Re: [PATCH v2 1/1] PCI: hv: Spread load across CPUs when selecting effective affinity CPU

[Naman Jain <namjain@linux.microsoft.com>]

On 10/26/2026 10:29 AM, Michael Kelley wrote:
> In a system with multiple devices with the same configuration (such as
> NVMe controllers), the affinity masks are the same for all such devices.
> Linux typicaly must select a single CPU from each mask to be the effective
> affinity. If a fixed algorithm is used, such as picking the first CPU
> in the mask, interrupts from multiple devices are assigned to the same
> CPUs, while other CPUs are not assigned any interrupts, producing an
> interrupt load imbalance.
> 
> On x86, the "APIC" IRQ chip avoids this problem by spreading the
> interrupts across the CPUs in an affinity mask. It tracks the number
> of IRQs assigned to each CPU, and chooses the CPU in the mask that
> has the fewest assignments. Because Hyper-V vPCI device IRQs eventually
> parent to APIC, the spreading done by APIC accrues to Hyper-V vPCI
> devices, so all is good in x86 systems.
> 
> On arm64, the "ITS" IRQ chip for the GICv3+ITS does the same kind of
> spreading for LPI interrupts. But Hyper-V provides guest VMs only with
> a virtual GICv3, not including an ITS or LPI support. IRQs for vPCI
> devices on arm64 are mapped to SPIs and parented to the "GICv3" IRQ
> chip. The GICv3 IRQ chip always chooses the first CPU in the affinity
> mask as the effective affinity, producing an interrupt load imbalance
> for Hyper-V vPCI devices in arm64 systems.
> 
> The Hyper-V vPCI driver already has an arm64-specific IRQ chip to
> allocate and managed SPIs for use by vPCI devices. Solve the imbalance
> problem by extending this IRQ chip to implement a "set affinity"
> function that tracks SPIs assigned to each CPU, and chooses the
> least-loaded CPU as the effective affinity. Then invoke the parent
> IRQ chip (GICv3) to actually set that single CPU as the effective
> affinity in the virtual GICv3 hardware. As a result, IRQs are spread
> across CPUs like on x86 and interrupt load imbalances are avoided.
> 
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>

<snip>

> +/*
> + * Track the count of SPIs assigned to each CPU. Pick a single CPU from the
> + * affinity mask based on the SPI loading data, and have the parent IRQ
> + * chip (should be "GICv3") set the effective affinity to that single CPU.
> + */
> +static int hv_pci_arm64_set_affinity(struct irq_data *d,
> +				     const struct cpumask *mask_val,
> +				     bool force)
> +{
> +	unsigned int cpu, prev_cpu;
> +	int ret;
> +
> +	prev_cpu = cpumask_first(irq_data_get_effective_affinity_mask(d));
> +
> +	scoped_guard(raw_spinlock_irqsave, &cpu_spi_count_lock) {
> +		if (prev_cpu < nr_cpu_ids)
> +			hv_pci_dec_spi_count(d, prev_cpu);

Nit: Would it be better to add this nr_cpu_ids check in 
hv_pci_dec_spi_count()/hv_pci_inc_spi_count() instead? I don't have a 
strong opinion on it.

> +
> +		if (force)
> +			cpu = hv_pci_arm64_get_min_load(d, mask_val);
> +		else
> +			cpu = hv_pci_arm64_select_cpu(d, mask_val);
> +
> +		if (cpu >= nr_cpu_ids) {

At some places like this, you may still need to retain it.

> +			if (prev_cpu < nr_cpu_ids)
> +				hv_pci_inc_spi_count(d, prev_cpu);
> +			return -EINVAL;
> +		}
> +
> +		hv_pci_inc_spi_count(d, cpu);
> +	}
> +
> +	ret = irq_chip_set_affinity_parent(d, cpumask_of(cpu), force);
> +	if (ret < 0) {
> +		guard(raw_spinlock_irqsave)(&cpu_spi_count_lock);
> +
> +		if (prev_cpu < nr_cpu_ids)
> +			hv_pci_inc_spi_count(d, prev_cpu);
> +		if (cpu < nr_cpu_ids)
> +			hv_pci_dec_spi_count(d, cpu);
> +	}
> +
> +	return ret;

<snip>

With or without above nit changes, for the patch -

Reviewed-by: Naman Jain <namjain@linux.microsoft.com>
Tested-by: Naman Jain <namjain@linux.microsoft.com>

Tested the patch on Azure ARM64 VM and verified the interrupt 
distribution and also saw soft lockups disappearing after optimal 
distribution of NVME - PCI interrupts produced by this patch.
Not repeating here, observations are similar to what I mentioned in the 
review of v1.

Regards,
Naman