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block, bfq: always inject I/O of queues blocked by wakers

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Suppose that I/O dispatch is plugged, to wait for new I/O for the
in-service bfq-queue, say bfqq.  Suppose then that there is a further
bfq_queue woken by bfqq, and that this woken queue has pending I/O. A
woken queue does not steal bandwidth from bfqq, because it remains
soon without I/O if bfqq is not served. So there is virtually no risk
of loss of bandwidth for bfqq if this woken queue has I/O dispatched
while bfqq is waiting for new I/O. In contrast, this extra I/O
injection boosts throughput. This commit performs this extra
injection.

Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Link: https://lore.kernel.org/r/20210304174627.161-2-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2021-03-04 18:46:22 +01:00 committed by Jens Axboe
parent 9cf1adc6d3
commit 2ec5a5c483
2 changed files with 35 additions and 5 deletions
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32
block/bfq-iosched.c
View File

@ -4491,9 +4491,15 @@ check_queue:
bfq_bfqq_busy(bfqq->bic->bfqq[0]) &&
bfqq->bic->bfqq[0]->next_rq ?
bfqq->bic->bfqq[0] : NULL;
struct bfq_queue *blocked_bfqq =
!hlist_empty(&bfqq->woken_list) ?
container_of(bfqq->woken_list.first,
struct bfq_queue,
woken_list_node)
: NULL;
/*
* The next three mutually-exclusive ifs decide
* The next four mutually-exclusive ifs decide
* whether to try injection, and choose the queue to
* pick an I/O request from.
*
@ -4526,7 +4532,15 @@ check_queue:
* next bfqq's I/O is brought forward dramatically,
* for it is not blocked for milliseconds.
*
* The third if checks whether bfqq is a queue for
* The third if checks whether there is a queue woken
* by bfqq, and currently with pending I/O. Such a
* woken queue does not steal bandwidth from bfqq,
* because it remains soon without I/O if bfqq is not
* served. So there is virtually no risk of loss of
* bandwidth for bfqq if this woken queue has I/O
* dispatched while bfqq is waiting for new I/O.
*
* The fourth if checks whether bfqq is a queue for
* which it is better to avoid injection. It is so if
* bfqq delivers more throughput when served without
* any further I/O from other queues in the middle, or
@ -4546,11 +4560,11 @@ check_queue:
* bfq_update_has_short_ttime(), it is rather likely
* that, if I/O is being plugged for bfqq and the
* waker queue has pending I/O requests that are
* blocking bfqq's I/O, then the third alternative
* blocking bfqq's I/O, then the fourth alternative
* above lets the waker queue get served before the
* I/O-plugging timeout fires. So one may deem the
* second alternative superfluous. It is not, because
* the third alternative may be way less effective in
* the fourth alternative may be way less effective in
* case of a synchronization. For two main
* reasons. First, throughput may be low because the
* inject limit may be too low to guarantee the same
@ -4559,7 +4573,7 @@ check_queue:
* guarantees (the second alternative unconditionally
* injects a pending I/O request of the waker queue
* for each bfq_dispatch_request()). Second, with the
* third alternative, the duration of the plugging,
* fourth alternative, the duration of the plugging,
* i.e., the time before bfqq finally receives new I/O,
* may not be minimized, because the waker queue may
* happen to be served only after other queues.
@ -4577,6 +4591,14 @@ check_queue:
bfq_bfqq_budget_left(bfqq->waker_bfqq)
)
bfqq = bfqq->waker_bfqq;
else if (blocked_bfqq &&
bfq_bfqq_busy(blocked_bfqq) &&
blocked_bfqq->next_rq &&
bfq_serv_to_charge(blocked_bfqq->next_rq,
blocked_bfqq) <=
bfq_bfqq_budget_left(blocked_bfqq)
)
bfqq = blocked_bfqq;
else if (!idling_boosts_thr_without_issues(bfqd, bfqq) &&
(bfqq->wr_coeff == 1 || bfqd->wr_busy_queues > 1 ||
!bfq_bfqq_has_short_ttime(bfqq)))

8
block/bfq-wf2q.c
View File

@ -1706,4 +1706,12 @@ void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq)
if (bfqq->wr_coeff > 1)
bfqd->wr_busy_queues++;
/* Move bfqq to the head of the woken list of its waker */
if (!hlist_unhashed(&bfqq->woken_list_node) &&
&bfqq->woken_list_node != bfqq->waker_bfqq->woken_list.first) {
hlist_del_init(&bfqq->woken_list_node);
hlist_add_head(&bfqq->woken_list_node,
&bfqq->waker_bfqq->woken_list);
}
}