Re: [PATCH v3 0/2] blk-mq: introduce tag starvation observability

[Aaron Tomlin]

On Thu, Mar 19, 2026 at 06:19:54PM -0400, Aaron Tomlin wrote:
> 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" atomic 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.

Hi Jens, Steve, Masami,

Just a friendly ping on this patch. 

Please let me know if there is any feedback, or if you need me to make any
adjustments.


Kind regards,
-- 
Aaron Tomlin

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