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

From: Shradha Gupta

Date: Sat Apr 25 2026 - 02:42:26 EST

On Fri, Apr 24, 2026 at 05:25:45PM -0400, Yury Norov wrote:
> 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: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Shradha Gupta <shradhagupta@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Erni Sri Satya Vennela <ernis@xxxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > ---
> > .../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.

That's right, let me change that.

>
> > + } 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
>

so, by design the nvec should never exceed num_online_cpus(). I only
added the WARN_ON as a defensive safegaurd. But I agree with your
suggestion to move this earlier before the allocations.

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