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

[Nilay Shroff]

On 11/3/26 8:27 PM, 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>

Looks good to me:
Reviewed-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>