blk-mq: add tracepoint block_rq_tag_wait

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 tracepoint in the tag
allocation slow-path. It triggers immediately before the task state
is altered to TASK_UNINTERRUPTIBLE (ensuring safety for PREEMPT_RT
locks). It exposes the exact hardware context (hctx) that is starved,
the specific pool experiencing starvation (driver, software scheduler,
or reserved), and the exact pool depth.

This provides storage engineers with a zero-configuration, low-overhead
mechanism to definitively identify shared-tag bottlenecks. For example,
userspace can trivially replicate tag starvation counters using bpftrace:

    # bpftrace -e 'tracepoint:block:block_rq_tag_wait { @tag_waits[cpu] = count(); }'
    Attaching 1 probe...
    ^C
    @tag_waits[4]: 12
    @tag_waits[12]: 87

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Link: https://patch.msgid.link/20260525005123.722277-1-atomlin@atomlin.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 59 insertions, 0 deletions

diff --git a/include/trace/events/block.h b/include/trace/events/block.h
index 6aa79e2d799c..9c97a16850b9 100644
--- a/include/trace/events/block.h
+++ b/include/trace/events/block.h
@@ -227,6 +227,65 @@ DECLARE_EVENT_CLASS(block_rq,
 );
 
 /**
+ * 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
+ * @alloc_flags: allocation flags dictating the specific tag pool
+ *
+ * Called immediately before the submitting context is forced to block due
+ * to the exhaustion of available tags (i.e., physical hardware driver
+ * tags, software scheduler tags, or reserved tags). This trace point
+ * indicates that the context will be placed into an uninterruptible state
+ * via sbitmap_prepare_to_wait(). If a tag is not acquired in the final
+ * lockless retry, the context will yield the CPU 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, unsigned int alloc_flags),
+
+	TP_ARGS(q, hctx, is_sched_tag, alloc_flags),
+
+	TP_STRUCT__entry(
+		__field( dev_t,		dev			)
+		__field( u32,		hctx_id			)
+		__field( u32,		nr_tags			)
+		__field( bool,		is_sched_tag		)
+		__field( bool,		is_reserved		)
+	),
+
+	TP_fast_assign(
+		__entry->dev		= q->disk ? disk_devt(q->disk) : 0;
+		__entry->hctx_id	= hctx->queue_num;
+		__entry->is_sched_tag	= is_sched_tag;
+		__entry->is_reserved	= alloc_flags & BLK_MQ_REQ_RESERVED;
+
+		if (__entry->is_reserved) {
+			__entry->nr_tags = is_sched_tag ?
+					   hctx->sched_tags->nr_reserved_tags :
+					   hctx->tags->nr_reserved_tags;
+		} else {
+			if (is_sched_tag)
+				__entry->nr_tags = hctx->sched_tags->nr_tags -
+						   hctx->sched_tags->nr_reserved_tags;
+			else
+				__entry->nr_tags = hctx->tags->nr_tags -
+						   hctx->tags->nr_reserved_tags;
+		}
+
+	),
+
+	TP_printk("%d,%d hctx=%u starved on %s%s tags (depth=%u)",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->hctx_id,
+		  __entry->is_sched_tag ? "scheduler" : "hardware",
+		  __entry->is_reserved ? " reserved" : "",
+		  __entry->nr_tags)
+);
+
+/**
  * block_rq_insert - insert block operation request into queue
  * @rq: block IO operation request
  *