linux-kernel - Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Date:   Fri, 28 Oct 2016 14:07:29 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Paolo Valente <paolo.valente@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Hannes Reinecke <hare@...e.de>,
        Grant Likely <grant.likely@...retlab.ca>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

On Thursday, October 27, 2016 8:13:08 PM CEST Ulf Hansson wrote:
> On 27 October 2016 at 19:43, Jens Axboe <axboe@...nel.dk> wrote:
> > On 10/27/2016 11:32 AM, Ulf Hansson wrote:
> >>
> >> [...]
> >>
> >>>
> >>> 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