On Wed, Jun 03, 2026 at 02:49:24PM -0700, Jacob Keller wrote:
> On 6/1/2026 3:27 AM, 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]>
> > Reviewed-by: Simon Horman <[email protected]>
> > ---
> > Changes in v3
> > * Optimize the comments in mana_gd_setup_dyn_irqs()
> > * add more details in the dev_dbg for extra IRQs
> > ---
> > 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 | 60 ++++++++++++++++---
> > 1 file changed, 53 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > index 712a0881d720..00a28b3ca0a6 100644
> > --- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > +++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> > @@ -197,6 +197,8 @@ 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);
> > }
> > @@ -1717,11 +1719,24 @@ 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--;
> > + }
> > +}
>
> I would find all of this a bit easier to follow if irq_setup_linear()
> and irq_setup() had a mana prefix so it was more obvious these are
> specific to the driver. Of course irq_setup is pre-existing, and its not
> my driver so do as you will :)
>
> > +
> > 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);
> > @@ -1729,6 +1744,8 @@ static int mana_gd_setup_dyn_irqs(struct pci_dev
> > *pdev, int nvec)
> > return -ENOMEM;
> >
> > /*
> > + * In this function, num_msix_usable = HWC IRQ + Queue IRQ.
> > + * nvec is only Queue IRQ (HWC already setup).
> > * While processing the next pci irq vector, we start with index 1,
> > * as IRQ vector at index 0 is already processed for HWC.
> > * However, the population of irqs array starts with index 0, to be
> > @@ -1767,13 +1784,42 @@ 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;
> > + }
> > + } else {
> > + /*
> > + * When num_msix_usable are more than num_online_cpus, our
> > + * queue IRQs should be equal to num of online vCPUs.
> > + * We try to make sure queue IRQs spread across all vCPUs.
> > + * In such a case NUMA or CPU core affinity does not matter.
> > + * 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, queue IRQs will be more than num_online_cpus().
> > + * In such cases remaining extra IRQs will retain their default
> > + * affinity.
> > + */
> > + int first_unassigned = num_online_cpus();
> > + if (nvec > first_unassigned) {
> > + char buf[32];
> > +
> > + if (first_unassigned == nvec - 1)
> > + snprintf(buf, sizeof(buf), "%d",
> > + first_unassigned);
> > + else
> > + snprintf(buf, sizeof(buf), "%d-%d",
> > + first_unassigned, nvec - 1);
> > +
> > + dev_dbg(&pdev->dev,
> > + "MANA IRQ indices #%s will retain the default
> > CPU affinity\n", buf);
> > + }
> >
> > - err = irq_setup(irqs, nvec, gc->numa_node, skip_first_cpu);
> > - if (err) {
> > - cpus_read_unlock();
> > - goto free_irq;
> > + irq_setup_linear(irqs, nvec);
>
> irq_setup() doesn't have a driver prefix, but is actually a static
> function in gdma_main.c, so its implementation is specific to this
> driver despite its name.
>
> So if I understand this change correctly, if the number of usable MSI-X
> vectors is smaller than the number of CPUs, you contineu to use the
> current irq_setup logic.. otherwise you switch to the simpler "linear"
> logic.
>
> I guess this means the logic and heuristic used in irq_setup() breaks
> down when the number of vectors is large and number of vCPU is small?
>
> Makes sense.
>
Hi Jacob,
Yes, that's the right understanding.
Regarding the function names, let me take that up in a seperate patch to
add prefixes to all such functions.
Thanks.
> > }
> >
> > cpus_read_unlock();
> >
> > base-commit: 8415598365503ced2e3d019491b0a2756c85c494
