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Sun, 19 Apr 2026 02:30:48 +0000 From: Aaron Tomlin To: axboe@kernel.dk, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com Cc: bvanassche@acm.org, johannes.thumshirn@wdc.com, kch@nvidia.com, dlemoal@kernel.org, ritesh.list@gmail.com, loberman@redhat.com, neelx@suse.com, sean@ashe.io, mproche@gmail.com, chjohnst@gmail.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: [PATCH v4 2/2] blk-mq: expose tag starvation counts via debugfs Date: Sat, 18 Apr 2026 22:30:36 -0400 Message-ID: <20260419023036.1419514-3-atomlin@atomlin.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260419023036.1419514-1-atomlin@atomlin.com> References: <20260419023036.1419514-1-atomlin@atomlin.com> Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: BN0PR04CA0138.namprd04.prod.outlook.com (2603:10b6:408:ed::23) To CWLP123MB3523.GBRP123.PROD.OUTLOOK.COM (2603:10a6:400:70::10) Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CWLP123MB3523:EE_|LO2P123MB5960:EE_ X-MS-Office365-Filtering-Correlation-Id: b6e47df7-06b7-45f1-d7bc-08de9dbbaab7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024|18096099003|18002099003|22082099003|56012099003; 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This patch introduces two new debugfs attributes for each block hardware queue: - /sys/kernel/debug/block/[device]/hctxN/wait_on_hw_tag - /sys/kernel/debug/block/[device]/hctxN/wait_on_sched_tag These files expose atomic counters that increment each time a submitting context is forced into an uninterruptible sleep via io_schedule() due to the complete exhaustion of physical driver tags or software scheduler tags, respectively. To ensure negligible performance overhead even in production environments where CONFIG_BLK_DEBUG_FS is actively enabled, this tracking logic utilises dynamically allocated per-CPU counters. When this configuration is disabled, the tracking logic compiles down to a safe no-op. Signed-off-by: Aaron Tomlin --- block/blk-mq-debugfs.c | 84 ++++++++++++++++++++++++++++++++++++++++++ block/blk-mq-debugfs.h | 7 ++++ block/blk-mq-tag.c | 4 ++ include/linux/blk-mq.h | 12 ++++++ 4 files changed, 107 insertions(+) diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c index 047ec887456b..a3effed55d90 100644 --- a/block/blk-mq-debugfs.c +++ b/block/blk-mq-debugfs.c @@ -7,6 +7,7 @@ #include #include #include +#include #include "blk.h" #include "blk-mq.h" @@ -484,6 +485,54 @@ static int hctx_dispatch_busy_show(void *data, struct seq_file *m) return 0; } +/** + * hctx_wait_on_hw_tag_show - display hardware tag starvation count + * @data: generic pointer to the associated hardware context (hctx) + * @m: seq_file pointer for debugfs output formatting + * + * Prints the cumulative number of times a submitting context was forced + * to block due to the exhaustion of physical hardware driver tags. + * + * Return: 0 on success. + */ +static int hctx_wait_on_hw_tag_show(void *data, struct seq_file *m) +{ + struct blk_mq_hw_ctx *hctx = data; + unsigned long count = 0; + int cpu; + + if (hctx->wait_on_hw_tag) { + for_each_possible_cpu(cpu) + count += *per_cpu_ptr(hctx->wait_on_hw_tag, cpu); + } + seq_printf(m, "%lu\n", count); + return 0; +} + +/** + * hctx_wait_on_sched_tag_show - display scheduler tag starvation count + * @data: generic pointer to the associated hardware context (hctx) + * @m: seq_file pointer for debugfs output formatting + * + * Prints the cumulative number of times a submitting context was forced + * to block due to the exhaustion of software scheduler tags. + * + * Return: 0 on success. + */ +static int hctx_wait_on_sched_tag_show(void *data, struct seq_file *m) +{ + struct blk_mq_hw_ctx *hctx = data; + unsigned long count = 0; + int cpu; + + if (hctx->wait_on_sched_tag) { + for_each_possible_cpu(cpu) + count += *per_cpu_ptr(hctx->wait_on_sched_tag, cpu); + } + seq_printf(m, "%lu\n", count); + return 0; +} + #define CTX_RQ_SEQ_OPS(name, type) \ static void *ctx_##name##_rq_list_start(struct seq_file *m, loff_t *pos) \ __acquires(&ctx->lock) \ @@ -599,6 +648,8 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_hctx_attrs[] = { {"active", 0400, hctx_active_show}, {"dispatch_busy", 0400, hctx_dispatch_busy_show}, {"type", 0400, hctx_type_show}, + {"wait_on_hw_tag", 0400, hctx_wait_on_hw_tag_show}, + {"wait_on_sched_tag", 0400, hctx_wait_on_sched_tag_show}, {}, }; @@ -670,6 +721,11 @@ void blk_mq_debugfs_register_hctx(struct request_queue *q, snprintf(name, sizeof(name), "hctx%u", hctx->queue_num); hctx->debugfs_dir = debugfs_create_dir(name, q->debugfs_dir); + if (!hctx->wait_on_hw_tag) + hctx->wait_on_hw_tag = alloc_percpu(unsigned long); + if (!hctx->wait_on_sched_tag) + hctx->wait_on_sched_tag = alloc_percpu(unsigned long); + debugfs_create_files(q, hctx->debugfs_dir, hctx, blk_mq_debugfs_hctx_attrs); @@ -684,6 +740,11 @@ void blk_mq_debugfs_unregister_hctx(struct blk_mq_hw_ctx *hctx) debugfs_remove_recursive(hctx->debugfs_dir); hctx->sched_debugfs_dir = NULL; hctx->debugfs_dir = NULL; + + free_percpu(hctx->wait_on_hw_tag); + hctx->wait_on_hw_tag = NULL; + free_percpu(hctx->wait_on_sched_tag); + hctx->wait_on_sched_tag = NULL; } void blk_mq_debugfs_register_hctxs(struct request_queue *q) @@ -815,3 +876,26 @@ void blk_mq_debugfs_unregister_sched_hctx(struct blk_mq_hw_ctx *hctx) debugfs_remove_recursive(hctx->sched_debugfs_dir); hctx->sched_debugfs_dir = NULL; } + +/** + * blk_mq_debugfs_inc_wait_tags - increment the tag starvation counters + * @hctx: hardware context associated with the tag allocation + * @is_sched: true if the starved pool is the software scheduler + * + * Evaluates the exhausted tag pool and safely increments the appropriate + * per-cpu debugfs starvation counter. + * + * Note: A race window exists during rapid device probe or CPU hotplug + * where I/O might be submitted before blk_mq_debugfs_register_hctx() has + * completed allocating the per-CPU counters. Therefore, the pointer is + * explicitly checked to prevent a NULL pointer dereference. + */ +void blk_mq_debugfs_inc_wait_tags(struct blk_mq_hw_ctx *hctx, + bool is_sched) +{ + unsigned long __percpu *tags = is_sched ? hctx->wait_on_sched_tag : + hctx->wait_on_hw_tag; + + if (likely(tags)) + this_cpu_inc(*tags); +} diff --git a/block/blk-mq-debugfs.h b/block/blk-mq-debugfs.h index 49bb1aaa83dc..a0094d004d08 100644 --- a/block/blk-mq-debugfs.h +++ b/block/blk-mq-debugfs.h @@ -17,6 +17,8 @@ struct blk_mq_debugfs_attr { const struct seq_operations *seq_ops; }; +void blk_mq_debugfs_inc_wait_tags(struct blk_mq_hw_ctx *hctx, + bool is_sched); int __blk_mq_debugfs_rq_show(struct seq_file *m, struct request *rq); int blk_mq_debugfs_rq_show(struct seq_file *m, void *v); @@ -35,6 +37,11 @@ void blk_mq_debugfs_unregister_sched_hctx(struct blk_mq_hw_ctx *hctx); void blk_mq_debugfs_register_rq_qos(struct request_queue *q); #else +static inline void blk_mq_debugfs_inc_wait_tags(struct blk_mq_hw_ctx *hctx, + bool is_sched) +{ +} + static inline void blk_mq_debugfs_register(struct request_queue *q) { } diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 66138dd043d4..3cc6a97a87a0 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -17,6 +17,7 @@ #include "blk.h" #include "blk-mq.h" #include "blk-mq-sched.h" +#include "blk-mq-debugfs.h" /* * Recalculate wakeup batch when tag is shared by hctx. @@ -191,6 +192,9 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data) trace_block_rq_tag_wait(data->q, data->hctx, data->rq_flags & RQF_SCHED_TAGS); + blk_mq_debugfs_inc_wait_tags(data->hctx, + data->rq_flags & RQF_SCHED_TAGS); + bt_prev = bt; io_schedule(); diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index ebc45557aee8..17cd6221bb93 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -453,6 +453,18 @@ struct blk_mq_hw_ctx { struct dentry *debugfs_dir; /** @sched_debugfs_dir: debugfs directory for the scheduler. */ struct dentry *sched_debugfs_dir; + /** + * @wait_on_hw_tag: Cumulative per-cpu counter incremented each + * time a submitting context is forced to block due to physical + * hardware tag exhaustion. + */ + unsigned long __percpu *wait_on_hw_tag; + /** + * @wait_on_sched_tag: Cumulative per-cpu counter incremented each + * time a submitting context is forced to block due to software + * scheduler tag exhaustion. + */ + unsigned long __percpu *wait_on_sched_tag; #endif /** -- 2.51.0