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Diffstat (limited to 'include/linux/lru_cache.h')
| -rw-r--r-- | include/linux/lru_cache.h | 294 |
1 files changed, 294 insertions, 0 deletions
diff --git a/include/linux/lru_cache.h b/include/linux/lru_cache.h new file mode 100644 index 00000000..7a71ffad --- /dev/null +++ b/include/linux/lru_cache.h @@ -0,0 +1,294 @@ +/* + lru_cache.c + + This file is part of DRBD by Philipp Reisner and Lars Ellenberg. + + Copyright (C) 2003-2008, LINBIT Information Technologies GmbH. + Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>. + Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>. + + drbd is free software; you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 2, or (at your option) + any later version. + + drbd is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with drbd; see the file COPYING. If not, write to + the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. + + */ + +#ifndef LRU_CACHE_H +#define LRU_CACHE_H + +#include <linux/list.h> +#include <linux/slab.h> +#include <linux/bitops.h> +#include <linux/string.h> /* for memset */ +#include <linux/seq_file.h> + +/* +This header file (and its .c file; kernel-doc of functions see there) + define a helper framework to easily keep track of index:label associations, + and changes to an "active set" of objects, as well as pending transactions, + to persistently record those changes. + + We use an LRU policy if it is necessary to "cool down" a region currently in + the active set before we can "heat" a previously unused region. + + Because of this later property, it is called "lru_cache". + As it actually Tracks Objects in an Active SeT, we could also call it + toast (incidentally that is what may happen to the data on the + backend storage uppon next resync, if we don't get it right). + +What for? + +We replicate IO (more or less synchronously) to local and remote disk. + +For crash recovery after replication node failure, + we need to resync all regions that have been target of in-flight WRITE IO + (in use, or "hot", regions), as we don't know wether or not those WRITEs have + made it to stable storage. + + To avoid a "full resync", we need to persistently track these regions. + + This is known as "write intent log", and can be implemented as on-disk + (coarse or fine grained) bitmap, or other meta data. + + To avoid the overhead of frequent extra writes to this meta data area, + usually the condition is softened to regions that _may_ have been target of + in-flight WRITE IO, e.g. by only lazily clearing the on-disk write-intent + bitmap, trading frequency of meta data transactions against amount of + (possibly unnecessary) resync traffic. + + If we set a hard limit on the area that may be "hot" at any given time, we + limit the amount of resync traffic needed for crash recovery. + +For recovery after replication link failure, + we need to resync all blocks that have been changed on the other replica + in the mean time, or, if both replica have been changed independently [*], + all blocks that have been changed on either replica in the mean time. + [*] usually as a result of a cluster split-brain and insufficient protection. + but there are valid use cases to do this on purpose. + + Tracking those blocks can be implemented as "dirty bitmap". + Having it fine-grained reduces the amount of resync traffic. + It should also be persistent, to allow for reboots (or crashes) + while the replication link is down. + +There are various possible implementations for persistently storing +write intent log information, three of which are mentioned here. + +"Chunk dirtying" + The on-disk "dirty bitmap" may be re-used as "write-intent" bitmap as well. + To reduce the frequency of bitmap updates for write-intent log purposes, + one could dirty "chunks" (of some size) at a time of the (fine grained) + on-disk bitmap, while keeping the in-memory "dirty" bitmap as clean as + possible, flushing it to disk again when a previously "hot" (and on-disk + dirtied as full chunk) area "cools down" again (no IO in flight anymore, + and none expected in the near future either). + +"Explicit (coarse) write intent bitmap" + An other implementation could chose a (probably coarse) explicit bitmap, + for write-intent log purposes, additionally to the fine grained dirty bitmap. + +"Activity log" + Yet an other implementation may keep track of the hot regions, by starting + with an empty set, and writing down a journal of region numbers that have + become "hot", or have "cooled down" again. + + To be able to use a ring buffer for this journal of changes to the active + set, we not only record the actual changes to that set, but also record the + not changing members of the set in a round robin fashion. To do so, we use a + fixed (but configurable) number of slots which we can identify by index, and + associate region numbers (labels) with these indices. + For each transaction recording a change to the active set, we record the + change itself (index: -old_label, +new_label), and which index is associated + with which label (index: current_label) within a certain sliding window that + is moved further over the available indices with each such transaction. + + Thus, for crash recovery, if the ringbuffer is sufficiently large, we can + accurately reconstruct the active set. + + Sufficiently large depends only on maximum number of active objects, and the + size of the sliding window recording "index: current_label" associations within + each transaction. + + This is what we call the "activity log". + + Currently we need one activity log transaction per single label change, which + does not give much benefit over the "dirty chunks of bitmap" approach, other + than potentially less seeks. + + We plan to change the transaction format to support multiple changes per + transaction, which then would reduce several (disjoint, "random") updates to + the bitmap into one transaction to the activity log ring buffer. +*/ + +/* this defines an element in a tracked set + * .colision is for hash table lookup. + * When we process a new IO request, we know its sector, thus can deduce the + * region number (label) easily. To do the label -> object lookup without a + * full list walk, we use a simple hash table. + * + * .list is on one of three lists: + * in_use: currently in use (refcnt > 0, lc_number != LC_FREE) + * lru: unused but ready to be reused or recycled + * (lc_refcnt == 0, lc_number != LC_FREE), + * free: unused but ready to be recycled + * (lc_refcnt == 0, lc_number == LC_FREE), + * + * an element is said to be "in the active set", + * if either on "in_use" or "lru", i.e. lc_number != LC_FREE. + * + * DRBD currently (May 2009) only uses 61 elements on the resync lru_cache + * (total memory usage 2 pages), and up to 3833 elements on the act_log + * lru_cache, totalling ~215 kB for 64bit architecture, ~53 pages. + * + * We usually do not actually free these objects again, but only "recycle" + * them, as the change "index: -old_label, +LC_FREE" would need a transaction + * as well. Which also means that using a kmem_cache to allocate the objects + * from wastes some resources. + * But it avoids high order page allocations in kmalloc. + */ +struct lc_element { + struct hlist_node colision; + struct list_head list; /* LRU list or free list */ + unsigned refcnt; + /* back "pointer" into lc_cache->element[index], + * for paranoia, and for "lc_element_to_index" */ + unsigned lc_index; + /* if we want to track a larger set of objects, + * it needs to become arch independend u64 */ + unsigned lc_number; + + /* special label when on free list */ +#define LC_FREE (~0U) +}; + +struct lru_cache { + /* the least recently used item is kept at lru->prev */ + struct list_head lru; + struct list_head free; + struct list_head in_use; + + /* the pre-created kmem cache to allocate the objects from */ + struct kmem_cache *lc_cache; + + /* size of tracked objects, used to memset(,0,) them in lc_reset */ + size_t element_size; + /* offset of struct lc_element member in the tracked object */ + size_t element_off; + + /* number of elements (indices) */ + unsigned int nr_elements; + /* Arbitrary limit on maximum tracked objects. Practical limit is much + * lower due to allocation failures, probably. For typical use cases, + * nr_elements should be a few thousand at most. + * This also limits the maximum value of lc_element.lc_index, allowing the + * 8 high bits of .lc_index to be overloaded with flags in the future. */ +#define LC_MAX_ACTIVE (1<<24) + + /* statistics */ + unsigned used; /* number of lelements currently on in_use list */ + unsigned long hits, misses, starving, dirty, changed; + + /* see below: flag-bits for lru_cache */ + unsigned long flags; + + /* when changing the label of an index element */ + unsigned int new_number; + + /* for paranoia when changing the label of an index element */ + struct lc_element *changing_element; + + void *lc_private; + const char *name; + + /* nr_elements there */ + struct hlist_head *lc_slot; + struct lc_element **lc_element; +}; + + +/* flag-bits for lru_cache */ +enum { + /* debugging aid, to catch concurrent access early. + * user needs to guarantee exclusive access by proper locking! */ + __LC_PARANOIA, + /* if we need to change the set, but currently there is a changing + * transaction pending, we are "dirty", and must deferr further + * changing requests */ + __LC_DIRTY, + /* if we need to change the set, but currently there is no free nor + * unused element available, we are "starving", and must not give out + * further references, to guarantee that eventually some refcnt will + * drop to zero and we will be able to make progress again, changing + * the set, writing the transaction. + * if the statistics say we are frequently starving, + * nr_elements is too small. */ + __LC_STARVING, +}; +#define LC_PARANOIA (1<<__LC_PARANOIA) +#define LC_DIRTY (1<<__LC_DIRTY) +#define LC_STARVING (1<<__LC_STARVING) + +extern struct lru_cache *lc_create(const char *name, struct kmem_cache *cache, + unsigned e_count, size_t e_size, size_t e_off); +extern void lc_reset(struct lru_cache *lc); +extern void lc_destroy(struct lru_cache *lc); +extern void lc_set(struct lru_cache *lc, unsigned int enr, int index); +extern void lc_del(struct lru_cache *lc, struct lc_element *element); + +extern struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr); +extern struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr); +extern struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr); +extern unsigned int lc_put(struct lru_cache *lc, struct lc_element *e); +extern void lc_changed(struct lru_cache *lc, struct lc_element *e); + +struct seq_file; +extern size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc); + +extern void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext, + void (*detail) (struct seq_file *, struct lc_element *)); + +/** + * lc_try_lock - can be used to stop lc_get() from changing the tracked set + * @lc: the lru cache to operate on + * + * Note that the reference counts and order on the active and lru lists may + * still change. Returns true if we acquired the lock. + */ +static inline int lc_try_lock(struct lru_cache *lc) +{ + return !test_and_set_bit(__LC_DIRTY, &lc->flags); +} + +/** + * lc_unlock - unlock @lc, allow lc_get() to change the set again + * @lc: the lru cache to operate on + */ +static inline void lc_unlock(struct lru_cache *lc) +{ + clear_bit(__LC_DIRTY, &lc->flags); + smp_mb__after_clear_bit(); +} + +static inline int lc_is_used(struct lru_cache *lc, unsigned int enr) +{ + struct lc_element *e = lc_find(lc, enr); + return e && e->refcnt; +} + +#define lc_entry(ptr, type, member) \ + container_of(ptr, type, member) + +extern struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i); +extern unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e); + +#endif |