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: [email protected]
> Co-developed-by: Erni Sri Satya Vennela <[email protected]>
> Signed-off-by: Erni Sri Satya Vennela <[email protected]>
> Signed-off-by: Shradha Gupta <[email protected]>
> Reviewed-by: Haiyang Zhang <[email protected]>
> ---
> 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 <[email protected]>
