On Thursday, October 27, 2016 8:13:08 PM CEST Ulf Hansson wrote:

On 27 October 2016 at 19:43, Jens Axboe [off-list ref] wrote:

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On 10/27/2016 11:32 AM, Ulf Hansson wrote:

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[...]

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I'm hesistant to add a new scheduler because it's very easy to add, very
difficult to get rid of. If we do add BFQ as a legacy scheduler now,
it'll take us years and years to get rid of it again. We should be
moving towards LESS moving parts in the legacy path, not more.

Jens, I think you are wrong here and let me try to elaborate on why.

1)
We already have legacy schedulers like CFQ, DEADLINE, etc - and most
block device drivers are still using the legacy blk interface.

I don't think that's an accurate statement. In terms of coverage, most
drivers do support blk-mq. Anything SCSI, nvme, virtio-blk, SATA runs on
(or can run on) top of blk-mq.

Well, I just used "git grep" and found that many drivers didn't use
blkmq. Apologize if I gave the wrong impressions.

To clarify, this seems to be a complete list:

$ git grep -wl '\(__\|\)blk_\(fetch\|end\|start\)_request' | xargs grep -L blk_mq
Documentation/scsi/scsi_eh.txt
arch/um/drivers/ubd_kern.c
block/blk-tag.c
block/bsg-lib.c
drivers/block/DAC960.c
drivers/block/amiflop.c
drivers/block/aoe/aoeblk.c
drivers/block/aoe/aoecmd.c
drivers/block/aoe/aoedev.c
drivers/block/ataflop.c
drivers/block/cciss.c
drivers/block/floppy.c
drivers/block/hd.c
drivers/block/mg_disk.c
drivers/block/osdblk.c
drivers/block/paride/pcd.c
drivers/block/paride/pd.c
drivers/block/paride/pf.c
drivers/block/ps3disk.c
drivers/block/skd_main.c
drivers/block/sunvdc.c
drivers/block/swim.c
drivers/block/swim3.c
drivers/block/sx8.c
drivers/block/xsysace.c
drivers/block/z2ram.c
drivers/cdrom/gdrom.c
drivers/ide/ide-atapi.c
drivers/ide/ide-io.c
drivers/ide/ide-pm.c
drivers/memstick/core/ms_block.c
drivers/memstick/core/mspro_block.c
drivers/mmc/card/block.c
drivers/mmc/card/queue.c
drivers/mtd/mtd_blkdevs.c
drivers/s390/block/dasd.c
drivers/s390/block/scm_blk.c
drivers/sbus/char/jsflash.c
drivers/scsi/osd/osd_initiator.c
drivers/scsi/scsi_transport_fc.c
drivers/scsi/scsi_transport_sas.c
samples/bpf/tracex3_kern.c

From what I can tell, most of these are hopelessly obsolete, but

there are some notable exceptions: aoe, osdblk, skd, sunvdc, mtdblk,
mmc, dasd and scm. I've never used any of the first four, but the
last four of the list are certainly important (for very different
reasons).

	Arnd