[RFC 0/1] block: export windowed IO P99 latency

[Diangang Li <diangangli@gmail.com>]

Production environments occasionally run into elevated tail latencies. The
source can be the underlying device, but it can also be higher in the
stack (filesystem contention/journaling, memory reclaim, writeback, etc.).
Existing block IO statistics only provide throughput and average latency,
which fail to capture the critical tail end of the latency distribution
that often causes user-visible performance problems.

This patch adds windowed P99 latency tracking for block IO operations,
exposing the 99th percentile latency in /proc/diskstats and
/sys/block/<dev>/stat. System administrators can now monitor tail latency
trends over time using tools like iostat, enabling quick validation or
elimination of disk hardware as the source of latency issues.

Implementation uses per-CPU sliced ring histograms (21 buckets, 8us..~8s
range) with minimal overhead. P99 values are computed by aggregating
recent 1-second slices when reading statistics, reported in microseconds
using bucket midpoints.

The added work on the IO path is intentionally small (bucket selection and
a per-CPU counter update, with occasional per-slice reset), and in our
testing it does not have a measurable impact on IO performance.

Diangang Li (1):
  block: export windowed IO P99 latency

 block/blk-core.c          |  5 ++-
 block/blk-flush.c         |  6 ++-
 block/blk-mq.c            |  5 ++-
 block/genhd.c             | 50 ++++++++++++++++++++++++-
 include/linux/part_stat.h | 79 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 139 insertions(+), 6 deletions(-)

-- 
2.39.5