Re: Virtio-scsi multiqueue irq affinity

[Thomas Gleixner <tglx@linutronix.de>]

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