[RFC 0/3] aio-posix: enable io_uring SINGLE_ISSUER and TASKRUN flags

[Stefan Hajnoczi <stefanha@redhat.com>]

Do not merge this series. The performance effects are not significant. I am
sharing this mainly to archive the patches and in case someone has ideas on how
to improve this.

Bernd Schubert mentioned io_uring_setup(2) flags that may improve performance:
- IORING_SETUP_SINGLE_ISSUER: optimization when only 1 thread uses an io_uring context
- IORING_SETUP_COOP_TASKRUN: avoids IPIs
- IORING_SETUP_TASKRUN_FLAG: makes COOP_TASKRUN work with userspace CQ ring polling

Suraj Shirvankar already started work on SINGLE_ISSUER in the past:
https://lore.kernel.org/qemu-devel/174293621917.22751.11381319865102029969-0@git.sr.ht/

Where this differs from Suraj's previous work is that I have worked around the
need for the main loop AioContext to be shared by multiple threads (vCPU
threads and the migration thread).

Here are the performance numbers for fio bs=4k in a 4 vCPU guest with 1
IOThread using a virtio-blk disk backed by a local NVMe drive:

                      IOPS               IOPS
Benchmark             SINGLE_ISSUER      SINGLE_ISSUER|COOP_TASKRUN|TASKRUN_FLAG
randread  iodepth=1   54,045 (+1.2%)     54,189 (+1.5%)
randread  iodepth=64  318,135 (+0.1%)    315,632 (-0.68%)
randwrite iodepth=1   141,918 (-0.44%)   143,337 (+0.55%)
randwrite iodepth=64  323,948 (-0.015%)  322,755 (-0.38%)

You can find detailed benchmarking results here including the fio
output, fio command-line, and guest libvirt domain XML:
https://gitlab.com/stefanha/virt-playbooks/-/tree/io_uring-flags/notebook/fio-output
https://gitlab.com/stefanha/virt-playbooks/-/blob/io_uring-flags/files/fio.sh
https://gitlab.com/stefanha/virt-playbooks/-/blob/io_uring-flags/files/test.xml.j2

Stefan Hajnoczi (3):
  iothread: create AioContext in iothread_run()
  aio-posix: enable IORING_SETUP_SINGLE_ISSUER
  aio-posix: enable IORING_SETUP_COOP_TASKRUN |
    IORING_SETUP_TASKRUN_FLAG

 include/system/iothread.h |   1 -
 iothread.c                | 140 +++++++++++++++++++++-----------------
 util/fdmon-io_uring.c     |  26 ++++++-
 3 files changed, 101 insertions(+), 66 deletions(-)

-- 
2.50.1