Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint

[Nitesh Lal]

On Mon, May 17, 2021 at 8:23 PM Nitesh Lal <nilal@xxxxxxxxxx> wrote:
>
> On Mon, May 17, 2021 at 8:04 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, May 17 2021 at 18:44, Nitesh Lal wrote:
> > > On Mon, May 17, 2021 at 4:48 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > >> 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.
> >
> > > 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.
> >
> > > 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.
>
> >
> > >> 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