[PATCH 0/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies

[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.

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: complete requests via blk_mq_complete_request() so rq_affinity
    applies
  selftests: ublk: add test for rq_affinity completion redirection

 drivers/block/ublk_drv.c                        | 72 +++++++++++++++++--------
 include/uapi/linux/ublk_cmd.h                   |  8 +++
 tools/testing/selftests/ublk/Makefile           |  1 +
 tools/testing/selftests/ublk/kublk.c            |  1 +
 tools/testing/selftests/ublk/test_generic_18.sh | 42 +++++++++++++++
 5 files changed, 103 insertions(+), 21 deletions(-)
 create mode 100755 tools/testing/selftests/ublk/test_generic_18.sh

--
2.50.1 (Apple Git-155)