.g. by only lazily clearing the on-disk write-intent
 54   bitmap, trading frequency of meta data transactions against amount of
 55   (possibly unnecessary) resync traffic.
 56 
 57   If we set a hard limit on the area that may be "hot" at any given time, we
 58   limit the amount of resync traffic needed for crash recovery.
 59 
 60 For recovery after replication link failure,
 61   we need to resync all blocks that have been changed on the other replica
 62   in the mean time, or, if both replica have been changed independently [*],
 63   all blocks that have been changed on either replica in the mean time.
 64   [*] usually as a result of a cluster split-brain and insufficient protection.
 65       but there are valid use cases to do this on purpose.
 66 
 67   Tracking those blocks can be implemented as "dirty bitmap".
 68   Having it fine-grained reduces the amount of resync traffic.
 69   It should also be persistent, to allow for reboots (or crashes)
 70   while the replication link is down.
 71 
 72 There are various possible implementations for persistently storing
 73 write intent log information, three of which are mentioned here.
 74 
 75 "Chunk dirtying"
 76   The on-disk "dirty bitmap" may be re-used as "write-intent" bitmap as well.
 77   To reduce the frequency of bitmap updates for write-intent log purposes,
 78   one could dirty "chunks" (of some size) at a time of the (fine grained)
 79   on-disk bitmap, while keeping the in-memory "dirty" bitmap as clean as
 80   possible, flushing it to disk again when a previously "hot" (and on-disk
 81   dirtied as full chunk) area "cools down" again (no IO in flight anymore,
 82   and none expected in the near future either).
 83 
 84 "Explicit (coarse) write intent bitmap"
 85   An other implementation could chose a (probably coarse) explicit bitmap,
 86   for write-intent log purposes, additionally to the fine grained dirty bitmap.
 87 
 88 "Activity log"
 89   Yet an other implementation may keep track of the hot regions, by starting
 90   with an empty set, and writing down a journal of region numbers that have
 91   become "hot", or have "cooled down" again.
 92 
 93   To be able to use a ring buffer for this journal of changes to the active
 94   set, we not only record the actual changes to that set, but also record the
 95   not changing members of the set in a round robin fashion. To do so, we use a
 96   fixed (but configurable) number of slots which we can identify by index, and
 97   associate region numbers (labels) with these indices.
 98   For each transaction recording a change to the active set, we record the
 99   change itself (index: -old_label, +new_label), and which index is associated
100   with which label (index: current_label) within a certain sliding window that
101   is moved further over the available indices with each such transaction.
102 
103   Thus, for crash recovery, if the ringbuffer is sufficiently large, we can
104   accurately reconstruct the active set.
105 
106   Sufficiently large depends only on maximum number of active objects, and the
107   size of the sliding window recording "index: current_label" associations within
108   each transaction.
109 
110   This is what we call the "activity log".
111 
112   Currently we need one activity log transaction per single label change, which
113   does not give much benefit over the "dirty chunks of bitmap" approach, other
114   than potentially less seeks.
115 
116   We plan to change the transaction format to support multiple changes per
117   transaction, which then would reduce several (disjoint, "random") updates to
118   the bitmap into one transaction to the activity log ring buffer.
119 */
120 
121 /* this defines an element in a tracked set
122  * .colision is for hash table lookup.
123  * When we process a new IO request, we know its sector, thus can deduce the
124  * region number (label) easily.  To do the label -> object lookup without a
125  * full list walk, we use a simple hash table.
126  *
127  * .list is on one of three lists:
128  *  in_use: currently in use (refcnt > 0, lc_number != LC_FREE)
129  *     lru: unused but ready to be reused or recycled
130  *          (lc_refcnt == 0, lc_number != LC_FREE),
131  *    free: unused but ready to be recycled
132  *          (lc_refcnt == 0, lc_number == LC_FREE),
133  *
134  * an element is said to be "in the active set",
135  * if either on "in_use" or "lru", i.e. lc_number != LC_FREE.
136  *
137  * DRBD currently (May 2009) only uses 61 elements on the resync lru_cache
138  * (total memory usage 2 pages), and up to 3833 elements on the act_log
139  * lru_cache, totalling ~215 kB for 64bit architecture, ~53 pages.
140  *
141  * We usually do not actually free these objects again, but only "recycle"
142  * them, as the change "index: -old_label, +LC_FREE" would need a transaction
143  * as well.  Which also means that using a kmem_cache to allocate the objects
144  * from wastes some resources.
145  * But it avoids high order page allocations in kmalloc.
146  */
147 struct lc_element {
148         struct hlist_node colision;
149         struct list_head list;           /* LRU list or free list */
150         unsigned refcnt;
151         /* back "pointer" into lc_cache->element[index],
152          * for paranoia, and for "lc_element_to_index" */
153         unsigned lc_index;
154         /* if we want to track a larger set of objects,
155          * it needs to become arch independend u64 */
156         unsigned lc_number;
157         /* special label when on free list */
158 #define LC_FREE (~0U)
159 
160         /* for pending changes */
161         unsigned lc_new_number;
162 };
163 
164 struct lru_cache {
165         /* the least recently used item is kept at lru->prev */
166         struct list_head lru;
167         struct list_head free;
168         struct list_head in_use;
169         struct list_head to_be_changed;
170 
171         /* the pre-created kmem cache to allocate the objects from */
172         struct kmem_cache *lc_cache;
173 
174         /* size of tracked objects, used to memset(,0,) them in lc_reset */
175         size_t element_size;
176         /* offset of struct lc_element member in the tracked object */
177         size_t element_off;
178 
179         /* number of elements (indices) */
180         unsigned int nr_elements;
181         /* Arbitrary limit on maximum tracked objects. Practical limit is much
182          * lower due to allocation failures, probably. For typical use cases,
183          * nr_elements should be a few thousand at most.
184          * This also limits the maximum value of lc_element.lc_index, allowing the
185          * 8 high bits of .lc_index to be overloaded with flags in the future. */
186 #define LC_MAX_ACTIVE   (1<<24)
187 
188         /* allow to accumulate a few (index:label) changes,
189          * but no more than max_pending_changes */
190         unsigned int max_pending_changes;
191         /* number of elements currently on to_be_changed list */
192         unsigned int pending_changes;
193 
194         /* statistics */
195         unsigned used; /* number of elements currently on in_use list */
196         unsigned long hits, misses, starving, locked, changed;
197 
198         /* see below: flag-bits for lru_cache */
199         unsigned long flags;
200 
201 
202         void  *lc_private;
203         const char *name;
204 
205         /* nr_elements there */
206         struct hlist_head *lc_slot;
207         struct lc_element **lc_element;
208 };
209 
210 
211 /* flag-bits for lru_cache */
212 enum {
213         /* debugging aid, to catch concurrent access early.
214          * user needs to guarantee exclusive access by proper locking! */
215         __LC_PARANOIA,
216 
217         /* annotate that the set is "dirty", possibly accumulating further
218          * changes, until a transaction is finally triggered */
219         __LC_DIRTY,
220 
221         /* Locked, no further changes allowed.
222          * Also used to serialize changing transactions. */
223         __LC_LOCKED,
224 
225         /* if we need to change the set, but currently there is no free nor
226          * unused element available, we are "starving", and must not give out
227          * further references, to guarantee that eventually some refcnt will
228          * drop to zero and we will be able to make progress again, changing
229          * the set, writing the transaction.
230          * if the statistics say we are frequently starving,
231          * nr_elements is too small. */
232         __LC_STARVING,
233 };
234 #define LC_PARANOIA (1<<__LC_PARANOIA)
235 #define LC_DIRTY    (1<<__LC_DIRTY)
236 #define LC_LOCKED   (1<<__LC_LOCKED)
237 #define LC_STARVING (1<<__LC_STARVING)
238 
239 extern struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
240                 unsigned max_pending_changes,
241                 unsigned e_count, size_t e_size, size_t e_off);
242 extern void lc_reset(struct lru_cache *lc);
243 extern void lc_destroy(struct lru_cache *lc);
244 extern void lc_set(struct lru_cache *lc, unsigned int enr, int index);
245 extern void lc_del(struct lru_cache *lc, struct lc_element *element);
246 
247 extern struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr);
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_committed(struct lru_cache *lc);
253 
254 struct seq_file;
255 extern void 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_for_transaction - can be used to stop lc_get() from changing the tracked set
262  * @lc: the lru cache to operate on
263  *
264  * Allows (expects) the set to be "dirty".  Note that the reference counts and
265  * order on the active and lru lists may still change.  Used to serialize
266  * changing transactions.  Returns true if we aquired the lock.
267  */
268 static inline int lc_try_lock_for_transaction(struct lru_cache *lc)
269 {
270         return !test_and_set_bit(__LC_LOCKED, &lc->flags);
271 }
272 
273 /**
274  * lc_try_lock - variant to stop lc_get() from changing the tracked set
275  * @lc: the lru cache to operate on
276  *
277  * Note that the reference counts and order on the active and lru lists may
278  * still change.  Only works on a "clean" set.  Returns true if we aquired the
279  * lock, which means there are no pending changes, and any further attempt to
280  * change the set will not succeed until the next lc_unlock().
281  */
282 extern int lc_try_lock(struct lru_cache *lc);
283 
284 /**
285  * lc_unlock - unlock @lc, allow lc_get() to change the set again
286  * @lc: the lru cache to operate on
287  */
288 static inline void lc_unlock(struct lru_cache *lc)
289 {
290         clear_bit(__LC_DIRTY, &lc->flags);
291         clear_bit_unlock(__LC_LOCKED, &lc->flags);
292 }
293 
294 extern bool lc_is_used(struct lru_cache *lc, unsigned int enr);
295 
296 #define lc_entry(ptr, type, member) \
297         container_of(ptr, type, member)
298 
299 extern struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i);
300 extern unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e);
301 
302 #endif
303 

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