cfq-iosched: speedup cic rb lookup

We often lookup the same queue many times in succession, so cache
the last looked up queue to avoid browsing the rbtree.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 94f53a1..64df3fa 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1165,6 +1165,8 @@ static void cfq_free_io_context(struct io_context *ioc)
        struct rb_node *n;
        int freed = 0;
 
+       ioc->ioc_data = NULL;
+
        while ((n = rb_first(&ioc->cic_root)) != NULL) {
                __cic = rb_entry(n, struct cfq_io_context, rb_node);
                rb_erase(&__cic->rb_node, &ioc->cic_root);
@@ -1228,10 +1230,11 @@ static void cfq_exit_io_context(struct io_context *ioc)
        struct cfq_io_context *__cic;
        struct rb_node *n;
 
+       ioc->ioc_data = NULL;
+
        /*
         * put the reference this task is holding to the various queues
         */
-
        n = rb_first(&ioc->cic_root);
        while (n != NULL) {
                __cic = rb_entry(n, struct cfq_io_context, rb_node);
@@ -1415,6 +1418,10 @@ static void
 cfq_drop_dead_cic(struct io_context *ioc, struct cfq_io_context *cic)
 {
        WARN_ON(!list_empty(&cic->queue_list));
+
+       if (ioc->ioc_data == cic)
+               ioc->ioc_data = NULL;
+
        rb_erase(&cic->rb_node, &ioc->cic_root);
        kmem_cache_free(cfq_ioc_pool, cic);
        elv_ioc_count_dec(ioc_count);
@@ -1430,6 +1437,13 @@ cfq_cic_rb_lookup(struct cfq_data *cfqd, struct io_context *ioc)
        if (unlikely(!ioc))
                return NULL;
 
+       /*
+        * we maintain a last-hit cache, to avoid browsing over the tree
+        */
+       cic = ioc->ioc_data;
+       if (cic && cic->key == cfqd)
+               return cic;
+
 restart:
        n = ioc->cic_root.rb_node;
        while (n) {
@@ -1445,8 +1459,10 @@ restart:
                        n = n->rb_left;
                else if (key > k)
                        n = n->rb_right;
-               else
+               else {
+                       ioc->ioc_data = cic;
                        return cic;
+               }
        }
 
        return NULL;