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block, bfq: do not consider interactive queues in srt filtering

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The speed at which a bfq_queue receives I/O is one of the parameters by
which bfq decides whether the queue is soft real-time (i.e., whether the
queue contains the I/O of a soft real-time application). In particular,
when a bfq_queue remains without outstanding I/O requests, bfq computes
the minimum time instant, named soft_rt_next_start, at which the next
request of the queue may arrive for the queue to be deemed as soft real
time.

Unfortunately this filtering may cause problems with a queue in
interactive weight raising. In fact, such a queue may be conveying the
I/O needed to load a soft real-time application. The latter will
actually exhibit a soft real-time I/O pattern after it finally starts
doing its job. But, if soft_rt_next_start is updated for an interactive
bfq_queue, and the queue has received a lot of service before remaining
with no outstanding request (likely to happen on a fast device), then
soft_rt_next_start is assigned such a high value that, for a very long
time, the queue is prevented from being possibly considered as soft real
time.

This commit removes the updating of soft_rt_next_start for bfq_queues in
interactive weight raising.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Paolo Valente 2019-01-29 12:06:25 +01:00 committed by Jens Axboe
parent 22cb4e6815
commit 20cd32450b
1 changed files with 29 additions and 10 deletions
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39
block/bfq-iosched.c
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@ -3274,16 +3274,32 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
* requests, then the request pattern is isochronous * requests, then the request pattern is isochronous
* (see the comments on the function * (see the comments on the function
* bfq_bfqq_softrt_next_start()). Thus we can compute * bfq_bfqq_softrt_next_start()). Thus we can compute
* soft_rt_next_start. If, instead, the queue still * soft_rt_next_start. And we do it, unless bfqq is in
* has outstanding requests, then we have to wait for * interactive weight raising. We do not do it in the
* the completion of all the outstanding requests to * latter subcase, for the following reason. bfqq may
* discover whether the request pattern is actually * be conveying the I/O needed to load a soft
* isochronous. * real-time application. Such an application will
* actually exhibit a soft real-time I/O pattern after
* it finally starts doing its job. But, if
* soft_rt_next_start is computed here for an
* interactive bfqq, and bfqq had received a lot of
* service before remaining with no outstanding
* request (likely to happen on a fast device), then
* soft_rt_next_start would be assigned such a high
* value that, for a very long time, bfqq would be
* prevented from being possibly considered as soft
* real time.
*
* If, instead, the queue still has outstanding
* requests, then we have to wait for the completion
* of all the outstanding requests to discover whether
* the request pattern is actually isochronous.
*/ */
if (bfqq->dispatched == 0) if (bfqq->dispatched == 0 &&
bfqq->wr_coeff != bfqd->bfq_wr_coeff)
bfqq->soft_rt_next_start = bfqq->soft_rt_next_start =
bfq_bfqq_softrt_next_start(bfqd, bfqq); bfq_bfqq_softrt_next_start(bfqd, bfqq);
else { else if (bfqq->dispatched > 0) {
/* /*
* Schedule an update of soft_rt_next_start to when * Schedule an update of soft_rt_next_start to when
* the task may be discovered to be isochronous. * the task may be discovered to be isochronous.
@ -4834,11 +4850,14 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
* isochronous, and both requisites for this condition to hold * isochronous, and both requisites for this condition to hold
* are now satisfied, then compute soft_rt_next_start (see the * are now satisfied, then compute soft_rt_next_start (see the
* comments on the function bfq_bfqq_softrt_next_start()). We * comments on the function bfq_bfqq_softrt_next_start()). We
* schedule this delayed check when bfqq expires, if it still * do not compute soft_rt_next_start if bfqq is in interactive
* has in-flight requests. * weight raising (see the comments in bfq_bfqq_expire() for
* an explanation). We schedule this delayed update when bfqq
* expires, if it still has in-flight requests.
*/ */
if (bfq_bfqq_softrt_update(bfqq) && bfqq->dispatched == 0 && if (bfq_bfqq_softrt_update(bfqq) && bfqq->dispatched == 0 &&
RB_EMPTY_ROOT(&bfqq->sort_list)) RB_EMPTY_ROOT(&bfqq->sort_list) &&
bfqq->wr_coeff != bfqd->bfq_wr_coeff)
bfqq->soft_rt_next_start = bfqq->soft_rt_next_start =
bfq_bfqq_softrt_next_start(bfqd, bfqq); bfq_bfqq_softrt_next_start(bfqd, bfqq);