[PATCH v6 0/2] blk-mq: introduce tag starvation observability

[Aaron Tomlin <atomlin@atomlin.com>]

Hi Jens, Steve, Masami,

In high-performance storage environments, particularly when utilising RAID
controllers with shared tag sets (BLK_MQ_F_TAG_HCTX_SHARED), severe latency
spikes can occur when fast devices are starved of available tags.
Currently, diagnosing this specific queue contention requires deploying
dynamic kprobes or inferring sleep states, which lacks a simple,
out-of-the-box diagnostic path.

This short series introduces dedicated, low-overhead observability for tag
exhaustion events in the block layer:

  - Patch 1 introduces the "block_rq_tag_wait" tracepoint in the tag
    allocation slow-path to capture precise, event-based starvation.

  - Patch 2 complements this by exposing "wait_on_hw_tag" and
    "wait_on_sched_tag" per-CPU counters via debugfs for quick,
    point-in-time cumulative polling.

Together, these provide storage engineers with zero-configuration
mechanisms to definitively identify shared-tag bottlenecks.

Please let me know your thoughts.


Changes since v5 [1]:
 - Replaced this_cpu_inc() with raw_cpu_inc() within
   blk_mq_debugfs_inc_wait_tags(). This resolves a preemption warning
   triggered under CONFIG_DEBUG_PREEMPT=y, as the routine is invoked from a
   preemptible context immediately prior to io_schedule(). This adjustment
   deliberately prioritises the reduction of execution overhead over
   absolute statistical precision for this diagnostic interface.

Changes since v4 [2]:
 - Prevented a NULL pointer dereference in the tracepoint fast-assign for
   disk-less request queues by safely checking q->disk before resolving the
   dev_t

 - Fixed a Use-After-Free (UAF) and permanent memory leak by decoupling
   the per-CPU counter allocation from the volatile debugfs lifecycle and
   tying it directly to the core hctx lifecycle (i.e., blk_mq_init_hctx()
   and blk_mq_exit_hctx())

 - Fixed a potential compiler double-fetch bug by wrapping the per-CPU
   pointer evaluations with READ_ONCE() in blk_mq_debugfs_inc_wait_tags()

 - Passed the appropriate gfp_t flags down to the allocation routines to
   maintain the strict GFP_NOIO context

 - Updated kernel-doc descriptions to clarify that the NULL pointer 
   checks guard against memory allocation failures under pressure, rather 
   than initialisation race conditions

Changes since v3 [3]:
 - Transitioned tracking architecture from shared atomic_t variables to
   dynamically allocated per-CPU counters to resolve cache line bouncing
   (Bart Van Assche)

Changes since v2 [4]:
 - Added "Reviewed-by:" and "Tested-by:" tags for patch 1

 - Evaluate is_sched_tag directly within TP_fast_assign (Steven Rostedt)

 - Introduced atomic counters via debugfs 

Changes since v1 [5]:
 - Improved the description of the trace point (Damien Le Moal)

 - Removed the redundant "active requests" (Laurence Oberman)

 - Introduced pool-specific starvation tracking

[1]: https://lore.kernel.org/lkml/20260427020142.358912-1-atomlin@atomlin.com/
[2]: https://lore.kernel.org/lkml/20260419023036.1419514-1-atomlin@atomlin.com/
[3]: https://lore.kernel.org/lkml/20260319221956.332770-1-atomlin@atomlin.com/
[4]: https://lore.kernel.org/lkml/20260319015300.287653-1-atomlin@atomlin.com/
[5]: https://lore.kernel.org/lkml/20260317182835.258183-1-atomlin@atomlin.com/


Aaron Tomlin (2):
  blk-mq: add tracepoint block_rq_tag_wait
  blk-mq: expose tag starvation counts via debugfs

 block/blk-mq-debugfs.c       | 109 +++++++++++++++++++++++++++++++++++
 block/blk-mq-debugfs.h       |  19 ++++++
 block/blk-mq-tag.c           |   8 +++
 block/blk-mq.c               |   5 ++
 include/linux/blk-mq.h       |  12 ++++
 include/trace/events/block.h |  43 ++++++++++++++
 6 files changed, 196 insertions(+)

-- 
2.51.0