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>

1 files changed, 29 insertions, 10 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index cd307767a134..c7a4a15c7c19 100644
--- a/block/bfq-iosched.c
+++ b/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);