linux-kernel - [PATCH BUGFIX IMPROVEMENT 13/14] block, bfq: do not overcharge writes in asymmetric scenarios

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Date:   Tue, 29 Jan 2019 12:06:37 +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,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        broonie@...nel.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, mancha@...er-research.com,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX IMPROVEMENT 13/14] block, bfq: do not overcharge writes in asymmetric scenarios

Writes tend to starve reads. bfq counters this problem by overcharging
writes with an inflated service w.r.t. the actual service (number of
sector written) they receive.

Yet his overcharging is useless, and actually causes unfairness in the
opposite direction, when bfq happens to be enforcing strong I/O
control. bfq does this enforcing when the scenario is asymmetric,
i.e., when some bfq_queue or group of bfq_queues is to be granted a
different bandwidth than some other bfq_queue or group of
bfq_queues. So, in such a scenario, this commit disables write
overcharging.

Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-iosched.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 2ab53d93ba12..06268449d2ca 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -888,7 +888,8 @@ static struct request *bfq_find_next_rq(struct bfq_data *bfqd,
 static unsigned long bfq_serv_to_charge(struct request *rq,
 					struct bfq_queue *bfqq)
 {
-	if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1)
+	if (bfq_bfqq_sync(bfqq) || bfqq->wr_coeff > 1 ||
+	    !bfq_symmetric_scenario(bfqq->bfqd))
 		return blk_rq_sectors(rq);

 	return blk_rq_sectors(rq) * bfq_async_charge_factor;
-- 
2.20.1