Re: [PATCH v6 01/29] irq/matrix: Expose functions to allocate the best CPU for new vectors

[Ricardo Neri]

On Fri, May 06, 2022 at 09:48:28PM +0200, Thomas Gleixner wrote:
> Ricardo,

Thank you very much for your feedback Thomas! I am sorry for my late reply, I
had been out of office.

> 
> On Thu, May 05 2022 at 16:59, Ricardo Neri wrote:
> > Certain types of interrupts, such as NMI, do not have an associated vector.
> > They, however, target specific CPUs. Thus, when assigning the destination
> > CPU, it is beneficial to select the one with the lowest number of
> > vectors.
> 
> Why is that beneficial especially in the context of a NMI watchdog which
> then broadcasts the NMI to all other CPUs?

My intent was not the NMI watchdog specifically but potential use cases that do
not involve NMI broadcasts. If the NMI targets a single CPU, it is best to
select the CPU with the lowest vector allocation count.

> 
> That's wishful thinking perhaps, but I don't see any benefit at all.
> 
> > Prepend the functions matrix_find_best_cpu_managed() and
> > matrix_find_best_cpu_managed()
> 
> The same function prepended twice becomes two functions :)
> 

Sorry, I missed this.

> > with the irq_ prefix and expose them for
> > IRQ controllers to use when allocating and activating vector-less IRQs.
> 
> There is no such thing like a vectorless IRQ. NMIs have a vector. Can we
> please describe facts and not pulled out of thin air concepts which do
> not exist?

Thank you for the clarification. I see your point. I wrote this patch because
maskable interrupts and NMIs have different entry points. As you state,
however, the also have a vector.

I can drop this patch.

BR,
Ricardo

> 
> Thanks,
> 
>         tglx