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SubjectRe: Virtio-scsi multiqueue irq affinity
Date
Yihang,

On Sat, May 08 2021 at 15:52, xuyihang wrote:
>
> We are dealing with a scenario which may need to assign a default
> irqaffinity for managed IRQ.
>
> Assume we have a full CPU usage RT thread running binded to a specific
> CPU.
>
> In the mean while, interrupt handler registered by a device which is
> ksoftirqd may never have a chance to run. (And we don't want to use
> isolate CPU)

A device cannot register and interrupt handler in ksoftirqd.

> There could be a couple way to deal with this problem:
>
> 1. Adjust priority of ksoftirqd or RT thread, so the interrupt handler
> could preempt
>
> RT thread. However, I am not sure whether it could have some side
> effects or not.
>
> 2. Adjust interrupt CPU affinity or RT thread affinity. But managed IRQ
> seems design to forbid user from manipulating interrupt affinity.
>
> It seems managed IRQ is coupled with user side application to me.
>
> Would you share your thoughts about this issue please?

Can you please provide a more detailed description of your system?

- Number of CPUs

- Kernel version
- Is NOHZ full enabled?
- Any isolation mechanisms enabled, and if so how are they
configured (e.g. on the kernel command line)?

- Number of queues in the multiqueue device

- Is the RT thread issuing I/O to the multiqueue device?

Thanks,

tglx

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