On 3/19/26 10:53, 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 (SSDs) are starved of hardware
> tags when sharing the same blk_mq_tag_set.
>
> Currently, diagnosing this specific hardware queue contention is
> difficult. When a CPU thread exhausts the tag pool, blk_mq_get_tag()
> forces the current thread to block uninterruptible via io_schedule().
> While this can be inferred via sched:sched_switch or dynamically
> traced by attaching a kprobe to blk_mq_mark_tag_wait(), there is no
> dedicated, out-of-the-box observability for this event.
>
> This patch introduces the block_rq_tag_wait static trace point in the
> tag allocation slow-path. It triggers immediately before the thread
> yields the CPU, exposing the exact hardware context (hctx) that is
> starved, the specific pool experiencing starvation (hardware or software
> scheduler), and the total pool depth.
>
> This provides storage engineers and performance monitoring agents
> with a zero-configuration, low-overhead mechanism to definitively
> identify shared-tag bottlenecks and tune I/O schedulers or cgroup
> throttling accordingly.
>
> Signed-off-by: Aaron Tomlin
> ---
> Changes in v1 [1]:
> - 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/[email protected]/
>
> block/blk-mq-tag.c | 4
> include/trace/events/block.h | 43
> 2 files changed, 47 insertions(+)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 33946cdb5716..a6691a4fe7a7 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -13,6 +13,7 @@
> #include
>
> #include
> +#include
> #include "blk.h"
> #include "blk-mq.h"
> #include "blk-mq-sched.h"
> @@ -187,6 +188,9 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data
> *data)
> if (tag != BLK_MQ_NO_TAG)
> break;
>
> + trace_block_rq_tag_wait(data->q, data->hctx,
> + !!(data->rq_flags & RQF_SCHED_TAGS));
I do not think that the "!!" is needed here.
Other than this, this looks OK to me.
Reviewed-by: Damien Le Moal
> +
> bt_prev = bt;
> io_schedule();
>
> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> index 6aa79e2d799c..f7708d0d7a0c 100644
> --- a/include/trace/events/block.h
> +++ b/include/trace/events/block.h
> @@ -226,6 +226,49 @@ DECLARE_EVENT_CLASS(block_rq,
> IOPRIO_PRIO_LEVEL(__entry->ioprio), __entry->comm)
> );
>
> +/**
> + * block_rq_tag_wait - triggered when a request is starved of a tag
> + * @q: request queue of the target device
> + * @hctx: hardware context of the request experiencing starvation
> + * @is_sched_tag: indicates whether the starved pool is the software
> scheduler
> + *
> + * Called immediately before the submitting context is forced to block due
> + * to the exhaustion of available tags (i.e., physical hardware driver tags
> + * or software scheduler tags). This trace point indicates that the context
> + * will be placed into an uninterruptible state via io_schedule() until an
> + * active request completes and relinquishes its assigned tag.
> + */
> +TRACE_EVENT(block_rq_tag_wait,
> +
> + TP_PROTO(struct request_queue *q, struct blk_mq_hw_ctx *hctx, bool
> is_sched_tag),
> +
> + TP_ARGS(q, hctx, is_sched_tag),
> +
> + TP_STRUCT__entry(
> + __field( dev_t, dev )
> + __field( u32, hctx_id )
> + __field( u32, nr_tags )
> + __field( bool, is_sched_tag)
> + ),
> +
> + TP_fast_assign(
> + __entry->dev= disk_devt(q->disk);
> + __entry->hctx_id= hctx->queue_num;
> + __entry->is_sched_tag = is_sched_tag;
> +
> + if (__entry->is_sched_tag)
> + __entry->nr_tags = hctx->sched_tags->nr_tags;
> + else
> + __entry->nr_tags = hctx->tags->nr_tags;
> + ),
> +
> + TP_printk("%d,%d hctx=%u starved on %s tags (depth=%u)",
> + MAJOR(__entry->dev), MINOR(__entry->dev),
> + __entry->hctx_id,
> + __entry->is_sched_tag ? "scheduler" : "hardware",
> + __entry->nr_tags)
> +);
> +
> /**
> * block_rq_insert - insert block operation request into queue
> * @rq: block IO operation request
--
Damien Le Moal
Western Digital Research
