On Wed, 2018-08-22 at 11:42 -0600, Jens Axboe wrote:

Any chance you can try 4.18 at least?

Of course, I tried the kernel.org sources now, sys-kernel/vanilla-sources-4.18.3 
on Gentoo. The issue is the same.

I wonder that nobody has noticed that issue that I can reproduce on many kernels
since 4.13 or even older. Is it a rare combination with my bcache, luks or
btrfs? Or hardware specific? Or scheduler specific, as even BFQ is not that well
tested on btrfs like the default one.

Schedulers are specific to the queuing infrastructure, so certain types
will only be available with MQ and certain types will only be available
on !MQ.

I noticed that. It's just a bit intransparent from the menuconfig, options don't
mention if a scheduler is working on MQ or not, like CONFIG_IOSCHED_BFQ.
There it's written in the description, while CONFIG_MQ_IOSCHED_KYBER has the
note in the name.
Actually MQ schedulers could depend on BLK_MQ to make it obvious.

Best regards,
Massimo