Consider a new I/O request that arrives for a bfq_queue bfqq. If, when
this happens, the only active bfq_queues are bfqq and either its waker
bfq_queue or one of its woken bfq_queues, then there is no point in
queueing this new I/O request in bfqq for service. In fact, the
in-service queue and bfqq agree on serving this new I/O request as
soon as possible. So this commit puts this new I/O request directly
into the dispatch list.
Tested-by: Jan Kara <j...@suse.cz>
Acked-by: Jan Kara <j...@suse.cz>
Signed-off-by: Paolo Valente <paolo.vale...@linaro.org>
---
block/bfq-iosched.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 43 insertions(+), 1 deletion(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a83149407336..a9c1a14b64f4 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5640,7 +5640,49 @@ static void bfq_insert_request(struct blk_mq_hw_ctx
*hctx, struct request *rq,
spin_lock_irq(&bfqd->lock);
bfqq = bfq_init_rq(rq);
- if (!bfqq || at_head || blk_rq_is_passthrough(rq)) {
+
+ /*
+ * Reqs with at_head or passthrough flags set are to be put
+ * directly into dispatch list. Additional case for putting rq
+ * directly into the dispatch queue: the only active
+ * bfq_queues are bfqq and either its waker bfq_queue or one
+ * of its woken bfq_queues. The rationale behind this
+ * additional condition is as follows:
+ * - consider a bfq_queue, say Q1, detected as a waker of
+ * another bfq_queue, say Q2
+ * - by definition of a waker, Q1 blocks the I/O of Q2, i.e.,
+ * some I/O of Q1 needs to be completed for new I/O of Q2
+ * to arrive. A notable example of waker is journald
+ * - so, Q1 and Q2 are in any respect the queues of two
+ * cooperating processes (or of two cooperating sets of
+ * processes): the goal of Q1's I/O is doing what needs to
+ * be done so that new Q2's I/O can finally be
+ * issued. Therefore, if the service of Q1's I/O is delayed,
+ * then Q2's I/O is delayed too. Conversely, if Q2's I/O is
+ * delayed, the goal of Q1's I/O is hindered.
+ * - as a consequence, if some I/O of Q1/Q2 arrives while
+ * Q2/Q1 is the only queue in service, there is absolutely
+ * no point in delaying the service of such an I/O. The
+ * only possible result is a throughput loss
+ * - so, when the above condition holds, the best option is to
+ * have the new I/O dispatched as soon as possible
+ * - the most effective and efficient way to attain the above
+ * goal is to put the new I/O directly in the dispatch
+ * list
+ * - as an additional restriction, Q1 and Q2 must be the only
+ * busy queues for this commit to put the I/O of Q2/Q1 in
+ * the dispatch list. This is necessary, because, if also
+ * other queues are waiting for service, then putting new
+ * I/O directly in the dispatch list may evidently cause a
+ * violation of service guarantees for the other queues
+ */
+ if (!bfqq ||
+ (bfqq != bfqd->in_service_queue &&
+ bfqd->in_service_queue != NULL &&
+ bfq_tot_busy_queues(bfqd) == 1 + bfq_bfqq_busy(bfqq) &&
+ (bfqq->waker_bfqq == bfqd->in_service_queue ||
+ bfqd->in_service_queue->waker_bfqq == bfqq)) ||
+ at_head || blk_rq_is_passthrough(rq)) {
if (at_head)
list_add(&rq->queuelist, &bfqd->dispatch);
else
--
2.20.1
