[PATCH v4 0/3] nvme: introduce the service-time iopolicy

[Guixin Liu]

The queue-depth iopolicy counts requests, so a 128K I/O weighs the same
as a 4K one, and it cannot express that one path is simply faster than
another. This series adds a service-time iopolicy for NVMe native
multipath, adapted from dm-ps-service-time: it picks the path with the
lowest estimated service time, in_flight_bytes / relative_throughput,
so balancing follows actual bytes in flight and per-path throughput
weights.

Weights live in a per-path relative_throughput sysfs attribute (0-100,
default 1, 0 makes the path a standby). Patch 3 documents the new
attributes in the stable sysfs ABI; patch 1 carries the benchmark data
in its commit log.

This version is rebased onto the nvme-7.4 branch of
git://git.infradead.org/nvme.git.

Changes since v3:

- Rebased onto nvme-7.4, which adds the iopolicies and path_state
  attributes and the discovery-subsystem "none" handling.

- The iopolicies ABI entry now lists service-time too. nvme-7.4
  already documents the attribute (with a hardcoded list of the
  three older policies); patch 3 extends that list along with the
  iopolicy accepted values.

Changes since v2:

- in_flight_bytes no longer leaks on failover: the accounted bytes
  are now released in nvme_failover_req() before blk_steal_bios()
  clears the request data length. Before, every failover (ANA
  transition, path error, queue_rq unwind) permanently left the
  request's bytes in the counter, skewing path selection until a
  controller reset. Reproduced with an ANA transition during load on
  the NVMe-oF/TCP test setup: the counter kept 512K of phantom bytes
  after all I/O completed; with the fix it returns to zero.

- The release clamps at zero instead of a plain subtraction. A
  request requeued across a controller reset is accounted against the
  pre-reset counter but completes after nvme_mpath_init_identify()
  re-zeroed it, and the plain subtraction would wrap negative, making
  the comparator treat the path as pathologically overloaded. This
  mirrors atomic_dec_if_positive() in the queue-depth accounting.

- The comparison operates on u64 instead of size_t, so the byte
  counts and the shift guard keep their magnitude on 32-bit builds,
  where size_t would truncate the atomic64 reads.

- Added "service-time" to the accepted values of the subsystem
  iopolicy ABI entry and to the policies listed by the iopolicies
  ABI entry.

No public v3 was posted; this version folds the review feedback from
the v2 discussion and a rebase onto nvme-7.4.

On the adaptive I/O policy dependency Hannes raised during v1: that
series has become the latency I/O policy and is still under
revision. The two are complementary: the latency policy infers path
speed from observed completions, service-time takes configured
weights and byte counts.

Guixin Liu (3):
  nvme-multipath: implement service-time iopolicy
  nvme-multipath: document service-time iopolicy
  nvme: document the service-time iopolicy sysfs attributes

 Documentation/ABI/stable/sysfs-nvme          |  35 +++-
 Documentation/admin-guide/nvme-multipath.rst |  31 +++-
 drivers/nvme/host/multipath.c                | 189 ++++++++++++++++++-
 drivers/nvme/host/nvme.h                     |   6 +
 drivers/nvme/host/sysfs.c                    |   7 +-
 5 files changed, 256 insertions(+), 12 deletions(-)

v2 of this series:
https://lore.kernel.org/r/20261019075607.808947-1-kanie@linux.alibaba.com

v1 of this series:
https://lore.kernel.org/r/20260617114602.2224074-1-kanie@linux.alibaba.com

latency I/O policy v7:
https://lore.kernel.org/r/20260809100825.2014133-1-nilay@linux.ibm.com
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