On Thu, Apr 23, 2026 at 11:17:00PM -0700, Shradha Gupta wrote:
> In mana driver, the number of IRQs allocated are 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.
>
> In such cases when many parallel TCP connections are tested, the
> throughput drops significantly
>
> Test envs:
> =======================================================
> Case 1: without this patch
> =======================================================
> 4 vcpu(2 cores), 5 MANA IRQs (1 HWC + 4 Queue)
>
> TYPE effective vCPU aff
> =======================================================
> IRQ0: HWC 0
> IRQ1: mana_q1 0
> IRQ2: mana_q2 2
> IRQ3: mana_q3 0
> IRQ4: mana_q4 3
>
> %soft on each vCPU(mpstat -P ALL 1) on receiver
> vCPU 0 1 2 3
> =======================================================
> pass 1: 38.85 0.03 24.89 24.65
> pass 2: 39.15 0.03 24.57 25.28
> pass 3: 40.36 0.03 23.20 23.17
>
> =======================================================
> Case 2: with this patch
> =======================================================
> 4 vcpu(2 cores), 5 MANA IRQs (1 HWC + 4 Queue)
>
> TYPE effective vCPU aff
> =======================================================
> IRQ0: HWC 0
> IRQ1: mana_q1 0
> IRQ2: mana_q2 1
> IRQ3: mana_q3 2
> IRQ4: mana_q4 3
>
> %soft on each vCPU(mpstat -P ALL 1) on receiver
> vCPU 0 1 2 3
> =======================================================
> pass 1: 15.42 15.85 14.99 14.51
> pass 2: 15.53 15.94 15.81 15.93
> pass 3: 16.41 16.35 16.40 16.36
>
> =======================================================
> Throughput Impact(in Gbps, same env)
> =======================================================
> TCP conn with patch w/o patch
> 20480 15.65 7.73
> 10240 15.63 8.93
> 8192 15.64 9.69
> 6144 15.64 13.16
> 4096 15.69 15.75
> 2048 15.69 15.83
> 1024 15.71 15.28
>
> Fixes: 755391121038 ("net: mana: Allocate MSI-X vectors dynamically")
> Cc: [email protected]
> Signed-off-by: Shradha Gupta <[email protected]>
> Signed-off-by: Erni Sri Satya Vennela <[email protected]>
> Reviewed-by: Haiyang Zhang <[email protected]>
> ---
> .../net/ethernet/microsoft/mana/gdma_main.c | 35 +++++++++++++++++--
> 1 file changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> index 098fbda0d128..433c044d53c6 100644
> --- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> +++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> @@ -1672,6 +1672,23 @@ static int irq_setup(unsigned int *irqs, unsigned int
> len, int node,
> return 0;
> }
>
> +static int irq_setup_linear(unsigned int *irqs, unsigned int len)
> +{
> + int cpu;
> +
> + rcu_read_lock();
> + for_each_online_cpu(cpu) {
> + if (len <= 0)
> + break;
> +
> + irq_set_affinity_and_hint(*irqs++, cpumask_of(cpu));
> + len--;
> + }
> + rcu_read_unlock();
> +
> + return 0;
> +}
> +
> static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
> {
> struct gdma_context *gc = pci_get_drvdata(pdev);
> @@ -1722,10 +1739,24 @@ static int mana_gd_setup_dyn_irqs(struct pci_dev
> *pdev, int nvec)
> * first CPU sibling group since they are already affinitized to HWC IRQ
> */
> cpus_read_lock();
> - if (gc->num_msix_usable <= num_online_cpus())
> + if (gc->num_msix_usable <= num_online_cpus()) {
> skip_first_cpu = true;
> + err = irq_setup(irqs, nvec, gc->numa_node, skip_first_cpu);
Then you don't need the 'skip_first_cpu' variable.
> + } else {
> + /*
> + * In case our IRQs are more than num_online_cpus, we try to
> + * make sure we are using all vcpus. In such a case NUMA or
> + * CPU core affinity does not matter.
> + * Note that in this case the total mana IRQ should always be
> + * num_online_cpu + 1. The first HWC IRQ is already handled
> + * in HWC setup calls
> + * So, the nvec value in this path should always be equal to
> + * num_online_cpu
> + */
> + WARN_ON(nvec > num_online_cpus());
That sounds weird. If you don't support IRQs more than CPUs , and want to
warn about it, you'd do that earlier in the function, and align the other
logic accordingly. For example:
if (WARN_ON(nvec > num_online_cpus()))
nvec = num_online_cpus();
irqs = kmalloc_objs(int, nvec);
if (!irqs)
return -ENOMEM;
...
So you'll decrease pressure on allocator.
What would happen with those IRQs beyond num_online_cpus()? Can you explain
it in the comment? I'm not an expert in your driver, but usually if you pass
a vector to function, and the function is able to handle only a part of it,
it returns the number of processed elements.
Thanks,
Yury
> + err = irq_setup_linear(irqs, nvec);
> + }
>
> - err = irq_setup(irqs, nvec, gc->numa_node, skip_first_cpu);
> if (err) {
> cpus_read_unlock();
> goto free_irq;
>
> base-commit: e728258debd553c95d2e70f9cd97c9fde27c7130
> --
> 2.34.1
