[PATCH v2 0/2] ublk: honor rq_affinity on request completion

[Yoav Cohen <yoav@nvidia.com>]

ublk ends successfully completed requests inline on the server thread,
so rq_affinity never redirects completion work back to the submitting
CPU the way it does for NVMe, SCSI, virtio-blk, loop, nbd and rnbd.
Patch 1 fixes that and adds a feature flag so servers can detect
support. Patch 2 adds a selftest for it.

v2, addressing Ming Lei's review of v1:
- v1 only redirected the classic-copy-mode successful-read completion;
  every write, flush/discard, error, and zero-copy/user-copy/
  auto-buf-reg/shmem-zc I/O still completed inline as before, so
  rq_affinity had no effect for most traffic. ublk_end_rq() now
  re-derives which completion path __ublk_complete_rq() took from
  @req/@io state, so every outcome goes through
  blk_mq_complete_request_remote().
- v1 ran the new completion path unconditionally on any patched
  kernel. v2 adds UBLK_F_SUPPORT_RQ_AFFINITY, which a server must
  request at ADD_DEV time; without it, behavior is unchanged.

With an SPDK-style polling user-space block server at queue depth 96,
rq_affinity=2 gives 7-8% higher IOPS than today's always-inline
completion by offloading a saturated server CPU; rq_affinity=0 keeps
the old behavior for anyone who wants to opt out.

Yoav Cohen (2):
  ublk: honor rq_affinity on request completion
  selftests: ublk: add rq_affinity test

 drivers/block/ublk_drv.c                      | 98 ++++++++++++++-----
 include/uapi/linux/ublk_cmd.h                 |  8 ++
 tools/testing/selftests/ublk/Makefile         |  1 +
 tools/testing/selftests/ublk/kublk.c          |  6 +-
 .../testing/selftests/ublk/test_generic_18.sh | 42 ++++++++
 5 files changed, 132 insertions(+), 23 deletions(-)
 create mode 100755 tools/testing/selftests/ublk/test_generic_18.sh

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