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 <[email protected]> --- 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 <linux/kmemleak.h> #include <linux/delay.h> +#include <trace/events/block.h> #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)); + 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 -- 2.51.0
