[PATCH] [BLOCK] cfq-iosched: change cfq io context linking from list to tree

On setups with many disks, we spend a considerable amount of time
looking up the process-disk mapping on each queue of io. Testing with
a NULL based block driver, this costs 40-50% reduction in throughput
for 1000 disks.

Signed-off-by: Jens Axboe <axboe@suse.de>

2 files changed, 108 insertions, 116 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index bde40a6ae665..bb43a1677626 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1190,19 +1190,19 @@ cfq_find_cfq_hash(struct cfq_data *cfqd, unsigned int key, unsigned short prio)
         return __cfq_find_cfq_hash(cfqd, key, prio, hash_long(key, CFQ_QHASH_SHIFT));
 }
 
-static void cfq_free_io_context(struct cfq_io_context *cic)
+static void cfq_free_io_context(struct io_context *ioc)
 {
         struct cfq_io_context *__cic;
-        struct list_head *entry, *next;
-        int freed = 1;
+        struct rb_node *n;
+        int freed = 0;
 
-        list_for_each_safe(entry, next, &cic->list) {
-                __cic = list_entry(entry, struct cfq_io_context, list);
+        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);
                 kmem_cache_free(cfq_ioc_pool, __cic);
                 freed++;
         }
 
-        kmem_cache_free(cfq_ioc_pool, cic);
         if (atomic_sub_and_test(freed, &ioc_count) && ioc_gone)
                 complete(ioc_gone);
 }
@@ -1210,8 +1210,7 @@ static void cfq_free_io_context(struct cfq_io_context *cic)
 static void cfq_trim(struct io_context *ioc)
 {
         ioc->set_ioprio = NULL;
-        if (ioc->cic)
-                cfq_free_io_context(ioc->cic);
+        cfq_free_io_context(ioc);
 }
 
 /*
@@ -1250,26 +1249,26 @@ static void cfq_exit_single_io_context(struct cfq_io_context *cic)
         spin_unlock(q->queue_lock);
 }
 
-static void cfq_exit_io_context(struct cfq_io_context *cic)
+static void cfq_exit_io_context(struct io_context *ioc)
 {
         struct cfq_io_context *__cic;
-        struct list_head *entry;
         unsigned long flags;
-
-        local_irq_save(flags);
+        struct rb_node *n;
 
         /*
          * put the reference this task is holding to the various queues
          */
-        read_lock(&cfq_exit_lock);
-        list_for_each(entry, &cic->list) {
-                __cic = list_entry(entry, struct cfq_io_context, list);
+        read_lock_irqsave(&cfq_exit_lock, flags);
+
+        n = rb_first(&ioc->cic_root);
+        while (n != NULL) {
+                __cic = rb_entry(n, struct cfq_io_context, rb_node);
+
                 cfq_exit_single_io_context(__cic);
+                n = rb_next(n);
         }
 
-        cfq_exit_single_io_context(cic);
-        read_unlock(&cfq_exit_lock);
-        local_irq_restore(flags);
+        read_unlock_irqrestore(&cfq_exit_lock, flags);
 }
 
 static struct cfq_io_context *
@@ -1278,10 +1277,10 @@ cfq_alloc_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
         struct cfq_io_context *cic = kmem_cache_alloc(cfq_ioc_pool, gfp_mask);
 
         if (cic) {
-                INIT_LIST_HEAD(&cic->list);
+                RB_CLEAR(&cic->rb_node);
+                cic->key = NULL;
                 cic->cfqq[ASYNC] = NULL;
                 cic->cfqq[SYNC] = NULL;
-                cic->key = NULL;
                 cic->last_end_request = jiffies;
                 cic->ttime_total = 0;
                 cic->ttime_samples = 0;
@@ -1373,15 +1372,17 @@ static inline void changed_ioprio(struct cfq_io_context *cic)
 static int cfq_ioc_set_ioprio(struct io_context *ioc, unsigned int ioprio)
 {
         struct cfq_io_context *cic;
+        struct rb_node *n;
 
         write_lock(&cfq_exit_lock);
 
-        cic = ioc->cic;
-
-        changed_ioprio(cic);
-
-        list_for_each_entry(cic, &cic->list, list)
+        n = rb_first(&ioc->cic_root);
+        while (n != NULL) {
+                cic = rb_entry(n, struct cfq_io_context, rb_node);
+ 
                 changed_ioprio(cic);
+                n = rb_next(n);
+        }
 
         write_unlock(&cfq_exit_lock);
 
@@ -1445,14 +1446,67 @@ out:
         return cfqq;
 }
 
+static struct cfq_io_context *
+cfq_cic_rb_lookup(struct cfq_data *cfqd, struct io_context *ioc)
+{
+        struct rb_node *n = ioc->cic_root.rb_node;
+        struct cfq_io_context *cic;
+        void *key = cfqd;
+
+        while (n) {
+                cic = rb_entry(n, struct cfq_io_context, rb_node);
+
+                if (key < cic->key)
+                        n = n->rb_left;
+                else if (key > cic->key)
+                        n = n->rb_right;
+                else
+                        return cic;
+        }
+
+        return NULL;
+}
+
+static inline void
+cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
+             struct cfq_io_context *cic)
+{
+        struct rb_node **p = &ioc->cic_root.rb_node;
+        struct rb_node *parent = NULL;
+        struct cfq_io_context *__cic;
+
+        read_lock(&cfq_exit_lock);
+
+        cic->ioc = ioc;
+        cic->key = cfqd;
+
+        ioc->set_ioprio = cfq_ioc_set_ioprio;
+
+        while (*p) {
+                parent = *p;
+                __cic = rb_entry(parent, struct cfq_io_context, rb_node);
+
+                if (cic->key < __cic->key)
+                        p = &(*p)->rb_left;
+                else if (cic->key > __cic->key)
+                        p = &(*p)->rb_right;
+                else
+                        BUG();
+        }
+
+        rb_link_node(&cic->rb_node, parent, p);
+        rb_insert_color(&cic->rb_node, &ioc->cic_root);
+        list_add(&cic->queue_list, &cfqd->cic_list);
+        read_unlock(&cfq_exit_lock);
+}
+
 /*
  * Setup general io context and cfq io context. There can be several cfq
  * io contexts per general io context, if this process is doing io to more
- * than one device managed by cfq. Note that caller is holding a reference to
- * cfqq, so we don't need to worry about it disappearing
+ * than one device managed by cfq.
  */
 static struct cfq_io_context *
-cfq_get_io_context(struct cfq_data *cfqd, pid_t pid, gfp_t gfp_mask)
+cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
 {
         struct io_context *ioc = NULL;
         struct cfq_io_context *cic;
@@ -1463,88 +1517,15 @@ cfq_get_io_context(struct cfq_data *cfqd, pid_t pid, gfp_t gfp_mask)
         if (!ioc)
                 return NULL;
 
-restart:
-        if ((cic = ioc->cic) == NULL) {
-                cic = cfq_alloc_io_context(cfqd, gfp_mask);
-
-                if (cic == NULL)
-                        goto err;
-
-                /*
-                 * manually increment generic io_context usage count, it
-                 * cannot go away since we are already holding one ref to it
-                 */
-                cic->ioc = ioc;
-                cic->key = cfqd;
-                read_lock(&cfq_exit_lock);
-                ioc->set_ioprio = cfq_ioc_set_ioprio;
-                ioc->cic = cic;
-                list_add(&cic->queue_list, &cfqd->cic_list);
-                read_unlock(&cfq_exit_lock);
-        } else {
-                struct cfq_io_context *__cic;
-
-                /*
-                 * the first cic on the list is actually the head itself
-                 */
-                if (cic->key == cfqd)
-                        goto out;
-
-                if (unlikely(!cic->key)) {
-                        read_lock(&cfq_exit_lock);
-                        if (list_empty(&cic->list))
-                                ioc->cic = NULL;
-                        else
-                                ioc->cic = list_entry(cic->list.next,
-                                                      struct cfq_io_context,
-                                                      list);
-                        read_unlock(&cfq_exit_lock);
-                        kmem_cache_free(cfq_ioc_pool, cic);
-                        atomic_dec(&ioc_count);
-                        goto restart;
-                }
-
-                /*
-                 * cic exists, check if we already are there. linear search
-                 * should be ok here, the list will usually not be more than
-                 * 1 or a few entries long
-                 */
-                list_for_each_entry(__cic, &cic->list, list) {
-                        /*
-                         * this process is already holding a reference to
-                         * this queue, so no need to get one more
-                         */
-                        if (__cic->key == cfqd) {
-                                cic = __cic;
-                                goto out;
-                        }
-                        if (unlikely(!__cic->key)) {
-                                read_lock(&cfq_exit_lock);
-                                list_del(&__cic->list);
-                                read_unlock(&cfq_exit_lock);
-                                kmem_cache_free(cfq_ioc_pool, __cic);
-                                atomic_dec(&ioc_count);
-                                goto restart;
-                        }
-                }
+        cic = cfq_cic_rb_lookup(cfqd, ioc);
+        if (cic)
+                goto out;
 
-                /*
-                 * nope, process doesn't have a cic assoicated with this
-                 * cfqq yet. get a new one and add to list
-                 */
-                __cic = cfq_alloc_io_context(cfqd, gfp_mask);
-                if (__cic == NULL)
-                        goto err;
-
-                __cic->ioc = ioc;
-                __cic->key = cfqd;
-                read_lock(&cfq_exit_lock);
-                list_add(&__cic->list, &cic->list);
-                list_add(&__cic->queue_list, &cfqd->cic_list);
-                read_unlock(&cfq_exit_lock);
-                cic = __cic;
-        }
+        cic = cfq_alloc_io_context(cfqd, gfp_mask);
+        if (cic == NULL)
+                goto err;
 
+        cfq_cic_link(cfqd, ioc, cic);
 out:
         return cic;
 err:
@@ -1965,7 +1946,7 @@ cfq_set_request(request_queue_t *q, struct request *rq, struct bio *bio,
 
         might_sleep_if(gfp_mask & __GFP_WAIT);
 
-        cic = cfq_get_io_context(cfqd, key, gfp_mask);
+        cic = cfq_get_io_context(cfqd, gfp_mask);
 
         spin_lock_irqsave(q->queue_lock, flags);
 
@@ -2133,11 +2114,14 @@ static void cfq_exit_queue(elevator_t *e)
         request_queue_t *q = cfqd->queue;
 
         cfq_shutdown_timer_wq(cfqd);
+
         write_lock(&cfq_exit_lock);
         spin_lock_irq(q->queue_lock);
+
         if (cfqd->active_queue)
                 __cfq_slice_expired(cfqd, cfqd->active_queue, 0);
-        while(!list_empty(&cfqd->cic_list)) {
+
+        while (!list_empty(&cfqd->cic_list)) {
                 struct cfq_io_context *cic = list_entry(cfqd->cic_list.next,
                                                         struct cfq_io_context,
                                                         queue_list);
@@ -2152,6 +2136,7 @@ static void cfq_exit_queue(elevator_t *e)
                 cic->key = NULL;
                 list_del_init(&cic->queue_list);
         }
+
         spin_unlock_irq(q->queue_lock);
         write_unlock(&cfq_exit_lock);
 
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 82469db25100..cb608768ca37 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -3539,11 +3539,15 @@ void put_io_context(struct io_context *ioc)
         BUG_ON(atomic_read(&ioc->refcount) == 0);
 
         if (atomic_dec_and_test(&ioc->refcount)) {
+                struct cfq_io_context *cic;
+
                 rcu_read_lock();
                 if (ioc->aic && ioc->aic->dtor)
                         ioc->aic->dtor(ioc->aic);
-                if (ioc->cic && ioc->cic->dtor)
-                        ioc->cic->dtor(ioc->cic);
+                if (ioc->cic_root.rb_node != NULL) {
+                        cic = rb_entry(rb_first(&ioc->cic_root), struct cfq_io_context, rb_node);
+                        cic->dtor(ioc);
+                }
                 rcu_read_unlock();
 
                 kmem_cache_free(iocontext_cachep, ioc);
@@ -3556,6 +3560,7 @@ void exit_io_context(void)
 {
         unsigned long flags;
         struct io_context *ioc;
+        struct cfq_io_context *cic;
 
         local_irq_save(flags);
         task_lock(current);
@@ -3567,9 +3572,11 @@ void exit_io_context(void)
 
         if (ioc->aic && ioc->aic->exit)
                 ioc->aic->exit(ioc->aic);
-        if (ioc->cic && ioc->cic->exit)
-                ioc->cic->exit(ioc->cic);
-
+        if (ioc->cic_root.rb_node != NULL) {
+                cic = rb_entry(rb_first(&ioc->cic_root), struct cfq_io_context, rb_node);
+                cic->exit(ioc);
+        }
+ 
         put_io_context(ioc);
 }
 
@@ -3598,7 +3605,7 @@ struct io_context *current_io_context(gfp_t gfp_flags)
                 ret->last_waited = jiffies; /* doesn't matter... */
                 ret->nr_batch_requests = 0; /* because this is 0 */
                 ret->aic = NULL;
-                ret->cic = NULL;
+                ret->cic_root.rb_node = NULL;
                 tsk->io_context = ret;
         }