block, bfq: update base request service times when possible

I/O injection gets reduced if it increases the request service times
of the victim queue beyond a certain threshold.  The threshold, in its
turn, is computed as a function of the base service time enjoyed by
the queue when it undergoes no injection.

As a consequence, for injection to work properly, the above base value
has to be accurate. In this respect, such a value may vary over
time. For example, it varies if the size or the spatial locality of
the I/O requests in the queue change. It is then important to update
this value whenever possible. This commit performs this update.

Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -5496,7 +5496,18 @@ static void bfq_update_inject_limit(struct bfq_data *bfqd,
 		 * start trying injection.
 		 */
 		bfqq->inject_limit = max_t(unsigned int, 1, old_limit);
-	}
+	} else if (!bfqd->rqs_injected && bfqd->rq_in_driver == 1)
+		/*
+		 * No I/O injected and no request still in service in
+		 * the drive: these are the exact conditions for
+		 * computing the base value of the total service time
+		 * for bfqq. So let's update this value, because it is
+		 * rather variable. For example, it varies if the size
+		 * or the spatial locality of the I/O requests in bfqq
+		 * change.
+		 */
+		bfqq->last_serv_time_ns = tot_time_ns;
+
 
 	/* update complete, not waiting for any request completion any longer */
 	bfqd->waited_rq = NULL;