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block, bfq: check waker only for queues with no in-flight I/O

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Consider two bfq_queues, say Q1 and Q2, with Q2 empty. If a request of
Q1 gets completed shortly before a new request arrives for Q2, then
BFQ flags Q1 as a candidate waker for Q2. Yet, the arrival of this new
request may have a different cause, in the following case. If also Q2
has requests in flight while waiting for the arrival of a new request,
then the completion of its own requests may be the actual cause of the
awakening of the process that sends I/O to Q2. So Q1 may be flagged
wrongly as a candidate waker.

This commit avoids this deceptive flagging, by disabling
candidate-waker flagging for Q2, if Q2 has in-flight I/O.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-7-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2021-06-19 16:09:47 +02:00 committed by Jens Axboe
parent bd3664b362
commit efc72524b3
1 changed files with 13 additions and 8 deletions
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21
block/bfq-iosched.c
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@ -1985,14 +1985,18 @@ static void bfq_update_io_intensity(struct bfq_queue *bfqq, u64 now_ns)
* Turning back to the detection of a waker queue, a queue Q is deemed
* as a waker queue for bfqq if, for three consecutive times, bfqq
* happens to become non empty right after a request of Q has been
* completed. In particular, on the first time, Q is tentatively set
* as a candidate waker queue, while on the third consecutive time
* that Q is detected, the field waker_bfqq is set to Q, to confirm
* that Q is a waker queue for bfqq. These detection steps are
* performed only if bfqq has a long think time, so as to make it more
* likely that bfqq's I/O is actually being blocked by a
* synchronization. This last filter, plus the above three-times
* requirement, make false positives less likely.
* completed. In this respect, even if bfqq is empty, we do not check
* for a waker if it still has some in-flight I/O. In fact, in this
* case bfqq is actually still being served by the drive, and may
* receive new I/O on the completion of some of the in-flight
* requests. In particular, on the first time, Q is tentatively set as
* a candidate waker queue, while on the third consecutive time that Q
* is detected, the field waker_bfqq is set to Q, to confirm that Q is
* a waker queue for bfqq. These detection steps are performed only if
* bfqq has a long think time, so as to make it more likely that
* bfqq's I/O is actually being blocked by a synchronization. This
* last filter, plus the above three-times requirement, make false
* positives less likely.
*
* NOTE
*
@ -2018,6 +2022,7 @@ static void bfq_check_waker(struct bfq_data *bfqd, struct bfq_queue *bfqq,
if (!bfqd->last_completed_rq_bfqq ||
bfqd->last_completed_rq_bfqq == bfqq ||
bfq_bfqq_has_short_ttime(bfqq) ||
bfqq->dispatched > 0 ||
now_ns - bfqd->last_completion >= 4 * NSEC_PER_MSEC ||
bfqd->last_completed_rq_bfqq == bfqq->waker_bfqq)
return;