Re: [PATCH v3] blk-mq: dispatch kworker to housekeeping CPU when hctx cpumask is empty

[Jens Axboe <axboe@kernel.dk>]

On 10/15/26 11:22 PM, Aaron Tomlin wrote:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 025a799f35b9..1a113679f15b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2241,6 +2241,21 @@ 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())) {
> +		int cpu;
> +
> +		guard(rcu)();
> +
> +		cpu = cpumask_any_and_distribute(housekeeping_cpumask(HK_TYPE_DOMAIN),
> +						 cpu_online_mask);
> +		if (cpu < nr_cpu_ids)
> +			return cpu;
> +	}
> +	return WORK_CPU_UNBOUND;
> +}

Not opposed to this, but let's please think about making the
common case the fast path. CPU isolation is not the common
path. That might mean annotating the hctx itself.

-- 
Jens Axboe