nvme: split bios issued in reverse order

[Keith Busch]

On Tue, May 24, 2022 at 09:25:20AM -0400, Jonathan Nicklin wrote:
> On Tue, May 24, 2022 at 8:58 AM Sagi Grimberg <sagi at grimberg.me> wrote:
> >
> >
> > > There seems to be an inconsistency in the order of writes that are
> > > issued after splitting a bio. Ordering depends on how the application
> > > write is submitted and the number of I/O queues configured.
> > >
> > > In our testing nvme/tcp,
> >
> > Is this specific to nvme-tcp?
> 
> No. This is not specific to nvme-tcp. I confirmed the same behavior
> directly to a pci device.

I just tried this on a qemu pci device with MDTS set to  '3' for 32k splits.
Both psync and libaio dispatch the submitted bio's in ascending LBA order. I'm
using kernel 5.18 with these fio commands:

 # fio --name=global --filename=/dev/nvme0n1 --bs=128k --direct=1 --size=128k --rw=read --ioengine=psync --name=job 
 # fio --name=global --filename=/dev/nvme0n1 --bs=128k --direct=1 --size=128k --rw=read --ioengine=libaio --name=job 

The first command produced this trace:

 fio-438 .. 57.799950: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=29184, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=0, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.799995: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=513, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=64, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800002: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=514, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=128, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800009: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=515, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=192, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800017: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=516, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=256, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800023: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=517, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=320, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800029: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=518, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=384, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-438 .. 57.800046: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=3, cmdid=519, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=448, len=63, ctrl=0x0, dsmgmt=0, reftag=0)

And the second command produced this:

 fio-445 ..  83.440824: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=32896, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=0, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.440870: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=28801, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=64, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.440920: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=24706, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=128, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.440931: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=20611, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=192, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.441246: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=36992, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=256, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.441259: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=32897, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=320, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.441287: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=28802, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=384, len=63, ctrl=0x0, dsmgmt=0, reftag=0)
 fio-445 ..  83.441348: nvme_setup_cmd: nvme0: disk=nvme0n1, qid=10, cmdid=41088, nsid=1, flags=0x0, meta=0x0, cmd=(nvme_cmd_read slba=448, len=63, ctrl=0x0, dsmgmt=0, reftag=0)