On Mon, May 17, 2021 at 8:23 PM Nitesh Lal [off-list ref] wrote:

On Mon, May 17, 2021 at 8:04 PM Thomas Gleixner [off-list ref] wrote:

quoted

On Mon, May 17 2021 at 18:44, Nitesh Lal wrote:

quoted

On Mon, May 17, 2021 at 4:48 PM Thomas Gleixner [off-list ref] wrote:

quoted

The hint was added so that userspace has a better understanding where it
should place the interrupt. So if irqbalanced ignores it anyway, then
what's the point of the hint? IOW, why is it still used drivers?

Took a quick look at the irqbalance repo and saw the following commit:

dcc411e7bf    remove affinity_hint infrastructure

The commit message mentions that "PJ is redesiging how affinity hinting
works in the kernel, the future model will just tell us to ignore an IRQ,
and the kernel will handle placement for us.  As such we can remove the
affinity_hint recognition entirely".

No idea who PJ is. I really love useful commit messages. Maybe Neil can
shed some light on that.

quoted

This does indicate that apparently, irqbalance moved away from the usage of
affinity_hint. However, the next question is what was this future
model?

I might have missed something in the last 5 years, but that's the first
time I hear about someone trying to cleanup that thing.

quoted

I don't know but I can surely look into it if that helps or maybe someone
here already knows about it?

I CC'ed Neil :)

Thanks, I have added PJ Waskiewicz as well who I think was referred in
that commit message as PJ.

quoted

quoted

quoted

Now there is another aspect to that. What happens if irqbalanced does
not run at all and a driver relies on the side effect of the hint
setting the initial affinity. Bah...

Right, but if they only rely on this API so that the IRQs are spread across
all the CPUs then that issue is already resolved and these other drivers
should not regress because of changing this behavior. Isn't it?

Is that true for all architectures?

Unfortunately, I don't know and that's probably why we have to be careful.

I think here to ensure that we are not breaking any of the drivers we have
to first analyze all the existing drivers and understand how they are using
this API.
AFAIK there are three possible scenarios:

- A driver use this API to spread the IRQs
  + For this case we should be safe considering the spreading is naturally
    done from the IRQ subsystem itself.

- A driver use this API to actually set the hint
  + These drivers should have no functional impact because of this revert

- Driver use this API to force a certain affinity mask
  + In this case we have to replace the API with the irq_force_affinity()

I can start looking into the individual drivers, however, testing them will
be a challenge.

Any thoughts?

--
Thanks
Nitesh