FreeBSD/Linux Kernel Cross Reference
sys/lib/lru_cache.c

     /*
        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.
    
     */
    
    #include <linux/module.h>
    #include <linux/bitops.h>
    #include <linux/slab.h>
    #include <linux/string.h> /* for memset */
    #include <linux/seq_file.h> /* for seq_printf */
    #include <linux/lru_cache.h>
    
    MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, "
                  "Lars Ellenberg <lars@linbit.com>");
    MODULE_DESCRIPTION("lru_cache - Track sets of hot objects");
    MODULE_LICENSE("GPL");
    
    /* this is developers aid only.
     * it catches concurrent access (lack of locking on the users part) */
    #define PARANOIA_ENTRY() do {           \
            BUG_ON(!lc);                    \
            BUG_ON(!lc->nr_elements);       \
            BUG_ON(test_and_set_bit(__LC_PARANOIA, &lc->flags)); \
    } while (0)
    
    #define RETURN(x...)     do { \
            clear_bit_unlock(__LC_PARANOIA, &lc->flags); \
            return x ; } while (0)
    
    /* BUG() if e is not one of the elements tracked by lc */
    #define PARANOIA_LC_ELEMENT(lc, e) do { \
            struct lru_cache *lc_ = (lc);   \
            struct lc_element *e_ = (e);    \
            unsigned i = e_->lc_index;      \
            BUG_ON(i >= lc_->nr_elements);  \
            BUG_ON(lc_->lc_element[i] != e_); } while (0)
    
    
    /* We need to atomically
     *  - try to grab the lock (set LC_LOCKED)
     *  - only if there is no pending transaction
     *    (neither LC_DIRTY nor LC_STARVING is set)
     * Because of PARANOIA_ENTRY() above abusing lc->flags as well,
     * it is not sufficient to just say
     *      return 0 == cmpxchg(&lc->flags, 0, LC_LOCKED);
     */
    int lc_try_lock(struct lru_cache *lc)
    {
            unsigned long val;
            do {
                    val = cmpxchg(&lc->flags, 0, LC_LOCKED);
            } while (unlikely (val == LC_PARANOIA));
            /* Spin until no-one is inside a PARANOIA_ENTRY()/RETURN() section. */
            return 0 == val;
    #if 0
            /* Alternative approach, spin in case someone enters or leaves a
             * PARANOIA_ENTRY()/RETURN() section. */
            unsigned long old, new, val;
            do {
                    old = lc->flags & LC_PARANOIA;
                    new = old | LC_LOCKED;
                    val = cmpxchg(&lc->flags, old, new);
            } while (unlikely (val == (old ^ LC_PARANOIA)));
            return old == val;
    #endif
    }
    
    /**
     * lc_create - prepares to track objects in an active set
     * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details
     * @max_pending_changes: maximum changes to accumulate until a transaction is required
     * @e_count: number of elements allowed to be active simultaneously
     * @e_size: size of the tracked objects
     * @e_off: offset to the &struct lc_element member in a tracked object
     *
     * Returns a pointer to a newly initialized struct lru_cache on success,
     * or NULL on (allocation) failure.
     */
    struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
                   unsigned max_pending_changes,
                   unsigned e_count, size_t e_size, size_t e_off)
   {
           struct hlist_head *slot = NULL;
           struct lc_element **element = NULL;
           struct lru_cache *lc;
           struct lc_element *e;
           unsigned cache_obj_size = kmem_cache_size(cache);
           unsigned i;
   
           WARN_ON(cache_obj_size < e_size);
           if (cache_obj_size < e_size)
                   return NULL;
   
           /* e_count too big; would probably fail the allocation below anyways.
            * for typical use cases, e_count should be few thousand at most. */
           if (e_count > LC_MAX_ACTIVE)
                   return NULL;
   
           slot = kcalloc(e_count, sizeof(struct hlist_head), GFP_KERNEL);
           if (!slot)
                   goto out_fail;
           element = kzalloc(e_count * sizeof(struct lc_element *), GFP_KERNEL);
           if (!element)
                   goto out_fail;
   
           lc = kzalloc(sizeof(*lc), GFP_KERNEL);
           if (!lc)
                   goto out_fail;
   
           INIT_LIST_HEAD(&lc->in_use);
           INIT_LIST_HEAD(&lc->lru);
           INIT_LIST_HEAD(&lc->free);
           INIT_LIST_HEAD(&lc->to_be_changed);
   
           lc->name = name;
           lc->element_size = e_size;
           lc->element_off = e_off;
           lc->nr_elements = e_count;
           lc->max_pending_changes = max_pending_changes;
           lc->lc_cache = cache;
           lc->lc_element = element;
           lc->lc_slot = slot;
   
           /* preallocate all objects */
           for (i = 0; i < e_count; i++) {
                   void *p = kmem_cache_alloc(cache, GFP_KERNEL);
                   if (!p)
                           break;
                   memset(p, 0, lc->element_size);
                   e = p + e_off;
                   e->lc_index = i;
                   e->lc_number = LC_FREE;
                   e->lc_new_number = LC_FREE;
                   list_add(&e->list, &lc->free);
                   element[i] = e;
           }
           if (i == e_count)
                   return lc;
   
           /* else: could not allocate all elements, give up */
           for (i--; i; i--) {
                   void *p = element[i];
                   kmem_cache_free(cache, p - e_off);
           }
           kfree(lc);
   out_fail:
           kfree(element);
           kfree(slot);
           return NULL;
   }
   
   void lc_free_by_index(struct lru_cache *lc, unsigned i)
   {
           void *p = lc->lc_element[i];
           WARN_ON(!p);
           if (p) {
                   p -= lc->element_off;
                   kmem_cache_free(lc->lc_cache, p);
           }
   }
   
   /**
    * lc_destroy - frees memory allocated by lc_create()
    * @lc: the lru cache to destroy
    */
   void lc_destroy(struct lru_cache *lc)
   {
           unsigned i;
           if (!lc)
                   return;
           for (i = 0; i < lc->nr_elements; i++)
                   lc_free_by_index(lc, i);
           kfree(lc->lc_element);
           kfree(lc->lc_slot);
           kfree(lc);
   }
   
   /**
    * lc_reset - does a full reset for @lc and the hash table slots.
    * @lc: the lru cache to operate on
    *
    * It is roughly the equivalent of re-allocating a fresh lru_cache object,
    * basically a short cut to lc_destroy(lc); lc = lc_create(...);
    */
   void lc_reset(struct lru_cache *lc)
   {
           unsigned i;
   
           INIT_LIST_HEAD(&lc->in_use);
           INIT_LIST_HEAD(&lc->lru);
           INIT_LIST_HEAD(&lc->free);
           INIT_LIST_HEAD(&lc->to_be_changed);
           lc->used = 0;
           lc->hits = 0;
           lc->misses = 0;
           lc->starving = 0;
           lc->locked = 0;
           lc->changed = 0;
           lc->pending_changes = 0;
           lc->flags = 0;
           memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
   
           for (i = 0; i < lc->nr_elements; i++) {
                   struct lc_element *e = lc->lc_element[i];
                   void *p = e;
                   p -= lc->element_off;
                   memset(p, 0, lc->element_size);
                   /* re-init it */
                   e->lc_index = i;
                   e->lc_number = LC_FREE;
                   e->lc_new_number = LC_FREE;
                   list_add(&e->list, &lc->free);
           }
   }
   
   /**
    * lc_seq_printf_stats - print stats about @lc into @seq
    * @seq: the seq_file to print into
    * @lc: the lru cache to print statistics of
    */
   size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
   {
           /* NOTE:
            * total calls to lc_get are
            * (starving + hits + misses)
            * misses include "locked" count (update from an other thread in
            * progress) and "changed", when this in fact lead to an successful
            * update of the cache.
            */
           return seq_printf(seq, "\t%s: used:%u/%u "
                   "hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu\n",
                   lc->name, lc->used, lc->nr_elements,
                   lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
   }
   
   static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
   {
           return  lc->lc_slot + (enr % lc->nr_elements);
   }
   
   
   static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr,
                   bool include_changing)
   {
           struct hlist_node *n;
           struct lc_element *e;
   
           BUG_ON(!lc);
           BUG_ON(!lc->nr_elements);
           hlist_for_each_entry(e, n, lc_hash_slot(lc, enr), colision) {
                   /* "about to be changed" elements, pending transaction commit,
                    * are hashed by their "new number". "Normal" elements have
                    * lc_number == lc_new_number. */
                   if (e->lc_new_number != enr)
                           continue;
                   if (e->lc_new_number == e->lc_number || include_changing)
                           return e;
                   break;
           }
           return NULL;
   }
   
   /**
    * lc_find - find element by label, if present in the hash table
    * @lc: The lru_cache object
    * @enr: element number
    *
    * Returns the pointer to an element, if the element with the requested
    * "label" or element number is present in the hash table,
    * or NULL if not found. Does not change the refcnt.
    * Ignores elements that are "about to be used", i.e. not yet in the active
    * set, but still pending transaction commit.
    */
   struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
   {
           return __lc_find(lc, enr, 0);
   }
   
   /**
    * lc_is_used - find element by label
    * @lc: The lru_cache object
    * @enr: element number
    *
    * Returns true, if the element with the requested "label" or element number is
    * present in the hash table, and is used (refcnt > 0).
    * Also finds elements that are not _currently_ used but only "about to be
    * used", i.e. on the "to_be_changed" list, pending transaction commit.
    */
   bool lc_is_used(struct lru_cache *lc, unsigned int enr)
   {
           struct lc_element *e = __lc_find(lc, enr, 1);
           return e && e->refcnt;
   }
   
   /**
    * lc_del - removes an element from the cache
    * @lc: The lru_cache object
    * @e: The element to remove
    *
    * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list,
    * sets @e->enr to %LC_FREE.
    */
   void lc_del(struct lru_cache *lc, struct lc_element *e)
   {
           PARANOIA_ENTRY();
           PARANOIA_LC_ELEMENT(lc, e);
           BUG_ON(e->refcnt);
   
           e->lc_number = e->lc_new_number = LC_FREE;
           hlist_del_init(&e->colision);
           list_move(&e->list, &lc->free);
           RETURN();
   }
   
   static struct lc_element *lc_prepare_for_change(struct lru_cache *lc, unsigned new_number)
   {
           struct list_head *n;
           struct lc_element *e;
   
           if (!list_empty(&lc->free))
                   n = lc->free.next;
           else if (!list_empty(&lc->lru))
                   n = lc->lru.prev;
           else
                   return NULL;
   
           e = list_entry(n, struct lc_element, list);
           PARANOIA_LC_ELEMENT(lc, e);
   
           e->lc_new_number = new_number;
           if (!hlist_unhashed(&e->colision))
                   __hlist_del(&e->colision);
           hlist_add_head(&e->colision, lc_hash_slot(lc, new_number));
           list_move(&e->list, &lc->to_be_changed);
   
           return e;
   }
   
   static int lc_unused_element_available(struct lru_cache *lc)
   {
           if (!list_empty(&lc->free))
                   return 1; /* something on the free list */
           if (!list_empty(&lc->lru))
                   return 1;  /* something to evict */
   
           return 0;
   }
   
   static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool may_change)
   {
           struct lc_element *e;
   
           PARANOIA_ENTRY();
           if (lc->flags & LC_STARVING) {
                   ++lc->starving;
                   RETURN(NULL);
           }
   
           e = __lc_find(lc, enr, 1);
           /* if lc_new_number != lc_number,
            * this enr is currently being pulled in already,
            * and will be available once the pending transaction
            * has been committed. */
           if (e && e->lc_new_number == e->lc_number) {
                   ++lc->hits;
                   if (e->refcnt++ == 0)
                           lc->used++;
                   list_move(&e->list, &lc->in_use); /* Not evictable... */
                   RETURN(e);
           }
   
           ++lc->misses;
           if (!may_change)
                   RETURN(NULL);
   
           /* It has been found above, but on the "to_be_changed" list, not yet
            * committed.  Don't pull it in twice, wait for the transaction, then
            * try again */
           if (e)
                   RETURN(NULL);
   
           /* To avoid races with lc_try_lock(), first, mark us dirty
            * (using test_and_set_bit, as it implies memory barriers), ... */
           test_and_set_bit(__LC_DIRTY, &lc->flags);
   
           /* ... only then check if it is locked anyways. If lc_unlock clears
            * the dirty bit again, that's not a problem, we will come here again.
            */
           if (test_bit(__LC_LOCKED, &lc->flags)) {
                   ++lc->locked;
                   RETURN(NULL);
           }
   
           /* In case there is nothing available and we can not kick out
            * the LRU element, we have to wait ...
            */
           if (!lc_unused_element_available(lc)) {
                   __set_bit(__LC_STARVING, &lc->flags);
                   RETURN(NULL);
           }
   
           /* It was not present in the active set.  We are going to recycle an
            * unused (or even "free") element, but we won't accumulate more than
            * max_pending_changes changes.  */
           if (lc->pending_changes >= lc->max_pending_changes)
                   RETURN(NULL);
   
           e = lc_prepare_for_change(lc, enr);
           BUG_ON(!e);
   
           clear_bit(__LC_STARVING, &lc->flags);
           BUG_ON(++e->refcnt != 1);
           lc->used++;
           lc->pending_changes++;
   
           RETURN(e);
   }
   
   /**
    * lc_get - get element by label, maybe change the active set
    * @lc: the lru cache to operate on
    * @enr: the label to look up
    *
    * Finds an element in the cache, increases its usage count,
    * "touches" and returns it.
    *
    * In case the requested number is not present, it needs to be added to the
    * cache. Therefore it is possible that an other element becomes evicted from
    * the cache. In either case, the user is notified so he is able to e.g. keep
    * a persistent log of the cache changes, and therefore the objects in use.
    *
    * Return values:
    *  NULL
    *     The cache was marked %LC_STARVING,
    *     or the requested label was not in the active set
    *     and a changing transaction is still pending (@lc was marked %LC_DIRTY).
    *     Or no unused or free element could be recycled (@lc will be marked as
    *     %LC_STARVING, blocking further lc_get() operations).
    *
    *  pointer to the element with the REQUESTED element number.
    *     In this case, it can be used right away
    *
    *  pointer to an UNUSED element with some different element number,
    *          where that different number may also be %LC_FREE.
    *
    *          In this case, the cache is marked %LC_DIRTY,
    *          so lc_try_lock() will no longer succeed.
    *          The returned element pointer is moved to the "to_be_changed" list,
    *          and registered with the new element number on the hash collision chains,
    *          so it is possible to pick it up from lc_is_used().
    *          Up to "max_pending_changes" (see lc_create()) can be accumulated.
    *          The user now should do whatever housekeeping is necessary,
    *          typically serialize on lc_try_lock_for_transaction(), then call
    *          lc_committed(lc) and lc_unlock(), to finish the change.
    *
    * NOTE: The user needs to check the lc_number on EACH use, so he recognizes
    *       any cache set change.
    */
   struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
   {
           return __lc_get(lc, enr, 1);
   }
   
   /**
    * lc_try_get - get element by label, if present; do not change the active set
    * @lc: the lru cache to operate on
    * @enr: the label to look up
    *
    * Finds an element in the cache, increases its usage count,
    * "touches" and returns it.
    *
    * Return values:
    *  NULL
    *     The cache was marked %LC_STARVING,
    *     or the requested label was not in the active set
    *
    *  pointer to the element with the REQUESTED element number.
    *     In this case, it can be used right away
    */
   struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
   {
           return __lc_get(lc, enr, 0);
   }
   
   /**
    * lc_committed - tell @lc that pending changes have been recorded
    * @lc: the lru cache to operate on
    *
    * User is expected to serialize on explicit lc_try_lock_for_transaction()
    * before the transaction is started, and later needs to lc_unlock() explicitly
    * as well.
    */
   void lc_committed(struct lru_cache *lc)
   {
           struct lc_element *e, *tmp;
   
           PARANOIA_ENTRY();
           list_for_each_entry_safe(e, tmp, &lc->to_be_changed, list) {
                   /* count number of changes, not number of transactions */
                   ++lc->changed;
                   e->lc_number = e->lc_new_number;
                   list_move(&e->list, &lc->in_use);
           }
           lc->pending_changes = 0;
           RETURN();
   }
   
   
   /**
    * lc_put - give up refcnt of @e
    * @lc: the lru cache to operate on
    * @e: the element to put
    *
    * If refcnt reaches zero, the element is moved to the lru list,
    * and a %LC_STARVING (if set) is cleared.
    * Returns the new (post-decrement) refcnt.
    */
   unsigned int lc_put(struct lru_cache *lc, struct lc_element *e)
   {
           PARANOIA_ENTRY();
           PARANOIA_LC_ELEMENT(lc, e);
           BUG_ON(e->refcnt == 0);
           BUG_ON(e->lc_number != e->lc_new_number);
           if (--e->refcnt == 0) {
                   /* move it to the front of LRU. */
                   list_move(&e->list, &lc->lru);
                   lc->used--;
                   clear_bit_unlock(__LC_STARVING, &lc->flags);
           }
           RETURN(e->refcnt);
   }
   
   /**
    * lc_element_by_index
    * @lc: the lru cache to operate on
    * @i: the index of the element to return
    */
   struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i)
   {
           BUG_ON(i >= lc->nr_elements);
           BUG_ON(lc->lc_element[i] == NULL);
           BUG_ON(lc->lc_element[i]->lc_index != i);
           return lc->lc_element[i];
   }
   
   /**
    * lc_index_of
    * @lc: the lru cache to operate on
    * @e: the element to query for its index position in lc->element
    */
   unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
   {
           PARANOIA_LC_ELEMENT(lc, e);
           return e->lc_index;
   }
   
   /**
    * lc_set - associate index with label
    * @lc: the lru cache to operate on
    * @enr: the label to set
    * @index: the element index to associate label with.
    *
    * Used to initialize the active set to some previously recorded state.
    */
   void lc_set(struct lru_cache *lc, unsigned int enr, int index)
   {
           struct lc_element *e;
           struct list_head *lh;
   
           if (index < 0 || index >= lc->nr_elements)
                   return;
   
           e = lc_element_by_index(lc, index);
           BUG_ON(e->lc_number != e->lc_new_number);
           BUG_ON(e->refcnt != 0);
   
           e->lc_number = e->lc_new_number = enr;
           hlist_del_init(&e->colision);
           if (enr == LC_FREE)
                   lh = &lc->free;
           else {
                   hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
                   lh = &lc->lru;
           }
           list_move(&e->list, lh);
   }
   
   /**
    * lc_dump - Dump a complete LRU cache to seq in textual form.
    * @lc: the lru cache to operate on
    * @seq: the &struct seq_file pointer to seq_printf into
    * @utext: user supplied "heading" or other info
    * @detail: function pointer the user may provide to dump further details
    * of the object the lc_element is embedded in.
    */
   void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext,
                void (*detail) (struct seq_file *, struct lc_element *))
   {
           unsigned int nr_elements = lc->nr_elements;
           struct lc_element *e;
           int i;
   
           seq_printf(seq, "\tnn: lc_number refcnt %s\n ", utext);
           for (i = 0; i < nr_elements; i++) {
                   e = lc_element_by_index(lc, i);
                   if (e->lc_number == LC_FREE) {
                           seq_printf(seq, "\t%2d: FREE\n", i);
                   } else {
                           seq_printf(seq, "\t%2d: %4u %4u    ", i,
                                      e->lc_number, e->refcnt);
                           detail(seq, e);
                   }
           }
   }
   
   EXPORT_SYMBOL(lc_create);
   EXPORT_SYMBOL(lc_reset);
   EXPORT_SYMBOL(lc_destroy);
   EXPORT_SYMBOL(lc_set);
   EXPORT_SYMBOL(lc_del);
   EXPORT_SYMBOL(lc_try_get);
   EXPORT_SYMBOL(lc_find);
   EXPORT_SYMBOL(lc_get);
   EXPORT_SYMBOL(lc_put);
   EXPORT_SYMBOL(lc_committed);
   EXPORT_SYMBOL(lc_element_by_index);
   EXPORT_SYMBOL(lc_index_of);
   EXPORT_SYMBOL(lc_seq_printf_stats);
   EXPORT_SYMBOL(lc_seq_dump_details);
   EXPORT_SYMBOL(lc_try_lock);
   EXPORT_SYMBOL(lc_is_used);

Cache object: cf2742a63e4f7b20594d9a26ee3e45ba