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: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -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.
+		 * 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)
+		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);