On Thu, May 07, 2026 at 10:51:05PM -0700, Shradha Gupta wrote:
...
> > > We can definately get our throughput run results on other suggestions
> > > you have. And about that, I just needed a bit more clarity on what to
> > > test against. Are you suggesting, with irq_setup() intact and in use, we
> > > configure the non-mana IRQs to say CPU0 and capture the numbers?
> >
> > Can you try this:
> >
> > while(len--)
> > // Or cpu_online_mask or cpu_all_mask?
> > irq_set_affinity_and_hint(*irqs++, NULL);
> >
> > And compare it to the linear version under your vCPU scenario?
> >
> > Can you run your throughput test alone and on parallel with some
> > IRQ torture test?
> >
> > stress-ng --timer 4 --timeout 60s
> >
> > And maybe pin the stress test to the default CPU. Assuming it's 0:
> >
> > taskset -c 0 stress-ng --timer 4 --timeout 60s
> >
> > Unless the 'linear' version is significantly faster, I'd stick to the
> > above.
> >
> > Thanks,
> > Yury
>
> Hey Yury,
>
> We tried a few tests with your suggestion, and throughput seems to be
> the same compared to the linear distribution approach. We stressed out
> CPU0 in both the cases and the results were similar. No IRQ migration
> was observed in either case and no throughput drop.
>
> But one observation I had was that " irq_set_affinity_and_hint(*irqs++,
> NULL);" is essentially a no-op and we end up relying on the initial
> placement from pci_alloc_irq_vectors().
Yes you are, assuming you're not binding them before in your call chain.
> Even though in these tests we
> were not able to reproduce it, but with this distribution there is a
> chance we end up clustering the mana queue IRQs, while other vCPUs are
> not running any network load.
That sounds like an IRQ balancer bug which you're unable to reproduce.
> It's because the placement depends on
> system-wide IRQ state at allocation time.
I don't understand this point. The
irq_set_affinity_and_hint(*irqs++, NULL);
simply means: I trust system IRQ balancer to pick the best CPU for my
IRQ at runtime. It doesn't refer any "IRQ state at allocation time".
> The linear approach however gaurantees each queue IRQ lands on a
> distinct vCPU regardless of system state. Even after stressing the cpus
> using stress-ng, we did not observe any significant throughput drop.
If you just do nothing, it would lead to the same numbers, right? What
does that "non-significant throughput drop" mean? It sounds like the
linear approach is slightly worse.
--
So, as you can't demonstrate solid benefit for the 'linear' IRQ placement,
I would just stick to the no-affinity logic.
Thanks,
Yury
