Re: Control of IRQ Affinities from Userspace

["bigeasy@linutronix.de" <bigeasy@linutronix.de>]

On 2025-11-25 12:32:39 [+0100], Florian Bezdeka wrote:
> > It seems that if you exclude certain CPUs from getting interrupt
> > handling than it should work fine. Then the driver would only balance
> > the interrupts among the CPUs that are left.
> 
> Sebastian, what exactly do you mean by "exclude certain CPUs from
> getting interrupt handling"? I mean, that is what we do by configuring
> the /proc/<irq>/smp_affinity_list interface.

Step #1
- figure out if isolcpus= is restricting the affinity of requested
  interrupts to housekeeping CPUs only

Step #2
- Yes
   => look for the matching knob in cgroup interface
      Knob found?
      - Yes
        => Use knob.
      - No.
        => Add knob.
- No
  This should be added as it breaks the expectation of an isolated
  system.

I *think* the driver should request as many interrupts as there are
available CPUs in the system to handle them. The number of available
CPUs/ CPU mask should be a configure knob by the user. Using the
housekeeping CPUs as a default mask seems reasonable.
The question is what should happen if the mask changes at runtime. Maybe
a device needs to reconfigure, maybe just move the interrupt away.
But this should also affect NOHZ_FULL workloads.

> To sum up: 
> - The IRQ balancing issue is not limited to a single driver / subsystem
> - The managed IRQ infrastructure seems very "static" so insufficient for
>   this problem. In addition we would have to migrate all affected
>   drivers to the managed IRQ infrastructure first.
> 
> We would love to hear further thoughts / ideas / comments about this
> problem. We're highly interested in fixing this issue properly.

If the "managed IRQ infrastructure" would help here then why not. Maybe
Frederic has some insight here.

> Thanks for all the comments so far!
> 
> Florian

Sebastian