Re: [PATCH v8 10/12] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Hannes Reinecke <hare@suse.de>]

On 9/5/25 16:59, Daniel Wagner wrote:
> 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@kernel.org>
> ---
>   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 8244ecf878358c0b8de84458dcd5100c2f360213..1e66882e4d5bd9f78d132f3a229a1577853f7a9f 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -17,12 +17,25 @@
>   #include "blk.h"
>   #include "blk-mq.h"
>   
> +static struct cpumask blk_hk_online_mask;
> +
>   static unsigned int blk_mq_num_queues(const struct cpumask *mask,
>   				      unsigned int max_queues)
>   {
>   	unsigned int num;
>   
> -	num = cpumask_weight(mask);
> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
> +		const struct cpumask *hk_mask;
> +		struct cpumask avail_mask;
> +
> +		hk_mask = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
> +		cpumask_and(&avail_mask, mask, hk_mask);
> +
> +		num = cpumask_weight(&avail_mask);
> +	} else {
> +		num = cpumask_weight(mask);
> +	}
> +
>   	return min_not_zero(num, max_queues);
>   }
>   
> @@ -31,9 +44,13 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
>    *
>    * Returns an affinity mask that represents the queue-to-CPU mapping
>    * requested by the block layer based on possible CPUs.
> + * This helper takes isolcpus settings into account.
>    */
>   const struct cpumask *blk_mq_possible_queue_affinity(void)
>   {
> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
> +		return housekeeping_cpumask(HK_TYPE_IO_QUEUE);
> +
>   	return cpu_possible_mask;
>   }
>   EXPORT_SYMBOL_GPL(blk_mq_possible_queue_affinity);
> @@ -46,6 +63,12 @@ EXPORT_SYMBOL_GPL(blk_mq_possible_queue_affinity);
>    */
>   const struct cpumask *blk_mq_online_queue_affinity(void)
>   {
> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
> +		cpumask_and(&blk_hk_online_mask, cpu_online_mask,
> +			    housekeeping_cpumask(HK_TYPE_IO_QUEUE));
> +		return &blk_hk_online_mask;

Can you explain the use of 'blk_hk_online_mask'?
Why is a static variable?
To my untrained eye it's being recalculated every time one calls
this function. And only the first invocation run on an empty mask,
all subsequent ones see a populated mask.

Is that the intention?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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