Commit 3a9a3f6c authored by Divyesh Shah's avatar Divyesh Shah Committed by Jens Axboe
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cfq-iosched: Allow RT requests to pre-empt ongoing BE timeslice 



This patch adds the ability to pre-empt an ongoing BE timeslice when a RT
request is waiting for the current timeslice to complete. This reduces the
wait time to disk for RT requests from an upper bound of 4 (current value
of cfq_quantum) to 1 disk request.

Applied Jens' suggeested changes to avoid the rb lookup and use !cfq_class_rt()
and retested.

Latency(secs) for the RT task when doing sequential reads from 10G file.
                       | only RT | RT + BE | RT + BE + this patch
small (512 byte) reads | 143     | 163     | 145
large (1Mb) reads      | 142     | 158     | 146

Signed-off-by: default avatarDivyesh Shah <dpshah@google.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent bc58ba94

block/cfq-iosched.c

+38 −1
Original line number Diff line number Diff line
@@ -84,6 +84,11 @@ struct cfq_data { 
	 */

	struct cfq_rb_root service_tree;

	unsigned int busy_queues;

	/*

	 * Used to track any pending rt requests so we can pre-empt current

	 * non-RT cfqq in service when this value is non-zero.

	 */

	unsigned int busy_rt_queues;



	int rq_in_driver;

	int sync_flight;
@@ -562,6 +567,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq) 
	BUG_ON(cfq_cfqq_on_rr(cfqq));

	cfq_mark_cfqq_on_rr(cfqq);

	cfqd->busy_queues++;

	if (cfq_class_rt(cfqq))

		cfqd->busy_rt_queues++;



	cfq_resort_rr_list(cfqd, cfqq);

}
@@ -581,6 +588,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq) 


	BUG_ON(!cfqd->busy_queues);

	cfqd->busy_queues--;

	if (cfq_class_rt(cfqq))

		cfqd->busy_rt_queues--;

}



/*
@@ -1004,6 +1013,20 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) 
	if (cfq_slice_used(cfqq))

		goto expire;



	/*

	 * If we have a RT cfqq waiting, then we pre-empt the current non-rt

	 * cfqq.

	 */

	if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues) {

		/*

		 * We simulate this as cfqq timed out so that it gets to bank

		 * the remaining of its time slice.

		 */

		cfq_log_cfqq(cfqd, cfqq, "preempt");

		cfq_slice_expired(cfqd, 1);

		goto new_queue;

	}



	/*

	 * The active queue has requests and isn't expired, allow it to

	 * dispatch.
@@ -1067,6 +1090,13 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq, 
		if (RB_EMPTY_ROOT(&cfqq->sort_list))

			break;



		/*

		 * If there is a non-empty RT cfqq waiting for current

		 * cfqq's timeslice to complete, pre-empt this cfqq

		 */

		if (!cfq_class_rt(cfqq) && cfqd->busy_rt_queues)

			break;



	} while (dispatched < max_dispatch);



	/*
@@ -1801,6 +1831,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq, 
	if (rq_is_meta(rq) && !cfqq->meta_pending)

		return 1;



	/*

	 * Allow an RT request to pre-empt an ongoing non-RT cfqq timeslice.

	 */

	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))

		return 1;



	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))

		return 0;
@@ -1870,7 +1906,8 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq, 
		/*

		 * not the active queue - expire current slice if it is

		 * idle and has expired it's mean thinktime or this new queue

		 * has some old slice time left and is of higher priority

		 * has some old slice time left and is of higher priority or

		 * this new queue is RT and the current one is BE

		 */

		cfq_preempt_queue(cfqd, cfqq);

		cfq_mark_cfqq_must_dispatch(cfqq);

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