linux-kernel - [QUESTION] v4.12-rc5 BFQ enabled but unavailable?

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Message-ID: <20170613052833.GB6311@shells.gnugeneration.com>
Date:   Mon, 12 Jun 2017 22:28:34 -0700
From:   vcaputo@...garu.com
To:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: [QUESTION] v4.12-rc5 BFQ enabled but unavailable?

Hello LKML,

Attempted to play with BFQ but after building with CONFIG_IOSCHED_BFQ=y I'm
still not seeing it:
 # cat /sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda/queue/scheduler
 noop deadline [cfq] 
 # 

Even though:
 # dmesg | grep sched
 [    0.514536] io scheduler noop registered
 [    0.514537] io scheduler deadline registered
 [    0.514577] io scheduler cfq registered (default)
 [    0.514613] io scheduler bfq registered
 #

Quick glance at block/elevator.c elv_iosched_show() reveals:
 1125         list_for_each_entry(__e, &elv_list, list) {
 1126                 if (elv && !strcmp(elv->elevator_name, __e->elevator_name)) {
 1127                         len += sprintf(name+len, "[%s] ", elv->elevator_name);
 1128                         continue;
 1129                 }
 1130                 if (__e->uses_mq && q->mq_ops && elv_support_iosched(q))
 1131                         len += sprintf(name+len, "%s ", __e->elevator_name);
 1132                 else if (!__e->uses_mq && !q->mq_ops)
 1133                         len += sprintf(name+len, "%s ", __e->elevator_name);
 1134         }

iosched_bfq_mq.uses_mq is true, so it appears bfq can be silently omitted from
elv_iosched_show() output if !q->mq_ops || !elv_support_iosched(q).

What magic incantation is recommended to get this stuff working and how is a
user like myself expected to discover it without digging through the code?

I glanced through Documentation/block/bfq-iosched.txt, but mq isn't mentioned:
 $ grep -ic mq Documentation/block/bfq-iosched.txt 
 0
 $

There are use_blk_mq parameters for dm_mod and scsi_mod, defaulting to N:
 # cd /sys/module && find -iname '*mq*' -print -exec cat {} \;
 ./dm_mod/parameters/use_blk_mq
 N
 ./dm_mod/parameters/dm_mq_nr_hw_queues
 1
 ./dm_mod/parameters/dm_mq_queue_depth
 2048
 ./scsi_mod/parameters/use_blk_mq
 N
 #

Is one expected to force use_blk_mq manually to make BFQ available?

Any advice appreciated, thanks!

Regards,
Vito Caputo