lru_cache: allow multiple changes per transaction
Allow multiple changes to the active set of elements in lru_cache.
The only current user of lru_cache, drbd, is driving this generalisation.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
diff --git a/lib/lru_cache.c b/lib/lru_cache.c
index 1762168..d71d894 100644
--- a/lib/lru_cache.c
+++ b/lib/lru_cache.c
@@ -55,9 +55,40 @@
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
@@ -66,6 +97,7 @@
* 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;
@@ -98,12 +130,13 @@
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->new_number = LC_FREE;
+ lc->max_pending_changes = max_pending_changes;
lc->lc_cache = cache;
lc->lc_element = element;
lc->lc_slot = slot;
@@ -117,6 +150,7 @@
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;
}
@@ -175,15 +209,15 @@
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->dirty = 0;
+ lc->locked = 0;
lc->changed = 0;
+ lc->pending_changes = 0;
lc->flags = 0;
- lc->changing_element = NULL;
- lc->new_number = LC_FREE;
memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
for (i = 0; i < lc->nr_elements; i++) {
@@ -194,6 +228,7 @@
/* re-init it */
e->lc_index = i;
e->lc_number = LC_FREE;
+ e->lc_new_number = LC_FREE;
list_add(&e->list, &lc->free);
}
}
@@ -208,14 +243,14 @@
/* NOTE:
* total calls to lc_get are
* (starving + hits + misses)
- * misses include "dirty" count (update from an other thread in
+ * 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 dirty:%lu changed:%lu\n",
+ "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->dirty, lc->changed);
+ lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
}
static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
@@ -224,6 +259,27 @@
}
+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
@@ -232,38 +288,28 @@
* 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)
{
- 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) {
- if (e->lc_number == enr)
- return e;
- }
- return NULL;
+ return __lc_find(lc, enr, 0);
}
-/* returned element will be "recycled" immediately */
-static struct lc_element *lc_evict(struct lru_cache *lc)
+/**
+ * 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 list_head *n;
- struct lc_element *e;
-
- if (list_empty(&lc->lru))
- return NULL;
-
- n = lc->lru.prev;
- e = list_entry(n, struct lc_element, list);
-
- PARANOIA_LC_ELEMENT(lc, e);
-
- list_del(&e->list);
- hlist_del(&e->colision);
- return e;
+ struct lc_element *e = __lc_find(lc, enr, 1);
+ return e && e->refcnt;
}
/**
@@ -280,22 +326,34 @@
PARANOIA_LC_ELEMENT(lc, e);
BUG_ON(e->refcnt);
- e->lc_number = LC_FREE;
+ 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_get_unused_element(struct lru_cache *lc)
+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))
- return lc_evict(lc);
+ if (!list_empty(&lc->free))
+ n = lc->free.next;
+ else if (!list_empty(&lc->lru))
+ n = lc->lru.prev;
+ else
+ return NULL;
- n = lc->free.next;
- list_del(n);
- return list_entry(n, struct lc_element, list);
+ 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)
@@ -318,8 +376,12 @@
RETURN(NULL);
}
- e = lc_find(lc, enr);
- if (e) {
+ 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++;
@@ -331,6 +393,24 @@
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 ...
*/
@@ -339,24 +419,19 @@
RETURN(NULL);
}
- /* it was not present in the active set.
- * we are going to recycle an unused (or even "free") element.
- * user may need to commit a transaction to record that change.
- * we serialize on flags & LC_DIRTY */
- if (test_and_set_bit(__LC_DIRTY, &lc->flags)) {
- ++lc->dirty;
+ /* 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_get_unused_element(lc);
+ e = lc_prepare_for_change(lc, enr);
BUG_ON(!e);
clear_bit(__LC_STARVING, &lc->flags);
BUG_ON(++e->refcnt != 1);
lc->used++;
-
- lc->changing_element = e;
- lc->new_number = enr;
+ lc->pending_changes++;
RETURN(e);
}
@@ -388,12 +463,15 @@
* 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 (blocking further changes),
- * and the returned element pointer is removed from the lru list and
- * hash collision chains. The user now should do whatever housekeeping
- * is necessary.
- * Then he must call lc_changed(lc,element_pointer), to finish
- * the change.
+ * 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.
@@ -425,22 +503,25 @@
}
/**
- * lc_changed - tell @lc that the change has been recorded
+ * lc_committed - tell @lc that pending changes have been recorded
* @lc: the lru cache to operate on
- * @e: the element pending label change
+ *
+ * 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_changed(struct lru_cache *lc, struct lc_element *e)
+void lc_committed(struct lru_cache *lc)
{
+ struct lc_element *e, *tmp;
+
PARANOIA_ENTRY();
- BUG_ON(e != lc->changing_element);
- PARANOIA_LC_ELEMENT(lc, e);
- ++lc->changed;
- e->lc_number = lc->new_number;
- list_add(&e->list, &lc->in_use);
- hlist_add_head(&e->colision, lc_hash_slot(lc, lc->new_number));
- lc->changing_element = NULL;
- lc->new_number = LC_FREE;
- clear_bit_unlock(__LC_DIRTY, &lc->flags);
+ 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();
}
@@ -459,7 +540,7 @@
PARANOIA_ENTRY();
PARANOIA_LC_ELEMENT(lc, e);
BUG_ON(e->refcnt == 0);
- BUG_ON(e == lc->changing_element);
+ 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);
@@ -504,16 +585,24 @@
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);
- e->lc_number = enr;
+ 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);
- hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
- list_move(&e->list, e->refcnt ? &lc->in_use : &lc->lru);
+ 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);
}
/**
@@ -553,8 +642,10 @@
EXPORT_SYMBOL(lc_find);
EXPORT_SYMBOL(lc_get);
EXPORT_SYMBOL(lc_put);
-EXPORT_SYMBOL(lc_changed);
+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);