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block, bfq: re-evaluate convenience of I/O plugging on rq arrivals 

Upon an I/O-dispatch attempt, BFQ may detect that it was better to
plug I/O dispatch, and to wait for a new request to arrive for the
currently in-service queue. But the arrival of a new request for an
empty bfq_queue, and thus the switch from idle to busy of the
bfq_queue, may cause the scenario to change, and make plugging no
longer needed for service guarantees, or more convenient for
throughput. In this case, keeping I/O-dispatch plugged would certainly
lower throughput.

To address this issue, this commit makes such a check, and stops
plugging I/O if it is better to stop plugging I/O.

Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2021-01-25 20:02:44 +01:00 committed by Jens Axboe
parent eb2fd80f9d
commit 7f1995c27b
1 changed files with 19 additions and 5 deletions
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24
block/bfq-iosched.c
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@ -1649,6 +1649,8 @@ static bool bfq_bfqq_higher_class_or_weight(struct bfq_queue *bfqq,
return bfqq_weight > in_serv_weight;
}
static bool bfq_better_to_idle(struct bfq_queue *bfqq);
static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
struct bfq_queue *bfqq,
int old_wr_coeff,
@ -1750,10 +1752,10 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
bfq_add_bfqq_busy(bfqd, bfqq);
/*
* Expire in-service queue only if preemption may be needed
* for guarantees. In particular, we care only about two
* cases. The first is that bfqq has to recover a service
* hole, as explained in the comments on
* Expire in-service queue if preemption may be needed for
* guarantees or throughput. As for guarantees, we care
* explicitly about two cases. The first is that bfqq has to
* recover a service hole, as explained in the comments on
* bfq_bfqq_update_budg_for_activation(), i.e., that
* bfqq_wants_to_preempt is true. However, if bfqq does not
* carry time-critical I/O, then bfqq's bandwidth is less
@ -1780,11 +1782,23 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
* timestamps of the in-service queue would need to be
* updated, and this operation is quite costly (see the
* comments on bfq_bfqq_update_budg_for_activation()).
*
* As for throughput, we ask bfq_better_to_idle() whether we
* still need to plug I/O dispatching. If bfq_better_to_idle()
* says no, then plugging is not needed any longer, either to
* boost throughput or to perserve service guarantees. Then
* the best option is to stop plugging I/O, as not doing so
* would certainly lower throughput. We may end up in this
* case if: (1) upon a dispatch attempt, we detected that it
* was better to plug I/O dispatch, and to wait for a new
* request to arrive for the currently in-service queue, but
* (2) this switch of bfqq to busy changes the scenario.
*/
if (bfqd->in_service_queue &&
((bfqq_wants_to_preempt &&
bfqq->wr_coeff >= bfqd->in_service_queue->wr_coeff) ||
bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue)) &&
bfq_bfqq_higher_class_or_weight(bfqq, bfqd->in_service_queue) ||
!bfq_better_to_idle(bfqd->in_service_queue)) &&
next_queue_may_preempt(bfqd))
bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
false, BFQQE_PREEMPTED);