diff mbox series

[BUGFIX/IMPROVEMENT,V2,5/6] block, bfq: keep shared queues out of the waker mechanism

Message ID 20210304174627.161-6-paolo.valente@linaro.org (mailing list archive)
State New, archived
Headers show
Series revised version of third and last batch of patches | expand

Commit Message

Paolo Valente March 4, 2021, 5:46 p.m. UTC
Shared queues are likely to receive I/O at a high rate. This may
deceptively let them be considered as wakers of other queues. But a
false waker will unjustly steal bandwidth to its supposedly woken
queue. So considering also shared queues in the waking mechanism may
cause more control troubles than throughput benefits. This commit
keeps shared queues out of the waker-detection mechanism.

Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
 block/bfq-iosched.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 8497d0803d74..c62dbbe9cc33 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5852,7 +5852,17 @@  static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
 			1UL<<(BFQ_RATE_SHIFT - 10))
 		bfq_update_rate_reset(bfqd, NULL);
 	bfqd->last_completion = now_ns;
-	bfqd->last_completed_rq_bfqq = bfqq;
+	/*
+	 * Shared queues are likely to receive I/O at a high
+	 * rate. This may deceptively let them be considered as wakers
+	 * of other queues. But a false waker will unjustly steal
+	 * bandwidth to its supposedly woken queue. So considering
+	 * also shared queues in the waking mechanism may cause more
+	 * control troubles than throughput benefits. Then do not set
+	 * last_completed_rq_bfqq to bfqq if bfqq is a shared queue.
+	 */
+	if (!bfq_bfqq_coop(bfqq))
+		bfqd->last_completed_rq_bfqq = bfqq;
 
 	/*
 	 * If we are waiting to discover whether the request pattern