[PATCH BUGFIX/IMPROVEMENT V2 3/6] block, bfq: make shared queues inherit wakers

[Paolo Valente]

Consider a bfq_queue bfqq that is about to be merged with another
bfq_queue new_bfqq. The processes associated with bfqq are cooperators
of the processes associated with new_bfqq. So, if bfqq has a waker,
then it is reasonable (and beneficial for throughput) to assume that
all these processes will be happy to let bfqq's waker freely inject
I/O when they have no I/O. So this commit makes new_bfqq inherit
bfqq's waker.

Tested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
---
 block/bfq-iosched.c | 42 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a9c1a14b64f4..4b3d4849f3f5 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2819,6 +2819,29 @@ bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
 		bfq_mark_bfqq_IO_bound(new_bfqq);
 	bfq_clear_bfqq_IO_bound(bfqq);
 
+	/*
+	 * The processes associated with bfqq are cooperators of the
+	 * processes associated with new_bfqq. So, if bfqq has a
+	 * waker, then assume that all these processes will be happy
+	 * to let bfqq's waker freely inject I/O when they have no
+	 * I/O.
+	 */
+	if (bfqq->waker_bfqq && !new_bfqq->waker_bfqq &&
+	    bfqq->waker_bfqq != new_bfqq) {
+		new_bfqq->waker_bfqq = bfqq->waker_bfqq;
+		new_bfqq->tentative_waker_bfqq = NULL;
+
+		/*
+		 * If the waker queue disappears, then
+		 * new_bfqq->waker_bfqq must be reset. So insert
+		 * new_bfqq into the woken_list of the waker. See
+		 * bfq_check_waker for details.
+		 */
+		hlist_add_head(&new_bfqq->woken_list_node,
+			       &new_bfqq->waker_bfqq->woken_list);
+
+	}
+
 	/*
 	 * If bfqq is weight-raised, then let new_bfqq inherit
 	 * weight-raising. To reduce false positives, neglect the case
@@ -6303,7 +6326,7 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
 	if (likely(!new_queue)) {
 		/* If the queue was seeky for too long, break it apart. */
 		if (bfq_bfqq_coop(bfqq) && bfq_bfqq_split_coop(bfqq)) {
-			bfq_log_bfqq(bfqd, bfqq, "breaking apart bfqq");
+			struct bfq_queue *old_bfqq = bfqq;
 
 			/* Update bic before losing reference to bfqq */
 			if (bfq_bfqq_in_large_burst(bfqq))
@@ -6312,11 +6335,24 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
 			bfqq = bfq_split_bfqq(bic, bfqq);
 			split = true;
 
-			if (!bfqq)
+			if (!bfqq) {
 				bfqq = bfq_get_bfqq_handle_split(bfqd, bic, bio,
 								 true, is_sync,
 								 NULL);
-			else
+				bfqq->waker_bfqq = old_bfqq->waker_bfqq;
+				bfqq->tentative_waker_bfqq = NULL;
+
+				/*
+				 * If the waker queue disappears, then
+				 * new_bfqq->waker_bfqq must be
+				 * reset. So insert new_bfqq into the
+				 * woken_list of the waker. See
+				 * bfq_check_waker for details.
+				 */
+				if (bfqq->waker_bfqq)
+					hlist_add_head(&bfqq->woken_list_node,
+						       &bfqq->waker_bfqq->woken_list);
+			} else
 				bfqq_already_existing = true;
 		}
 	}
-- 
2.20.1