Index: linux-2.6-block/block/cfq-iosched.c
===================================================================
--- linux-2.6-block.orig/block/cfq-iosched.c	2011-06-28 16:32:45.672242961 -0400
+++ linux-2.6-block/block/cfq-iosched.c	2011-06-28 16:35:35.571928038 -0400
@@ -75,6 +75,8 @@ static DEFINE_IDA(cic_index_ida);
 #define sample_valid(samples)	((samples) > 80)
 #define rb_entry_cfqg(node)	rb_entry((node), struct cfq_group, rb_node)
 
+static void cfq_put_request(struct request *rq);
+
 /*
  * Most of our rbtree usage is for sorting with min extraction, so
  * if we cache the leftmost node we don't have to walk down the tree
@@ -148,6 +150,12 @@ struct cfq_queue {
 	struct cfq_group *cfqg;
 	/* Number of sectors dispatched from queue in single dispatch round */
 	unsigned long nr_sectors;
+
+	/*
+	 * This cfqq's further IO is dependent on other IO queued in other
+	 * cfqq pointed by dep_on_cfqq.
+	 */
+	struct cfq_queue *depends_on;
 };
 
 /*
@@ -447,7 +455,7 @@ static inline int cfqg_busy_async_queues
 		+ cfqg->service_trees[BE_WORKLOAD][ASYNC_WORKLOAD].count;
 }
 
-static void cfq_dispatch_insert(struct request_queue *, struct request *);
+static void cfq_dispatch_insert(struct request_queue *, struct request *, bool);
 static struct cfq_queue *cfq_get_queue(struct cfq_data *, bool,
 				       struct io_context *, gfp_t);
 static struct cfq_io_context *cfq_cic_lookup(struct cfq_data *,
@@ -2043,16 +2051,23 @@ static void cfq_arm_slice_timer(struct c
 
 /*
  * Move request from internal lists to the request queue dispatch list.
+ * @rq_dequeued: rq to be dispatched has already been removed from associated
+ * 	         cfqq. This is useful when rq from dependent queue is being
+ * 	         dispatched in current queue context.
  */
-static void cfq_dispatch_insert(struct request_queue *q, struct request *rq)
+static void cfq_dispatch_insert(struct request_queue *q, struct request *rq,
+			bool rq_dequeued)
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
 	struct cfq_queue *cfqq = RQ_CFQQ(rq);
 
 	cfq_log_cfqq(cfqd, cfqq, "dispatch_insert");
 
-	cfqq->next_rq = cfq_find_next_rq(cfqd, cfqq, rq);
-	cfq_remove_request(rq);
+	if (!rq_dequeued) {
+		cfqq->next_rq = cfq_find_next_rq(cfqd, cfqq, rq);
+		cfq_remove_request(rq);
+	}
+
 	cfqq->dispatched++;
 	(RQ_CFQG(rq))->dispatched++;
 	elv_dispatch_sort(q, rq);
@@ -2332,6 +2347,10 @@ static struct cfq_queue *cfq_select_queu
 	if (!RB_EMPTY_ROOT(&cfqq->sort_list))
 		goto keep_queue;
 
+	/* There are reuquests in the cfqq we depend on. Allow dispatch */
+	if (cfqq->depends_on && !RB_EMPTY_ROOT(&cfqq->depends_on->sort_list))
+		goto keep_queue;
+
 	/*
 	 * If another queue has a request waiting within our mean seek
 	 * distance, let it run.  The expire code will check for close
@@ -2402,7 +2421,7 @@ static int __cfq_forced_dispatch_cfqq(st
 	int dispatched = 0;
 
 	while (cfqq->next_rq) {
-		cfq_dispatch_insert(cfqq->cfqd->queue, cfqq->next_rq);
+		cfq_dispatch_insert(cfqq->cfqd->queue, cfqq->next_rq, false);
 		dispatched++;
 	}
 
@@ -2534,6 +2553,77 @@ static bool cfq_may_dispatch(struct cfq_
 }
 
 /*
+ * This queue was not active and we might expire it becaue its request got
+ * dispatched in some other queue's context and it is an empty queue now
+ */
+static void
+cfq_expire_inactive_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
+{
+	/*
+	 * If this cfqq is shared between multiple processes, check to
+	 * make sure that those processes are still issuing I/Os within
+	 * the mean seek distance.  If not, it may be time to break the
+	 * queues apart again.
+	 */
+	if (cfq_cfqq_coop(cfqq) && CFQQ_SEEKY(cfqq))
+		cfq_mark_cfqq_split_coop(cfqq);
+
+	cfq_del_cfqq_rr(cfqd, cfqq);
+}
+
+static void cfq_set_dispatch_dependent_request(struct cfq_data *cfqd,
+				struct cfq_queue *cfqq)
+{
+	struct cfq_queue *dep_cfqq;
+	int rw;
+	struct request *rq;
+
+	dep_cfqq = cfqq->depends_on;
+
+	cfq_log_cfqq(cfqd, cfqq, "dispatch from dependent"
+			" queue pid=%d\n", dep_cfqq->pid);
+	/*
+	 * Select a request from the queue we are dependent on. Dequeue
+	 * the request from other queue and make rq belong to this
+	 * queue and dispatch in this queue's context
+	 */
+	rq = cfq_check_fifo(dep_cfqq);
+	if (!rq)
+		rq = dep_cfqq->next_rq;
+	cfq_remove_request(rq);
+	if (RB_EMPTY_ROOT(&dep_cfqq->sort_list))
+		cfq_expire_inactive_queue(cfqd, dep_cfqq);
+	/*
+	 * Change the identity of request to belong to current cfqq
+	 * cfqg and cic. Drop references to old cfqq, cfqg and cic.
+	 */
+	cfqq->ref++;
+	rw = rq_data_dir(rq);
+	cfqq->allocated[rw]++;
+
+	/*
+	 * If we are here that means we are idling on the queue and we
+	 * must have dispatched atleast one request and that must have
+	 * set the cfqd->active_cic. Use that
+	 */
+	BUG_ON(!cfqd->active_cic);
+	atomic_long_inc(&cfqd->active_cic->ioc->refcount);
+
+	cfq_put_request(rq);
+
+	rq->elevator_private[0] = cfqd->active_cic;
+	rq->elevator_private[1] = cfqq;
+	rq->elevator_private[2] = cfq_ref_get_cfqg(cfqq->cfqg);
+
+	if (cfq_cfqq_wait_request(cfqq))
+		cfq_del_timer(cfqd, cfqq);
+
+	cfq_clear_cfqq_wait_request(cfqq);
+
+	cfq_dispatch_insert(cfqd->queue, rq, true);
+}
+
+/*
  * Dispatch a request from cfqq, moving them to the request queue
  * dispatch list.
  */
@@ -2541,22 +2631,26 @@ static bool cfq_dispatch_request(struct 
 {
 	struct request *rq;
 
-	BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list));
+	BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list) && !cfqq->depends_on);
 
 	if (!cfq_may_dispatch(cfqd, cfqq))
 		return false;
 
-	/*
-	 * follow expired path, else get first next available
-	 */
-	rq = cfq_check_fifo(cfqq);
-	if (!rq)
-		rq = cfqq->next_rq;
+	if (RB_EMPTY_ROOT(&cfqq->sort_list))
+		cfq_set_dispatch_dependent_request(cfqd, cfqq);
+	else {
+		/*
+		 * follow expired path, else get first next available
+		 */
+		rq = cfq_check_fifo(cfqq);
+		if (!rq)
+			rq = cfqq->next_rq;
 
-	/*
-	 * insert request into driver dispatch list
-	 */
-	cfq_dispatch_insert(cfqd->queue, rq);
+		/*
+		 * insert request into driver dispatch list
+		 */
+		cfq_dispatch_insert(cfqd->queue, rq, false);
+	}
 
 	if (!cfqd->active_cic) {
 		struct cfq_io_context *cic = RQ_CIC(rq);
@@ -2640,6 +2734,11 @@ static void cfq_put_queue(struct cfq_que
 	}
 
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
+
+	/* This cfqq is going away. If there is a dependent queue, drop ref */
+	if (cfqq->depends_on)
+		cfq_put_queue(cfqq->depends_on);
+
 	kmem_cache_free(cfq_pool, cfqq);
 	cfq_put_cfqg(cfqg);
 }
@@ -3670,6 +3769,111 @@ static int cfq_may_queue(struct request_
 }
 
 /*
+ * Calling task depends on "tsk" for further IO. It is caller's responsibility
+ * to make sure tsk pointer is valid during the execution of call
+ */
+static void
+cfq_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
+{
+	struct cfq_io_context *cic, *tsk_cic;
+	struct cfq_data *cfqd = q->elevator->elevator_data;
+	struct cfq_queue *cfqq, *tsk_cfqq, *__cfqq;
+
+	if (unlikely(!tsk))
+		return;
+
+	if (!tsk->io_context || !current->io_context)
+		return;
+
+	/*
+	 * If two processes belong to same cgroup, no need to do this as
+	 * both journalling thread and fsync process will go on same
+	 * service tree and be able to preempt each other
+	 */
+	rcu_read_lock();
+	if (task_blkio_cgroup(current) == task_blkio_cgroup(tsk)) {
+		rcu_read_unlock();
+		return;
+	}
+	rcu_read_unlock();
+
+	cic = cfq_cic_lookup(cfqd, current->io_context);
+	if (!cic)
+		return;
+
+	task_lock(tsk);
+	spin_lock_irq(q->queue_lock);
+
+	cfqq = cic_to_cfqq(cic, 1);
+	if (!cfqq)
+		goto out_unlock;
+
+	/* If cfqq already has a dependent queue, ignore this new queue */
+	if (cfqq->depends_on) {
+		cfq_log_cfqq(cfqd, cfqq, "depends on queue already set"
+			" old_pid=%d", cfqq->depends_on->pid);
+		goto out_unlock;
+	}
+
+	if (!tsk->io_context)
+		goto out_unlock;
+
+	tsk_cic = cfq_cic_lookup(cfqd, tsk->io_context);
+	if (!tsk_cic)
+		goto out_unlock;
+
+	tsk_cfqq = cic_to_cfqq(tsk_cic, 1);
+
+	if (!tsk_cfqq)
+		goto out_unlock;
+
+	/* Don't allow circular dependency among a group of queues */
+	__cfqq = tsk_cfqq;
+
+	while((__cfqq = __cfqq->depends_on)) {
+		if (__cfqq  == cfqq)
+			goto out_unlock;
+	}
+
+	/* Take reference on tasks' cfqq */
+	tsk_cfqq->ref++;
+	cfqq->depends_on = tsk_cfqq;
+
+	cfq_log_cfqq(cfqd, cfqq, "set depends on queue pid= %d", tsk->pid);
+
+out_unlock:
+	spin_unlock_irq(q->queue_lock);
+	task_unlock(tsk);
+}
+
+static void cfq_reset_depends_on_task(struct request_queue *q)
+{
+	struct cfq_io_context *cic;
+	struct cfq_data *cfqd = q->elevator->elevator_data;
+	struct cfq_queue *cfqq;
+
+	cic = cfq_cic_lookup(cfqd, current->io_context);
+	if (!cic)
+		return;
+
+	spin_lock_irq(q->queue_lock);
+
+	cfqq = cic_to_cfqq(cic, 1);
+	if (!cfqq || !cfqq->depends_on) {
+		spin_unlock_irq(q->queue_lock);
+		return;
+	}
+
+	cfq_log_cfqq(cfqd, cfqq, "reset depends on queue");
+
+	/* Drop refenrece on tasks's cfqq */
+	cfq_put_queue(cfqq->depends_on);
+	cfqq->depends_on = NULL;
+	spin_unlock_irq(q->queue_lock);
+
+}
+
+/*
  * queue lock held here
  */
 static void cfq_put_request(struct request *rq)
@@ -4206,6 +4410,8 @@ static struct elevator_type iosched_cfq 
 		.elevator_set_req_fn =		cfq_set_request,
 		.elevator_put_req_fn =		cfq_put_request,
 		.elevator_may_queue_fn =	cfq_may_queue,
+		.elevator_set_depends_on_task_fn = cfq_set_depends_on_task,
+		.elevator_reset_depends_on_task_fn = cfq_reset_depends_on_task,
 		.elevator_init_fn =		cfq_init_queue,
 		.elevator_exit_fn =		cfq_exit_queue,
 		.trim =				cfq_free_io_context,
Index: linux-2.6-block/block/blk-core.c
===================================================================
--- linux-2.6-block.orig/block/blk-core.c	2011-06-28 10:02:23.544839889 -0400
+++ linux-2.6-block/block/blk-core.c	2011-06-28 16:33:13.703491049 -0400
@@ -398,6 +398,19 @@ static int blk_init_free_list(struct req
 	return 0;
 }
 
+static void
+generic_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
+{
+	if (q->elevator)
+		elv_set_depends_on_task(q, tsk);
+}
+
+static void generic_reset_depends_on_task(struct request_queue *q)
+{
+	if (q->elevator)
+		elv_reset_depends_on_task(q);
+}
+
 struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
 {
 	return blk_alloc_queue_node(gfp_mask, -1);
@@ -534,6 +547,8 @@ blk_init_allocated_queue_node(struct req
 	q->prep_rq_fn		= NULL;
 	q->unprep_rq_fn		= NULL;
 	q->queue_flags		= QUEUE_FLAG_DEFAULT;
+	q->set_depends_on_task_fn	= generic_set_depends_on_task;
+	q->reset_depends_on_task_fn	= generic_reset_depends_on_task;
 
 	/* Override internal queue lock with supplied lock pointer */
 	if (lock)
@@ -2766,6 +2781,33 @@ void blk_finish_plug(struct blk_plug *pl
 }
 EXPORT_SYMBOL(blk_finish_plug);
 
+/*
+ * Give a hint to CFQ that current task is dependent on IO coming from
+ * "tsk". In such cases, CFQ will allow dispatch from "tsk" queue in
+ * the time slice of "current" process and this can cut down on
+ * unnecessarily queue idling.
+ *
+ * This function will return with interrupts disabled in case of CFQ.
+ * (task_lock()/task_unlock() pair).
+ */
+void blk_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
+{
+	if (q->set_depends_on_task_fn)
+		q->set_depends_on_task_fn(q, tsk);
+}
+EXPORT_SYMBOL(blk_set_depends_on_task);
+
+/*
+ * Tear down the any dependent task mapping previously set up by the current
+ * task.
+ */
+void blk_reset_depends_on_task(struct request_queue *q)
+{
+	if (q->reset_depends_on_task_fn)
+		q->reset_depends_on_task_fn(q);
+}
+EXPORT_SYMBOL(blk_reset_depends_on_task);
+
 int __init blk_dev_init(void)
 {
 	BUILD_BUG_ON(__REQ_NR_BITS > 8 *
Index: linux-2.6-block/block/elevator.c
===================================================================
--- linux-2.6-block.orig/block/elevator.c	2011-06-28 16:32:45.673243005 -0400
+++ linux-2.6-block/block/elevator.c	2011-06-28 16:33:13.705491137 -0400
@@ -783,6 +783,22 @@ int elv_may_queue(struct request_queue *
 	return ELV_MQUEUE_MAY;
 }
 
+void elv_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
+{
+	struct elevator_queue *e = q->elevator;
+
+	if (e->ops->elevator_set_depends_on_task_fn)
+		e->ops->elevator_set_depends_on_task_fn(q, tsk);
+}
+
+void elv_reset_depends_on_task(struct request_queue *q)
+{
+	struct elevator_queue *e = q->elevator;
+
+	if (e->ops->elevator_reset_depends_on_task_fn)
+		e->ops->elevator_reset_depends_on_task_fn(q);
+}
+
 void elv_abort_queue(struct request_queue *q)
 {
 	struct request *rq;
Index: linux-2.6-block/include/linux/blkdev.h
===================================================================
--- linux-2.6-block.orig/include/linux/blkdev.h	2011-06-28 16:32:46.020258283 -0400
+++ linux-2.6-block/include/linux/blkdev.h	2011-06-28 16:33:13.706491181 -0400
@@ -209,6 +209,8 @@ typedef int (merge_bvec_fn) (struct requ
 typedef void (softirq_done_fn)(struct request *);
 typedef int (dma_drain_needed_fn)(struct request *);
 typedef int (lld_busy_fn) (struct request_queue *q);
+typedef void (set_depends_on_task_fn) (struct request_queue *q, struct task_struct *tsk);
+typedef void (reset_depends_on_task_fn) (struct request_queue *q);
 
 enum blk_eh_timer_return {
 	BLK_EH_NOT_HANDLED,
@@ -283,7 +285,8 @@ struct request_queue
 	rq_timed_out_fn		*rq_timed_out_fn;
 	dma_drain_needed_fn	*dma_drain_needed;
 	lld_busy_fn		*lld_busy_fn;
-
+	set_depends_on_task_fn	*set_depends_on_task_fn;
+	reset_depends_on_task_fn	*reset_depends_on_task_fn;
 	/*
 	 * Dispatch queue sorting
 	 */
@@ -895,6 +898,9 @@ static inline bool blk_needs_flush_plug(
 	return plug && (!list_empty(&plug->list) || !list_empty(&plug->cb_list));
 }
 
+extern void blk_set_depends_on_task(struct request_queue *q, struct task_struct *tsk);
+extern void blk_reset_depends_on_task(struct request_queue *q);
+
 /*
  * tag stuff
  */
Index: linux-2.6-block/include/linux/elevator.h
===================================================================
--- linux-2.6-block.orig/include/linux/elevator.h	2011-06-28 16:32:46.022258371 -0400
+++ linux-2.6-block/include/linux/elevator.h	2011-06-28 16:33:13.707491225 -0400
@@ -23,6 +23,8 @@ typedef void (elevator_add_req_fn) (stru
 typedef struct request *(elevator_request_list_fn) (struct request_queue *, struct request *);
 typedef void (elevator_completed_req_fn) (struct request_queue *, struct request *);
 typedef int (elevator_may_queue_fn) (struct request_queue *, int);
+typedef void (elevator_set_depends_on_task_fn) (struct request_queue *, struct task_struct *);
+typedef void (elevator_reset_depends_on_task_fn) (struct request_queue *);
 
 typedef int (elevator_set_req_fn) (struct request_queue *, struct request *, gfp_t);
 typedef void (elevator_put_req_fn) (struct request *);
@@ -54,6 +56,8 @@ struct elevator_ops
 	elevator_put_req_fn *elevator_put_req_fn;
 
 	elevator_may_queue_fn *elevator_may_queue_fn;
+	elevator_set_depends_on_task_fn *elevator_set_depends_on_task_fn;
+	elevator_reset_depends_on_task_fn *elevator_reset_depends_on_task_fn;
 
 	elevator_init_fn *elevator_init_fn;
 	elevator_exit_fn *elevator_exit_fn;
@@ -114,6 +118,8 @@ extern struct request *elv_latter_reques
 extern int elv_register_queue(struct request_queue *q);
 extern void elv_unregister_queue(struct request_queue *q);
 extern int elv_may_queue(struct request_queue *, int);
+extern void elv_set_depends_on_task(struct request_queue *q, struct task_struct *tsk);
+extern void elv_reset_depends_on_task(struct request_queue *q);
 extern void elv_abort_queue(struct request_queue *);
 extern void elv_completed_request(struct request_queue *, struct request *);
 extern int elv_set_request(struct request_queue *, struct request *, gfp_t);