On Fri, May 01, 2026 at 12:22:20PM -0400, Yury Norov wrote:
> 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.
> >
> > 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]
> > 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()
> > ---
> > .../net/ethernet/microsoft/mana/gdma_main.c | 47 ++++++++++++++++---
> > 1 file changed, 40 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > index 098fbda0d128..d740d1dc43da 100644
> > --- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > +++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > @@ -167,6 +167,8 @@ static int mana_gd_query_max_resources(struct pci_dev
> > *pdev)
> > } else {
> > /* If dynamic allocation is enabled we have already allocated
> > * hwc msi
> > + * Also, we make sure in this case the following is always true
> > + * (num_msix_usable - 1 HWC) <= num_online_cpus()
> > */
> > gc->num_msix_usable = min(resp.max_msix, num_online_cpus() + 1);
> > }
> > @@ -1672,11 +1674,24 @@ static int irq_setup(unsigned int *irqs, unsigned
> > int len, int node,
> > return 0;
> > }
> >
> > +/* should be called with cpus_read_lock() held */
> > +static void irq_setup_linear(unsigned int *irqs, unsigned int len)
> > +{
> > + int cpu;
> > +
> > + for_each_online_cpu(cpu) {
> > + if (len == 0)
> > + break;
> > +
> > + irq_set_affinity_and_hint(*irqs++, cpumask_of(cpu));
> > + len--;
> > + }
> > +}
> > +
> > static int mana_gd_setup_dyn_irqs(struct pci_dev *pdev, int nvec)
> > {
> > struct gdma_context *gc = pci_get_drvdata(pdev);
> > struct gdma_irq_context *gic;
> > - bool skip_first_cpu = false;
> > int *irqs, irq, err, i;
> >
> > irqs = kmalloc_objs(int, nvec);
>
> So what about WARN_ON() and nvec adjustment before kmalloc?
Hey Yury,
I am still a bit unsure about the WARN_ON() before kmalloc, as after
that also, in the same function till we take the cpus_read_lock() the
num_online_cpus() can change(or reduce). That's why I introduced the
dev_dbg() to capture hot-remove edge case.
Do you still think it adds more value?
>
> > @@ -1722,13 +1737,31 @@ 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())
> > - skip_first_cpu = true;
> > + if (gc->num_msix_usable <= num_online_cpus()) {
> > + err = irq_setup(irqs, nvec, gc->numa_node, true);
> > + if (err) {
> > + cpus_read_unlock();
> > + goto free_irq;
>
> One thing puzzles me: if you skip first CPU with this 'true', and the
> gc->num_msix_usable == num_online_cpus(), it's one more than you can
> distribute. What do I miss?
>
Let me explain this case a bit better then,
- num_msix_usable = HWC IRQ + Queue IRQ
- nvec in this functions is only Queue IRQ (HWC already setup)
When num_online_cpus == num_msix_usable:
- nvec = num_online_cpus - 1
- first CPU is already assigned to HWC IRQ, so skip it
- Queue IRQs fit in the remaining CPUs
please let me know if I did not get your question right
> > + }
> > + } else {
> > + /*
> > + * When num_msix_usable 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.
>
> If it doesn't matter, why don't you assign each IRQ to all CPUs then?
> In theory, the system would have most of flexibility to balance them.
>
Okay, let me fix the comment and elaborate on this. It doesn't matter
because in such a case we want to anyway exhaust and distribute the
Queue IRQs to all vCPUs.
We don't want to rely on the system's balancer in this case as it could
be skewed by other devices' IRQ weights
> > + * Note: in this case the total mana IRQ should always be
> > + * num_online_cpus + 1. The first HWC IRQ is already handled
> > + * in HWC setup calls
> > + * However, if CPUs went offline since num_msix_usable was
> > + * computed, nvec count will be more than num_online_cpus().
> > + * In such cases remaining extra IRQs will retain their default
> > + * affinity.
> > + */
> > + if (nvec > num_online_cpus())
> > + dev_dbg(&pdev->dev,
> > + "IRQ count %d exceeds online CPU count %d. Some
> > IRQs will share CPU\n",
>
> I'd better say 'some IRQs will share the default CPU', and in the
> perfect world, I'd like to see:
>
> 'The IRQs #4-12 will share the default CPU #0'
>
> type of message.
Sure, that makes more sense. Will make the change in next version
>
> > + nvec, num_online_cpus());
>
> It's not that straightforward as it should be. In one case
>
> nvec > num_online_cpus()
>
> is a problem, while in another - not. It looks already suspicious. So
> when you throw a warning, you should mention it, I believe.
>
> In the
>
> gc->num_msix_usable <= num_online_cpus()
>
> case, when nvec is too big, would'n 'some IRQs share some CPU' just
> as well? If so, you again should throw a message.
let me try to cleanly address this too, in the next version. May be even
a seperate patch.
Thanks Yury.
>
> > - err = irq_setup(irqs, nvec, gc->numa_node, skip_first_cpu);
> > - if (err) {
> > - cpus_read_unlock();
> > - goto free_irq;
> > + irq_setup_linear(irqs, nvec);
> > }
> >
> > cpus_read_unlock();
> >
> > base-commit: e728258debd553c95d2e70f9cd97c9fde27c7130
> > --
> > 2.34.1
