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

[Aaron Tomlin]

On Wed, Oct 14, 2026 at 10:02:25PM -0400, Aaron Tomlin wrote:
> 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@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxx/
> ---
>  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
> 

Apologies.

The proper approach is to query housekeeping_cpumask(HK_TYPE_TIMER)
directly and select an online core via cpumask_any_and_distribute(). This
avoids invoking smp_processor_id() altogether, and round-robins delayed
work cleanly across available housekeeping cores. Furthermore, because CPU
hotplug explicitly enforces that at least one housekeeping core must remain
online on a running system, the intersection is guaranteed never to be
empty:


    static inline int blk_mq_hctx_unbound_cpu(void)
    {
        if (cpu_is_isolated(raw_smp_processor_id()))
            return cpumask_any_and_distribute(housekeeping_cpumask(HK_TYPE_TIMER),
                                              cpu_online_mask);
        return WORK_CPU_UNBOUND;
    }
      

Kind regards,
-- 
Aaron Tomlin