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From: Paolo Valente <paolo.valente@linaro.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	mariottiluca1@hotmail.it, holger@applied-asynchrony.com,
	pedroni.pietro.96@gmail.com,
	Paolo Valente <paolo.valente@linaro.org>
Subject: [PATCH FIXES/IMPROVEMENTS 3/7] block, bfq: consider also creation time in delayed stable merge
Date: Sat, 19 Jun 2021 16:09:44 +0200	[thread overview]
Message-ID: <20210619140948.98712-4-paolo.valente@linaro.org> (raw)
In-Reply-To: <20210619140948.98712-1-paolo.valente@linaro.org>

Since commit 430a67f9d616 ("block, bfq: merge bursts of newly-created
queues"), BFQ may schedule a merge between a newly created sync
bfq_queue and the last sync bfq_queue created. Such a merging is not
performed immediately, because BFQ needs first to find out whether the
newly created queue actually reaches a higher throughput if not merged
at all (and in that case BFQ will not perform any stable merging). To
check that, a little time must be waited after the creation of the new
queue, so that some I/O can flow in the queue, and statistics on such
I/O can be computed.

Yet, to evaluate the above waiting time, the last split time is
considered as start time, instead of the creation time of the
queue. This is a mistake, because considering the split time is
correct only in the following scenario.

The queue undergoes a non-stable merges on the arrival of its very
first I/O request, due to close I/O with some other queue. While the
queue is merged for close I/O, stable merging is not considered. Yet
the queue may then happen to be split, if the close I/O finishes (or
happens to be a false positive). From this time on, the queue can
again be considered for stable merging. But, again, a little time must
elapse, to let some new I/O flow in the queue and to get updated
statistics. To wait for this time, the split time is to be taken into
account.

Yet, if the queue does not undergo a non-stable merge on the arrival
of its very first request, then BFQ immediately checks whether the
stable merge is to be performed. It happens because the split time for
a queue is initialized to minus infinity when the queue is created.

This commit fixes this mistake by adding the missing condition. Now
the check for delayed stable-merge is performed after a little time is
elapsed not only from the last queue split time, but also from the
creation time of the queue.

Fixes: 430a67f9d616 ("block, bfq: merge bursts of newly-created queues")
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
 block/bfq-iosched.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index c5c0e74977d4..2a5c1a660f3b 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2711,7 +2711,9 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		if (bic->stable_merge_bfqq &&
 		    !bfq_bfqq_just_created(bfqq) &&
 		    time_is_before_jiffies(bfqq->split_time +
-					  msecs_to_jiffies(200))) {
+					  msecs_to_jiffies(200)) &&
+		    time_is_before_jiffies(bfqq->creation_time +
+					   msecs_to_jiffies(200))) {
 			struct bfq_queue *stable_merge_bfqq =
 				bic->stable_merge_bfqq;
 			int proc_ref = min(bfqq_process_refs(bfqq),
-- 
2.20.1


  parent reply	other threads:[~2021-06-19 14:10 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-19 14:09 [PATCH FIXES/IMPROVEMENTS 0/7] block, bfq: preserve control, boost throughput, fix bugs Paolo Valente
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 1/7] block, bfq: let also stably merged queues enjoy weight raising Paolo Valente
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 2/7] block, bfq: fix delayed stable merge check Paolo Valente
2021-06-19 14:09 ` Paolo Valente [this message]
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 4/7] block, bfq: boost throughput by extending queue-merging times Paolo Valente
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 5/7] block, bfq: avoid delayed merge of async queues Paolo Valente
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 6/7] block, bfq: check waker only for queues with no in-flight I/O Paolo Valente
2021-06-19 14:09 ` [PATCH FIXES/IMPROVEMENTS 7/7] block, bfq: reset waker pointer with shared queues Paolo Valente
2021-06-21 16:08 ` [PATCH FIXES/IMPROVEMENTS 0/7] block, bfq: preserve control, boost throughput, fix bugs Jens Axboe
2021-06-21 19:55 ` Oleksandr Natalenko
2021-06-21 20:03   ` Piotr Górski
2021-06-22  7:08   ` Paolo Valente
2021-06-22  7:35     ` Oleksandr Natalenko
2021-06-22 16:29       ` Jan Kara
2021-06-22 17:26         ` Oleksandr Natalenko
2021-07-02 22:07         ` Oleksandr Natalenko
2021-08-02 20:40           ` Oleksandr Natalenko
2021-08-03 10:45             ` Jan Kara

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