linux-kernel - [PATCH BUGFIX/IMPROVEMENT 5/6] block, bfq: keep shared queues out of the waker mechanism

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Date:   Tue, 26 Jan 2021 11:51:01 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Valente <paolo.valente@...aro.org>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH BUGFIX/IMPROVEMENT 5/6] block, bfq: keep shared queues out of the waker mechanism

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@...e.cz>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0c7e203085f1..23d0dd7bd90f 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5825,7 +5825,17 @@ 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
-- 
2.20.1