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Subject[PATCH BUGFIX IMPROVEMENT 01/14] block, bfq: do not consider interactive queues in srt filtering
Date
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>
---
block/bfq-iosched.c | 39 +++++++++++++++++++++++++++++----------
1 file changed, 29 insertions(+), 10 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index cd307767a134..c7a4a15c7c19 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -3274,16 +3274,32 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
* requests, then the request pattern is isochronous
* (see the comments on the function
* bfq_bfqq_softrt_next_start()). Thus we can compute
- * soft_rt_next_start. 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.
+ * soft_rt_next_start. And we do it, unless bfqq is in
+ * interactive weight raising. We do not do it in the
+ * latter subcase, for the following reason. bfqq may
+ * be conveying the I/O needed to load a soft
+ * 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 =
bfq_bfqq_softrt_next_start(bfqd, bfqq);
- else {
+ else if (bfqq->dispatched > 0) {
/*
* Schedule an update of soft_rt_next_start to when
* 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
* are now satisfied, then compute soft_rt_next_start (see the
* comments on the function bfq_bfqq_softrt_next_start()). We
- * schedule this delayed check when bfqq expires, if it still
- * has in-flight requests.
+ * do not compute soft_rt_next_start if bfqq is in interactive
+ * 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 &&
- RB_EMPTY_ROOT(&bfqq->sort_list))
+ RB_EMPTY_ROOT(&bfqq->sort_list) &&
+ bfqq->wr_coeff != bfqd->bfq_wr_coeff)
bfqq->soft_rt_next_start =
bfq_bfqq_softrt_next_start(bfqd, bfqq);

--
2.20.1
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