linux-kernel - [PATCH 4.14 68/81] block, bfq: increase idling for weight-raised queues

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20190613075653.988319178@linuxfoundation.org>
Date:   Thu, 13 Jun 2019 10:33:51 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Holger Hoffstätte 
        <holger@...lied-asynchrony.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.14 68/81] block, bfq: increase idling for weight-raised queues

[ Upstream commit 778c02a236a8728bb992de10ed1f12c0be5b7b0e ]

If a sync bfq_queue has a higher weight than some other queue, and
remains temporarily empty while in service, then, to preserve the
bandwidth share of the queue, it is necessary to plug I/O dispatching
until a new request arrives for the queue. In addition, a timeout
needs to be set, to avoid waiting for ever if the process associated
with the queue has actually finished its I/O.

Even with the above timeout, the device is however not fed with new
I/O for a while, if the process has finished its I/O. If this happens
often, then throughput drops and latencies grow. For this reason, the
timeout is kept rather low: 8 ms is the current default.

Unfortunately, such a low value may cause, on the opposite end, a
violation of bandwidth guarantees for a process that happens to issue
new I/O too late. The higher the system load, the higher the
probability that this happens to some process. This is a problem in
scenarios where service guarantees matter more than throughput. One
important case are weight-raised queues, which need to be granted a
very high fraction of the bandwidth.

To address this issue, this commit lower-bounds the plugging timeout
for weight-raised queues to 20 ms. This simple change provides
relevant benefits. For example, on a PLEXTOR PX-256M5S, with which
gnome-terminal starts in 0.6 seconds if there is no other I/O in
progress, the same applications starts in
- 0.8 seconds, instead of 1.2 seconds, if ten files are being read
  sequentially in parallel
- 1 second, instead of 2 seconds, if, in parallel, five files are
  being read sequentially, and five more files are being written
  sequentially

Tested-by: Holger Hoffstätte <holger@...lied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@...alenko.name>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 block/bfq-iosched.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 3b44bd28fc45..7d45ac451745 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2282,6 +2282,8 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd)
 	if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
 	    bfq_symmetric_scenario(bfqd))
 		sl = min_t(u64, sl, BFQ_MIN_TT);
+	else if (bfqq->wr_coeff > 1)
+		sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);

 	bfqd->last_idling_start = ktime_get();
 	hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),
-- 
2.20.1