Arch Linux User Repository

@graysky: See https://groups.google.com/forum/#!msg/bfq-iosched/j4QUK4IPOhw/CY1LmmhcAwAJ, as previously mentioned - apparently other kernel devs have removed compile-time scheduler defaults.

I've been able to "fix" it for now with a udev rule, slightly modified from what @vp1981 posted:

ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="bfq"

(The wildcard was causing several false positives and errors to show up in the system log while booting.)

@mrkline - Interesting... I get the same.
EDIT: https://ck-hack.blogspot.com/2017/08/linux-412-ck2-muqss-version-0160-for.html?showComment=1502832329876#c3067299809122898386

Scratch that, 4.12.7-2 seems to have the same issue.

Apparently ck-2 contains changes to make BFQ the default scheduler: http://ck-hack.blogspot.com/2017/08/linux-412-ck2-muqss-version-0160-for.html

So we should be good to go as soon as @graysky bumps this.

@mrkline, you may ask post-factum to translate new on linux.org.ru, see https://www.linux.org.ru/news/kernel/13581989, but this is receipt from the news:

[russian]
Планировщики ввода-вывода при использовании blk-mq нельзя указывать через командную строку ядра в загрузчике. Вместо этого используйте правило udev, например:

ACTION=="add|change", KERNEL=="sd[a-z]*", ATTR{queue/scheduler}="bfq"

Чтобы включить подсистему blk-mq, ядро нужно собрать с опцией CONFIG_SCSI_MQ_DEFAULT=y или передать параметр scsi_mod.use_blk_mq=1 из загрузчика.
[/russian]

(my translation)
... use udev rule, for example:

ACTION=="add|change", KERNEL=="sd[a-z]*", ATTR{queue/scheduler}="bfq"

In order to enable blk-mq subsystem the kernel has to be configured with option CONFIG_SCSI_MQ_DEFAULT=y or pass parameter scsi_mod.use_blk_mq=1 in boot loader.

P.S. Gee, I found the news on post-factum site:
https://pf.natalenko.name/news/, see "v4.12-pf2 is out" (https://pf.natalenko.name/news/?p=291)

@graysky 4.12.7-1 boots and runs fine on several of my boxen.

The only curious tidbit is that even though BFQ is set as the default:
$ zcat /proc/config.gz | grep -i BFQ
CONFIG_DEFAULT_BFQ=y
CONFIG_DEFAULT_IOSCHED="bfq"
CONFIG_IOSCHED_BFQ=y
CONFIG_BFQ_GROUP_IOSCHED=y

The kernel is defaulting to mq-deadline:
$ cat /sys/block/sda/queue/scheduler
[mq-deadline] kyber bfq none

Is this what @kwe was discussing below?

EDIT: See https://groups.google.com/d/msg/bfq-iosched/j4QUK4IPOhw/CY1LmmhcAwAJ
What's the simplest way to switch all disks to BFQ on boot in the meantime?

Building 4.12.7-1. Apart for the changes in config so you have to configure lots more than before we are missing the CPU optimizations. Update: Got a non-bootable kernel. Trying a second build, config seems better now as expected (I'll blame ccache which I disabled now). #2 Update: Non bootable again, stuck at "Triggering udevents"

4.12.7-1 works like a charm.

@graysky That config option will disappear in the future because the behavior will be enabled by default:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/scsi?id=5c279bd9e40624f4ab6e688671026d6005b066fa&h=master

Apparently this is planned for 4.13:

linux/drivers/scsi$ git tag --contains 5c279bd9e40624f4ab6e688671026d6005b066fa
v4.13-rc1
v4.13-rc2
v4.13-rc3
v4.13-rc4
v4.13-rc5

So, when it comes around, you might want to remove it again. :)

4.12.7-1 boots, everything seems kosher so far.