Commit 20cd3245 authored by Paolo Valente's avatar Paolo Valente Committed by Jens Axboe
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block, bfq: do not consider interactive queues in srt filtering 



The speed at which a bfq_queue receives I/O is one of the parameters by
which bfq decides whether the queue is soft real-time (i.e., whether the
queue contains the I/O of a soft real-time application). In particular,
when a bfq_queue remains without outstanding I/O requests, bfq computes
the minimum time instant, named soft_rt_next_start, at which the next
request of the queue may arrive for the queue to be deemed as soft real
time.

Unfortunately this filtering may cause problems with a queue in
interactive weight raising. In fact, such a queue may be conveying the
I/O needed to load a soft real-time application. The latter will
actually exhibit a soft real-time I/O pattern after it finally starts
doing its job. But, if soft_rt_next_start is updated for an interactive
bfq_queue, and the queue has received a lot of service before remaining
with no outstanding request (likely to happen on a fast device), then
soft_rt_next_start is assigned such a high value that, for a very long
time, the queue is prevented from being possibly considered as soft real
time.

This commit removes the updating of soft_rt_next_start for bfq_queues in
interactive weight raising.

Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 22cb4e68

block/bfq-iosched.c

+29 −10
Original line number Diff line number Diff line
@@ -3274,16 +3274,32 @@ void bfq_bfqq_expire(struct bfq_data *bfqd, 
		 * requests, then the request pattern is isochronous

		 * (see the comments on the function

		 * bfq_bfqq_softrt_next_start()). Thus we can compute

		 * soft_rt_next_start. If, instead, the queue still

		 * has outstanding requests, then we have to wait for

		 * the completion of all the outstanding requests to

		 * discover whether the request pattern is actually

		 * isochronous.

		 */

		if (bfqq->dispatched == 0)

		 * soft_rt_next_start. And we do it, unless bfqq is in

		 * interactive weight raising. We do not do it in the

		 * latter subcase, for the following reason. bfqq may

		 * be conveying the I/O needed to load a soft

		 * real-time application. Such an application will

		 * actually exhibit a soft real-time I/O pattern after

		 * it finally starts doing its job. But, if

		 * soft_rt_next_start is computed here for an

		 * interactive bfqq, and bfqq had received a lot of

		 * service before remaining with no outstanding

		 * request (likely to happen on a fast device), then

		 * soft_rt_next_start would be assigned such a high

		 * value that, for a very long time, bfqq would be

		 * prevented from being possibly considered as soft

		 * real time.

		 *

		 * If, instead, the queue still has outstanding

		 * requests, then we have to wait for the completion

		 * of all the outstanding requests to discover whether

		 * the request pattern is actually isochronous.

		 */

		if (bfqq->dispatched == 0 &&

		    bfqq->wr_coeff != bfqd->bfq_wr_coeff)

			bfqq->soft_rt_next_start =

				bfq_bfqq_softrt_next_start(bfqd, bfqq);

		else {

		else if (bfqq->dispatched > 0) {

			/*

			 * Schedule an update of soft_rt_next_start to when

			 * the task may be discovered to be isochronous.
@@ -4834,11 +4850,14 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd) 
	 * isochronous, and both requisites for this condition to hold

	 * are now satisfied, then compute soft_rt_next_start (see the

	 * comments on the function bfq_bfqq_softrt_next_start()). We

	 * schedule this delayed check when bfqq expires, if it still

	 * has in-flight requests.

	 * do not compute soft_rt_next_start if bfqq is in interactive

	 * weight raising (see the comments in bfq_bfqq_expire() for

	 * an explanation). We schedule this delayed update when bfqq

	 * expires, if it still has in-flight requests.

	 */

	if (bfq_bfqq_softrt_update(bfqq) && bfqq->dispatched == 0 &&

	    RB_EMPTY_ROOT(&bfqq->sort_list))

	    RB_EMPTY_ROOT(&bfqq->sort_list) &&

	    bfqq->wr_coeff != bfqd->bfq_wr_coeff)

		bfqq->soft_rt_next_start =

			bfq_bfqq_softrt_next_start(bfqd, bfqq);

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