[PATCH BUGFIX/IMPROVEMENT 5/6] block, bfq: avoid spurious switches to soft_rt of interactive queues

[Paolo Valente]

BFQ tags some bfq_queues as interactive or soft_rt if it deems that
these bfq_queues contain the I/O of, respectively, interactive or soft
real-time applications. BFQ privileges both these special types of
bfq_queues over normal bfq_queues. To privilege a bfq_queue, BFQ
mainly raises the weight of the bfq_queue. In particular, soft_rt
bfq_queues get a higher weight than interactive bfq_queues.

A bfq_queue may turn from interactive to soft_rt. And this leads to a
tricky issue. Soft real-time applications usually start with an
I/O-bound, interactive phase, in which they load themselves into main
memory. BFQ correctly detects this phase, and keeps the bfq_queues
associated with the application in interactive mode for a
while. Problems arise when the I/O pattern of the application finally
switches to soft real-time. One of the conditions for a bfq_queue to
be deemed as soft_rt is that the bfq_queue does not consume too much
bandwidth. But the bfq_queues associated with a soft real-time
application consume as much bandwidth as they can in the loading phase
of the application. So, after the application becomes truly soft
real-time, a lot of time should pass before the average bandwidth
consumed by its bfq_queues finally drops to a value acceptable for
soft_rt bfq_queues. As a consequence, there might be a time gap during
which the application is not privileged at all, because its bfq_queues
are not interactive any longer, but cannot be deemed as soft_rt yet.

To avoid this problem, BFQ pretends that an interactive bfq_queue
consumes zero bandwidth, and allows an interactive bfq_queue to switch
to soft_rt. Yet, this fake zero-bandwidth consumption easily causes
the bfq_queue to often switch to soft_rt deceptively, during its
loading phase. As in soft_rt mode, the bfq_queue gets its bandwidth
correctly computed, and therefore soon switches back to
interactive. Then it switches again to soft_rt, and so on. These
spurious fluctuations usually cause losses of throughput, because they
deceive BFQ's mechanisms for boosting throughput (injection,
I/O-plugging avoidance, ...).

This commit addresses this issue as follows:
1) It does compute actual bandwidth consumption also for interactive
   bfq_queues. This avoids the above false positives.
2) When a bfq_queue switches from interactive to normal mode, the
   consumed bandwidth is reset (forgotten). This allows the
   bfq_queue to enjoy soft_rt very quickly. In particular, two
   alternatives are possible in this switch:
    - the bfq_queue still has backlog, and therefore there is a budget
      already scheduled to serve the bfq_queue; in this case, the
      scheduling of the current budget of the bfq_queue is not
      hindered, because only the scheduling of the next budget will
      be affected by the weight drop. After that, if the bfq_queue is
      actually in a soft_rt phase, and becomes empty during the
      service of its current budget, which is the natural behavior of
      a soft_rt bfq_queue, then the bfq_queue will be considered as
      soft_rt when its next I/O arrives. If, in contrast, the
      bfq_queue remains constantly non-empty, then its next budget
      will be scheduled with a low weight, which is the natural
      treatment for an I/O-bound (non soft_rt) bfq_queue.
    - the bfq_queue is empty; in this case, the bfq_queue may be
      considered unjustly soft_rt when its new I/O arrives. Yet
      the problem is now much smaller than before, because it is
      unlikely that more than one spurious fluctuation occurs.

Tested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
---
 block/bfq-iosched.c | 57 +++++++++++++++++++++++++++++----------------
 1 file changed, 37 insertions(+), 20 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 161badb744d6..003c96fa01ad 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2356,6 +2356,24 @@ static void bfq_requests_merged(struct request_queue *q, struct request *rq,
 /* Must be called with bfqq != NULL */
 static void bfq_bfqq_end_wr(struct bfq_queue *bfqq)
 {
+	/*
+	 * If bfqq has been enjoying interactive weight-raising, then
+	 * reset soft_rt_next_start. We do it for the following
+	 * reason. bfqq may have been conveying the I/O needed to load
+	 * a soft real-time application. Such an application actually
+	 * exhibits a soft real-time I/O pattern after it finishes
+	 * loading, and finally starts doing its job. But, if bfqq has
+	 * been receiving a lot of bandwidth so far (likely to happen
+	 * on a fast device), then soft_rt_next_start now contains a
+	 * high value that. So, without this reset, bfqq would be
+	 * prevented from being possibly considered as soft_rt for a
+	 * very long time.
+	 */
+
+	if (bfqq->wr_cur_max_time !=
+	    bfqq->bfqd->bfq_wr_rt_max_time)
+		bfqq->soft_rt_next_start = jiffies;
+
 	if (bfq_bfqq_busy(bfqq))
 		bfqq->bfqd->wr_busy_queues--;
 	bfqq->wr_coeff = 1;
@@ -3956,30 +3974,15 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
 		 * If we get here, and there are no outstanding
 		 * 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. 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.
+		 * bfq_bfqq_softrt_next_start()). Therefore 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 &&
-		    bfqq->wr_coeff != bfqd->bfq_wr_coeff)
+		if (bfqq->dispatched == 0)
 			bfqq->soft_rt_next_start =
 				bfq_bfqq_softrt_next_start(bfqd, bfqq);
 		else if (bfqq->dispatched > 0) {
@@ -4563,9 +4566,21 @@ static void bfq_update_wr_data(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 						bfqq->wr_cur_max_time)) {
 			if (bfqq->wr_cur_max_time != bfqd->bfq_wr_rt_max_time ||
 			time_is_before_jiffies(bfqq->wr_start_at_switch_to_srt +
-					       bfq_wr_duration(bfqd)))
+					       bfq_wr_duration(bfqd))) {
+				/*
+				 * Either in interactive weight
+				 * raising, or in soft_rt weight
+				 * raising with the
+				 * interactive-weight-raising period
+				 * elapsed (so no switch back to
+				 * interactive weight raising).
+				 */
 				bfq_bfqq_end_wr(bfqq);
-			else {
+			} else { /*
+				  * soft_rt finishing while still in
+				  * interactive period, switch back to
+				  * interactive weight raising
+				  */
 				switch_back_to_interactive_wr(bfqq, bfqd);
 				bfqq->entity.prio_changed = 1;
 			}
@@ -5016,6 +5031,8 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 	}
 
 	bfqq->entity.new_weight = bfq_ioprio_to_weight(bfqq->new_ioprio);
+	bfq_log_bfqq(bfqd, bfqq, "new_ioprio %d new_weight %d",
+		     bfqq->new_ioprio, bfqq->entity.new_weight);
 	bfqq->entity.prio_changed = 1;
 }
 
-- 
2.20.1