From 2d31c684a053b46f75b230899d00c0f56708243d Mon Sep 17 00:00:00 2001 From: Davide Zini Date: Tue, 3 Jan 2023 15:55:02 +0100 Subject: block, bfq: inject I/O to underutilized actuators The main service scheme of BFQ for sync I/O is serving one sync bfq_queue at a time, for a while. In particular, BFQ enforces this scheme when it deems the latter necessary to boost throughput or to preserve service guarantees. Unfortunately, when BFQ enforces this policy, only one actuator at a time gets served for a while, because each bfq_queue contains I/O only for one actuator. The other actuators may remain underutilized. Actually, BFQ may serve (inject) extra I/O, taken from other bfq_queues, in parallel with that of the in-service queue. This injection mechanism may provide the ground for dealing also with the above actuator-underutilization problem. Yet BFQ does not take the actuator load into account when choosing which queue to pick extra I/O from. In addition, BFQ may happen to inject extra I/O only when the in-service queue is temporarily empty. In view of these facts, this commit extends the injection mechanism in such a way that the latter: (1) takes into account also the actuator load; (2) checks such a load on each dispatch, and injects I/O for an underutilized actuator, if there is one and there is I/O for it. To perform the check in (2), this commit introduces a load threshold, currently set to 4. A linear scan of each actuator is performed, until an actuator is found for which the following two conditions hold: the load of the actuator is below the threshold, and there is at least one non-in-service queue that contains I/O for that actuator. If such a pair (actuator, queue) is found, then the head request of that queue is returned for dispatch, instead of the head request of the in-service queue. We have set the threshold, empirically, to the minimum possible value for which an actuator is fully utilized, or close to be fully utilized. By doing so, injected I/O 'steals' as few drive-queue slots as possibile to the in-service queue. This reduces as much as possible the probability that the service of I/O from the in-service bfq_queue gets delayed because of slot exhaustion, i.e., because all the slots of the drive queue are filled with I/O injected from other queues (NCQ provides for 32 slots). This new mechanism also counters actuator underutilization in the case of asymmetric configurations of bfq_queues. Namely if there are few bfq_queues containing I/O for some actuators and many bfq_queues containing I/O for other actuators. Or if the bfq_queues containing I/O for some actuators have lower weights than the other bfq_queues. Reviewed-by: Damien Le Moal Signed-off-by: Paolo Valente Signed-off-by: Davide Zini Link: https://lore.kernel.org/r/20230103145503.71712-8-paolo.valente@linaro.org Signed-off-by: Jens Axboe --- block/bfq-cgroup.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'block/bfq-cgroup.c') diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c index 5f081f4d51fb..b42956ab5550 100644 --- a/block/bfq-cgroup.c +++ b/block/bfq-cgroup.c @@ -706,7 +706,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq, bfq_activate_bfqq(bfqd, bfqq); } - if (!bfqd->in_service_queue && !bfqd->rq_in_driver) + if (!bfqd->in_service_queue && !bfqd->tot_rq_in_driver) bfq_schedule_dispatch(bfqd); /* release extra ref taken above, bfqq may happen to be freed now */ bfq_put_queue(bfqq); -- cgit v1.2.3