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@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 2ab53d93ba12a0ca1e14f6098e9329b8ec05f7c2..06268449d2cadd751069cea7ed06c5700bf5af0a 100644 (file)
--- 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;