[PATCH v2] blk-mq: dispatch kworker to housekeeping CPU when hctx cpumask is empty

[Aaron Tomlin <atomlin@atomlin.com>]

Commit a46c27026da1 ("blk-mq: don't schedule block kworker on isolated
CPUs") excluded isolated CPUs from hctx->cpumask to prevent worker
threads from executing block work on isolated CPUs. When all CPUs mapped
to a hardware queue are isolated (i.e. blk_mq_hctx_empty_cpumask(hctx)
evaluates to true), blk_mq_hctx_next_cpu() returns WORK_CPU_UNBOUND with
the intention of offloading execution.

However, kblockd_workqueue is allocated as a per-CPU workqueue
(WQ_PERCPU). When WORK_CPU_UNBOUND is queued on a per-CPU workqueue, the
workqueue core resolves the target CPU via raw_smp_processor_id().
Consequently, if a task executing on an isolated CPU submits I/O and
triggers an asynchronous queue dispatch with zero delay (e.g. upon
hardware queue congestion, tag/resource exhaustion returning
BLK_STS_RESOURCE, or plugging), &hctx->run_work is queued directly onto
the local isolated CPU's worker pool. This causes the local per-CPU
high-priority kworker (e.g. "kworker/6:1H-kblockd") to be scheduled on
the isolated CPU. In real-time environments, an active SCHED_FIFO workload
will starve this SCHED_NORMAL worker, causing hardware dispatch to stall,
in addition to introducing unwanted latency jitter on nohz_full cores,
thereby defeating the original objective of commit a46c27026da1.

Introduce blk_mq_hctx_unbound_cpu() to select an online housekeeping CPU
via housekeeping_any_cpu(HK_TYPE_TIMER) when executing on an isolated
CPU, and otherwise fall back to WORK_CPU_UNBOUND. Use this helper across
the unbound fallback paths in blk_mq_hctx_next_cpu().

This ensures that:
    1.  When executing on an isolated CPU, deferred queue work is
        cleanly offloaded to an online housekeeping CPU across single-queue
        devices, hardware queues with empty cpumasks, and offline CPU
        fallback paths.

    2.  The per-CPU high-priority kworker on the isolated CPU is not
        scheduled, avoiding priority inversion and latency jitter.

    3.  When executing on a non-isolated CPU, WORK_CPU_UNBOUND is
        preserved, maintaining optimal local execution with zero CPU
        bouncing.

Fixes: a46c27026da1 ("blk-mq: don't schedule block kworker on isolated CPUs")
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
Changes since v1:

 - Introduce the blk_mq_hctx_unbound_cpu() helper to steer deferred work
   to an online housekeeping CPU only when executing on an isolated
   core, preserving WORK_CPU_UNBOUND and local cache locality for
   housekeeping cores

 - Apply blk_mq_hctx_unbound_cpu() across all unbound fallback paths in
   blk_mq_hctx_next_cpu(), comprehensively covering single-queue devices
   (nr_hw_queues == 1), empty hardware queue cpumasks, and the offline
   CPU fallback path.

 - Elaborate in the commit log on the triggers for asynchronous queue
   dispatch (e.g. tag exhaustion, BLK_STS_RESOURCE backpressure, and
   plugging) and the SCHED_FIFO worker starvation hazard on isolated
   cores

 - Link to v1: https://lore.kernel.org/lkml/20261012012905.267920-1-atomlin@atomlin.com/
---
 block/blk-mq.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 025a799f35b9..df6103b91e60 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2241,6 +2241,13 @@ static bool blk_mq_hctx_empty_cpumask(struct blk_mq_hw_ctx *hctx)
         return hctx->next_cpu >= nr_cpu_ids;
 }
 
+static inline int blk_mq_hctx_unbound_cpu(void)
+{
+	if (cpu_is_isolated(raw_smp_processor_id()))
+		return housekeeping_any_cpu(HK_TYPE_TIMER);
+	return WORK_CPU_UNBOUND;
+}
+
 /*
  * It'd be great if the workqueue API had a way to pass
  * in a mask and had some smarts for more clever placement.
@@ -2254,7 +2261,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 
 	/* Switch to unbound if no allowable CPUs in this hctx */
 	if (hctx->queue->nr_hw_queues == 1 || blk_mq_hctx_empty_cpumask(hctx))
-		return WORK_CPU_UNBOUND;
+		return blk_mq_hctx_unbound_cpu();
 
 	if (--hctx->next_cpu_batch <= 0) {
 select_cpu:
@@ -2281,7 +2288,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 		 */
 		hctx->next_cpu = next_cpu;
 		hctx->next_cpu_batch = 1;
-		return WORK_CPU_UNBOUND;
+		return blk_mq_hctx_unbound_cpu();
 	}
 
 	hctx->next_cpu = next_cpu;
-- 
2.55.0