Re: [PATCH] net/mlx5: Update mlx5_irq.mask when IRQ affinity changes

[Tariq Toukan <tariqt@nvidia.com>]

On 14/05/2026 10:42, Yi Li wrote:
> mlx5_irq.mask is used for:
> 1) Setting IRQ affinity_hint in mlx5_irq_alloc()
> 2) Determining mlx5e_channel.cpu in mlx5e_open_channel(), which in turn
>     decides the NUMA node for queue allocations.
> 
> When a user modifies IRQ affinity, mlx5_irq.mask remains unchanged.
> Consequently even if mlx5e_open_channel() is invoked again, queues are
> still allocated on the original NUMA node instead of the newly
> preferred one.
> 
> Fix this by registering an irq_set_affinity_notifier to update
> mlx5_irq.mask when /proc/irq/N/smp_affinity is modified.
> Therefore subsequent queue allocations reflect the updated affinity.
> 
> Signed-off-by: Yi Li <liyi@hygon.cn>
> ---

Hi,

Thanks for the patch. Looking at the proposal, I want to discuss two 
distinct aspects:

NAPI Execution Location: We already track effective affinity closely 
through irq_get_effective_affinity_mask(); NAPI processing is 
dynamically moved accordingly, including a forced NAPI cycle break if 
needed.

Memory Allocation Location: This is the core focus of your patch.

I have serious comments on the proposed implementation, but first I want 
to discuss the idea.

We investigated a similar approach a few years ago but ultimately 
decided against upstreaming it due to stability concerns:

High Volatility: The "current affinity" value can be extremely dynamic, 
potentially shifting multiple times per second depending on system load 
and tuning.

Performance Risk: Sampling a highly volatile "current" value to allocate 
relatively permanent resources (like channel queues) risks severe 
worst-case performance regressions if the affinity shifts immediately 
after allocation.

Because of this, we have historically relied on numa-distance logic for 
channel allocations to ensure a predictable baseline.

Do you have any benchmark data or specific use cases showing a clear net 
benefit over the existing numa-distance logic?

Best regards,
Tariq