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

[sashiko-bot@kernel.org]

> 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.
> 
> [ ... ]
> 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>

Sashiko has reviewed this patch and found no issues.

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