Re: [PATCH v9 10/13] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Ming Lei]

On Mon, Mar 30, 2026 at 06:10:44PM -0400, Aaron Tomlin wrote:
> From: Daniel Wagner <wagi@xxxxxxxxxx>
> 
> Extend the capabilities of the generic CPU to hardware queue (hctx)
> mapping code, so it maps houskeeping CPUs and isolated CPUs to the
> hardware queues evenly.
> 
> A hctx is only operational when there is at least one online
> housekeeping CPU assigned (aka active_hctx). Thus, check the final
> mapping that there is no hctx which has only offline housekeeing CPU and
> online isolated CPUs.
> 
> Example mapping result:
> 
>   16 online CPUs
> 
>   isolcpus=io_queue,2-3,6-7,12-13
> 
> Queue mapping:
>         hctx0: default 0 2
>         hctx1: default 1 3
>         hctx2: default 4 6
>         hctx3: default 5 7
>         hctx4: default 8 12
>         hctx5: default 9 13
>         hctx6: default 10
>         hctx7: default 11
>         hctx8: default 14
>         hctx9: default 15
> 
> IRQ mapping:
>         irq 42 affinity 0 effective 0  nvme0q0
>         irq 43 affinity 0 effective 0  nvme0q1
>         irq 44 affinity 1 effective 1  nvme0q2
>         irq 45 affinity 4 effective 4  nvme0q3
>         irq 46 affinity 5 effective 5  nvme0q4
>         irq 47 affinity 8 effective 8  nvme0q5
>         irq 48 affinity 9 effective 9  nvme0q6
>         irq 49 affinity 10 effective 10  nvme0q7
>         irq 50 affinity 11 effective 11  nvme0q8
>         irq 51 affinity 14 effective 14  nvme0q9
>         irq 52 affinity 15 effective 15  nvme0q10
> 
> A corner case is when the number of online CPUs and present CPUs
> differ and the driver asks for less queues than online CPUs, e.g.
> 
>   8 online CPUs, 16 possible CPUs
> 
>   isolcpus=io_queue,2-3,6-7,12-13
>   virtio_blk.num_request_queues=2
> 
> Queue mapping:
>         hctx0: default 0 1 2 3 4 5 6 7 8 12 13
>         hctx1: default 9 10 11 14 15
> 
> IRQ mapping
>         irq 27 affinity 0 effective 0 virtio0-config
>         irq 28 affinity 0-1,4-5,8 effective 5 virtio0-req.0
>         irq 29 affinity 9-11,14-15 effective 0 virtio0-req.1
> 
> Noteworthy is that for the normal/default configuration (!isoclpus) the
> mapping will change for systems which have non hyperthreading CPUs. The
> main assignment loop will completely rely that group_mask_cpus_evenly to
> do the right thing. The old code would distribute the CPUs linearly over
> the hardware context:
> 
> queue mapping for /dev/nvme0n1
>         hctx0: default 0 8
>         hctx1: default 1 9
>         hctx2: default 2 10
>         hctx3: default 3 11
>         hctx4: default 4 12
>         hctx5: default 5 13
>         hctx6: default 6 14
>         hctx7: default 7 15
> 
> The assign each hardware context the map generated by the
> group_mask_cpus_evenly function:
> 
> queue mapping for /dev/nvme0n1
>         hctx0: default 0 1
>         hctx1: default 2 3
>         hctx2: default 4 5
>         hctx3: default 6 7
>         hctx4: default 8 9
>         hctx5: default 10 11
>         hctx6: default 12 13
>         hctx7: default 14 15
> 
> In case of hyperthreading CPUs, the resulting map stays the same.
> 
> Signed-off-by: Daniel Wagner <wagi@xxxxxxxxxx>
> ---
>  block/blk-mq-cpumap.c | 177 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 158 insertions(+), 19 deletions(-)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index 8244ecf87835..3b4fa3b291c9 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -22,7 +22,18 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
>  {
>  	unsigned int num;
>  
> -	num = cpumask_weight(mask);
> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
> +		const struct cpumask *hk_mask;
> +		struct cpumask avail_mask;

This may overflow kernel stack.


Thanks,
Ming