Re: [PATCH v2] blk-mq: use a housekeeping CPU for passthrough requests on isolated hctxs

[Aaron Tomlin]

On Tue, Nov 03, 2026 at 03:57:22PM +0100, Daniel Wagner wrote:
> blk_mq_alloc_request_hctx uses the first online CPU in hctx->cpumask to
> select the software context for a request.
> 
> Since commit a46c27026da1 ("blk-mq: don't schedule block kworker on
> isolated CPUs"), hctx->cpumask excludes isolated CPUs. If a hardware
> context is mapped only to isolated CPUs, the mask is empty and
> blk_mq_alloc_request_hctx fails with -EXDEV.
> 
> The NVMe fabrics transports use blk_mq_alloc_request_hctx to allocate
> the Connect command for each I/O queue. As a result, connecting to a
> controller can fail when using isolcpus=domain or an isolated cpuset
> partition.
> 
> Passthrough requests bypass the software queues and go directly to
> hctx->dispatch. Their software context is only used to select the
> completion CPU and as a hint when freeing the tag.
> 
> Use the software context of an online housekeeping CPU for passthrough
> requests.
> 
> Fixes: a46c27026da1 ("blk-mq: don't schedule block kworker on isolated CPUs")
> Tested-by: Maurizio Lombardi <mlombard@xxxxxxxxxx>
> Signed-off-by: Daniel Wagner <dwagner@xxxxxxx>
> ---
> Connecting a fabrics controller fails when all CPUs of a hardware
> context are isolated with isolcpus=domain or an isolated cpuset
> partition. Since commit a46c27026da1 ("blk-mq: don't schedule block
> kworker on isolated CPUs") the CPU mask of such a hardware context is
> empty and blk_mq_alloc_request_hctx() fails the Connect command with
> -EXDEV. This patch lets passthrough requests use the software context
> of a housekeeping CPU instead.
> 
> The nvme fabrics fixes from v1 (patches 1 and 3-5) are in nvme-7.4, so
> only the blk-mq patch is left. The blktest for this patch is nvme/070,
> posted separately.
> ---
> Changes in v2:
> - Only the blk-mq patch left, the nvme patches are in nvme-7.4.
> - Reword the commit message to describe what the fix does (Hannes).
> - Add Tested-by from Maurizio.
> - Link to v1: https://patch.msgid.link/20261014-nvme-fabrics-isolcpu-v1-0-355156be27b2@xxxxxxxxxx
> ---
>  block/blk-mq.c | 37 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 5fca78778c31..77685e6283fa 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -717,6 +717,25 @@ struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
>  }
>  EXPORT_SYMBOL(blk_mq_alloc_request);
>  
> +/*
> + * Return an online CPU which is neither isolated from the scheduler domains
> + * nor from managed interrupts, or nr_cpu_ids if there is none.
> + */
> +static unsigned int blk_mq_housekeeping_cpu(void)
> +{
> +	unsigned int cpu;
> +
> +	rcu_read_lock();
> +	for_each_online_cpu(cpu) {
> +		if (!cpu_is_isolated(cpu) &&
> +		    housekeeping_test_cpu(cpu, HK_TYPE_MANAGED_IRQ))
> +			break;
> +	}
> +	rcu_read_unlock();
> +
> +	return cpu;
> +}
> +
>  struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>  	blk_opf_t opf, blk_mq_req_flags_t flags, unsigned int hctx_idx)
>  {
> @@ -767,8 +786,22 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>  	if (!blk_mq_hw_queue_mapped(data.hctx))
>  		goto out_queue_exit;
>  	cpu = cpumask_first_and(data.hctx->cpumask, cpu_online_mask);
> -	if (cpu >= nr_cpu_ids)
> -		goto out_queue_exit;
> +	if (cpu >= nr_cpu_ids) {
> +		/*
> +		 * Isolated CPUs are not part of hctx->cpumask, so it can be
> +		 * empty although online CPUs are mapped to this hardware
> +		 * context. Passthrough requests are inserted directly into
> +		 * hctx->dispatch, so use the software context of a
> +		 * housekeeping CPU for them. Inactive hardware contexts have
> +		 * no online CPU left.
> +		 */
> +		if (!blk_op_is_passthrough(opf) ||
> +		    test_bit(BLK_MQ_S_INACTIVE, &data.hctx->state))
> +			goto out_queue_exit;
> +		cpu = blk_mq_housekeeping_cpu();
> +		if (cpu >= nr_cpu_ids)
> +			goto out_queue_exit;
> +	}
>  	data.ctx = __blk_mq_get_ctx(q, cpu);
>  
>  	if (q->elevator)
> 
> ---
> base-commit: c8c53d669d4e45b3a804013251a34387d7e11bfb
> change-id: 20261013-nvme-fabrics-isolcpu-5b4948362f88
> 
> Best regards,
> --  
> Daniel Wagner <dwagner@xxxxxxx>
> 

Thank you.

Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>

-- 
Aaron Tomlin