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

[Ming Lei]

On Sun, Oct 11, 2026 at 8:29 PM Aaron Tomlin <atomlin@xxxxxxxxxxx> 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().

Then it looks like you need to fix WORK_CPU_UNBOUND, which has more wide uses,
if you think it is one problem.

> Consequently, if a task executing on an isolated CPU submits I/O and
> triggers an asynchronous queue dispatch with zero delay, &hctx->run_work
> is queued directly onto the local isolated CPU's worker pool. This

If your theory is correct, why do botter to change to
housekeeping_any_cpu(HK_TYPE_TIMER)
if the user is submitting IO from an isolated CPU?

> causes the local per-CPU high-priority kworker
> (e.g. "kworker/6:1H-kblockd") to be scheduled on the isolated CPU,
> introducing unwanted latency jitter and execution delays, thereby
> defeating the original objective of commit a46c27026da1.
>
> Rectify this by selecting an online housekeeping CPU via
> housekeeping_any_cpu(HK_TYPE_TIMER) when blk_mq_hctx_empty_cpumask(hctx)
> is true. This ensures that:
>     1.  The delayed work is cleanly offloaded to a housekeeping CPU.
>
>     2.  The per-CPU high-priority kworker on the isolated CPU is not
>         scheduled.
>
>     3.  Hardware queues with valid mapped CPUs preserve their per-CPU
>         locality and batch round-robin behaviour.
>
> 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>
> ---
>  block/blk-mq.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 025a799f35b9..9480c3aac898 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2252,8 +2252,11 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>         bool tried = false;
>         int next_cpu = hctx->next_cpu;
>
> -       /* Switch to unbound if no allowable CPUs in this hctx */
> -       if (hctx->queue->nr_hw_queues == 1 || blk_mq_hctx_empty_cpumask(hctx))
> +       /* Switch to housekeeping CPU if no allowable CPUs in this hctx */
> +       if (blk_mq_hctx_empty_cpumask(hctx))
> +               return housekeeping_any_cpu(HK_TYPE_TIMER);
> +
> +       if (hctx->queue->nr_hw_queues == 1)
>                 return WORK_CPU_UNBOUND;

Why is SQ so special here to keep WORK_CPU_UNBOUND?


Thanks,
Ming Lei