linux-kernel - [PATCH BUGFIX/IMPROVEMENT 2/6] block, bfq: set next_rq to waker_bfqq->next

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Date:   Fri, 22 Jan 2021 19:19:44 +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,
        Jia Cheng Hu <jia.jiachenghu@...il.com>,
        Jan Kara <jack@...e.cz>,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX/IMPROVEMENT 2/6] block, bfq: set next_rq to waker_bfqq->next_rq in waker injection

From: Jia Cheng Hu <jia.jiachenghu@...il.com>

Since commit c5089591c3ba ("block, bfq: detect wakers and
unconditionally inject their I/O"), when the in-service bfq_queue, say
Q, is temporarily empty, BFQ checks whether there are I/O requests to
inject (also) from the waker bfq_queue for Q. To this goal, the value
pointed by bfqq->waker_bfqq->next_rq must be controlled. However, the
current implementation mistakenly looks at bfqq->next_rq, which
instead points to the next request of the currently served queue.

This mistake evidently causes losses of throughput in scenarios with
waker bfq_queues.

This commit corrects this mistake.

Fixes: c5089591c3ba ("block, bfq: detect wakers and unconditionally inject their I/O")
Signed-off-by: Jia Cheng Hu <jia.jiachenghu@...il.com>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index eb2ca32d5b63..fdc5e163b2fe 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -4499,7 +4499,7 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
 			bfqq = bfqq->bic->bfqq[0];
 		else if (bfq_bfqq_has_waker(bfqq) &&
 			   bfq_bfqq_busy(bfqq->waker_bfqq) &&
-			   bfqq->next_rq &&
+			   bfqq->waker_bfqq->next_rq &&
 			   bfq_serv_to_charge(bfqq->waker_bfqq->next_rq,
 					      bfqq->waker_bfqq) <=
 			   bfq_bfqq_budget_left(bfqq->waker_bfqq)
-- 
2.20.1