block, bfq: unconditionally plug I/O in asymmetric scenarios

bfq detects the creation of multiple bfq_queues shortly after each
other, namely a burst of queue creations in the terminology used in the
code. If the burst is large, then no queue in the burst is granted
- either I/O-dispatch plugging when the queue remains temporarily idle
  while in service;
- or weight raising, because it causes even longer plugging.

In fact, such a plugging tends to lower throughput, while these bursts
are typically due to applications or services that spawn multiple
processes, to reach a common goal as soon as possible. Examples are a
"git grep" or the booting of a system.

Unfortunately, disabling plugging may cause a loss of service guarantees
in asymmetric scenarios, i.e., if queue weights are differentiated or if
more than one group is active.

This commit addresses this issue by no longer disabling I/O-dispatch
plugging for queues in large bursts.

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

block/bfq-iosched.c

@@ -3479,15 +3479,12 @@ static bool idling_boosts_thr_without_issues(struct bfq_data *bfqd,
 		bfqd->wr_busy_queues == 0;
 }
 
-static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
-						 struct bfq_queue *bfqq)
-{
 /*
  * There is a case where idling must be performed not for
  * throughput concerns, but to preserve service guarantees.
  *
  * To introduce this case, we can note that allowing the drive
-	 * to enqueue more than one request at a time, and thereby
+ * to enqueue more than one request at a time, and hence
  * delegating de facto final scheduling decisions to the
  * drive's internal scheduler, entails loss of control on the
  * actual request service order. In particular, the critical
@@ -3644,26 +3641,13 @@ static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
  * to let requests be served in the desired order until all
  * the requests already queued in the device have been served.
  */
-	bool asymmetric_scenario = (bfqq->wr_coeff > 1 &&
+static bool idling_needed_for_service_guarantees(struct bfq_data *bfqd,
+						 struct bfq_queue *bfqq)
+{
+	return (bfqq->wr_coeff > 1 &&
 		bfqd->wr_busy_queues <
 		bfq_tot_busy_queues(bfqd)) ||
 		!bfq_symmetric_scenario(bfqd);
-
-	/*
-	 * Finally, there is a case where maximizing throughput is the
-	 * best choice even if it may cause unfairness toward
-	 * bfqq. Such a case is when bfqq became active in a burst of
-	 * queue activations. Queues that became active during a large
-	 * burst benefit only from throughput, as discussed in the
-	 * comments on bfq_handle_burst. Thus, if bfqq became active
-	 * in a burst and not idling the device maximizes throughput,
-	 * then the device must no be idled, because not idling the
-	 * device provides bfqq and all other queues in the burst with
-	 * maximum benefit. Combining this and the above case, we can
-	 * now establish when idling is actually needed to preserve
-	 * service guarantees.
-	 */
-	return asymmetric_scenario && !bfq_bfqq_in_large_burst(bfqq);
 }
 
 /*