Re: [PATCH net v2] net: mana: Optimize irq affinity for low vcpu configs

[Simon Horman <horms@kernel.org>]

On Wed, Apr 29, 2026 at 02:06:37AM -0700, Shradha Gupta wrote:
> In mana driver, the number of IRQs allocated is capped by the
> min(num_cpu + 1, queue count). In cases, where the IRQ count is greater
> than the vcpu count, we want to utilize all the vCPUs, irrespective of
> their NUMA/core bindings.
> 
> This is important, especially in the envs where number of vCPUs are so
> few that the softIRQ handling overhead on two IRQs on the same vCPU is
> much more than their overheads if they were spread across sibling vCPUs.
> 
> This behaviour is more evident with dynamic IRQ allocation. Since MANA
> IRQs are assigned at a later stage compared to static allocation, other
> device IRQs may already be affinitized to the vCPUs. As a result, IRQ
> weights become imbalanced, causing multiple MANA IRQs to land on the
> same vCPU, while some vCPUs have none.
> 
> In such cases when many parallel TCP connections are tested, the
> throughput drops significantly.

...

> Fixes: 755391121038 ("net: mana: Allocate MSI-X vectors dynamically")
> Cc: stable@vger.kernel.org
> Co-developed-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
> Signed-off-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
> Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
> Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
> ---
> Changes in v2
>  * Removed the unused skip_first_cpu variable
>  * fixed exit condition in irq_setup_linear() with len == 0
>  * changed return type of irq_setup_linear() as it will always be 0
>  * removed the unnecessary rcu_read_lock() in irq_setup_linear()
>  * added appropriate comments to indicate expected behaviour when
>    IRQs are more than or equal to num_online_cpus()

Reviewed-by: Simon Horman <horms@kernel.org>