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Subject[PATCH BUGFIX IMPROVEMENT 07/14] block, bfq: do not plug I/O of in-service queue when harmful
Date
If the in-service bfq_queue is sync and remains temporarily idle, then
I/O dispatching (from other queues) may be plugged. It may be dome for
two reasons: either to boost throughput, or to preserve the bandwidth
share of the in-service queue. In the first case, if the I/O of the
in-service queue, when it finally arrives, consists only of one small
I/O request, then it makes sense to plug even the I/O of the
in-service queue. In fact, serving such a small request immediately is
likely to lower throughput instead of boosting it, whereas waiting a
little bit is likely to let that request grow, thanks to request
merging, and become more profitable in terms of throughput (this is
likely to happen exactly because the I/O of the queue has been
detected to boost throughput).

On the opposite end, if I/O dispatching is being plugged only to
preserve the bandwidth of the in-service queue, then it would be
better not to plug also the I/O of the in-service queue, because such
a plugging is likely to cause only loss of bandwidth for the queue.

Unfortunately, no distinction is made between the two cases, and the
I/O of the in-service queue is always plugged in case just a small I/O
request arrives. This commit draws this missing distinction and does
not perform harmful plugging.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
block/bfq-iosched.c | 31 +++++++++++++++++--------------
1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 2756f4b1432b..a6fe60114ade 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -4599,28 +4599,31 @@ static void bfq_rq_enqueued(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bool budget_timeout = bfq_bfqq_budget_timeout(bfqq);

/*
- * There is just this request queued: if the request
- * is small and the queue is not to be expired, then
- * just exit.
+ * There is just this request queued: if
+ * - the request is small, and
+ * - we are idling to boost throughput, and
+ * - the queue is not to be expired,
+ * then just exit.
*
* In this way, if the device is being idled to wait
* for a new request from the in-service queue, we
* avoid unplugging the device and committing the
- * device to serve just a small request. On the
- * contrary, we wait for the block layer to decide
- * when to unplug the device: hopefully, new requests
- * will be merged to this one quickly, then the device
- * will be unplugged and larger requests will be
- * dispatched.
+ * device to serve just a small request. In contrast
+ * we wait for the block layer to decide when to
+ * unplug the device: hopefully, new requests will be
+ * merged to this one quickly, then the device will be
+ * unplugged and larger requests will be dispatched.
*/
- if (small_req && !budget_timeout)
+ if (small_req && idling_boosts_thr_without_issues(bfqd, bfqq) &&
+ !budget_timeout)
return;

/*
- * A large enough request arrived, or the queue is to
- * be expired: in both cases disk idling is to be
- * stopped, so clear wait_request flag and reset
- * timer.
+ * A large enough request arrived, or idling is being
+ * performed to preserve service guarantees, or
+ * finally the queue is to be expired: in all these
+ * cases disk idling is to be stopped, so clear
+ * wait_request flag and reset timer.
*/
bfq_clear_bfqq_wait_request(bfqq);
hrtimer_try_to_cancel(&bfqd->idle_slice_timer);
--
2.20.1
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