[Intel-wired-lan] [PATCH] genirq: Provide new interfaces for affinity hints
Nitesh Lal
nilal at redhat.com
Thu May 27 13:06:04 UTC 2021
On Thu, May 27, 2021 at 6:03 AM Shung-Hsi Yu <shung-hsi.yu at suse.com> wrote:
>
> Hi,
>
> On Fri, May 21, 2021 at 02:03:06PM +0200, Thomas Gleixner wrote:
> > The discussion about removing the side effect of irq_set_affinity_hint() of
> > actually applying the cpumask (if not NULL) as affinity to the interrupt,
> > unearthed a few unpleasantries:
> >
> > 1) The modular perf drivers rely on the current behaviour for the very
> > wrong reasons.
> >
> > 2) While none of the other drivers prevents user space from changing
> > the affinity, a cursorily inspection shows that there are at least
> > expectations in some drivers.
> >
> > #1 needs to be cleaned up anyway, so that's not a problem
> >
> > #2 might result in subtle regressions especially when irqbalanced (which
> > nowadays ignores the affinity hint) is disabled.
> >
> > Provide new interfaces:
> >
> > irq_update_affinity_hint() - Only sets the affinity hint pointer
> > irq_apply_affinity_hint() - Set the pointer and apply the affinity to
> > the interrupt
> >
> > Make irq_set_affinity_hint() a wrapper around irq_apply_affinity_hint() and
> > document it to be phased out.
>
> Is there recommended way to retrieve the CPU number that the interrupt has
> affinity?
>
> Previously a driver (I'm looking at drivers/net/ethernet/amazon/ena) that
> uses irq_set_affinity_hint() to spread out IRQ knows the corresponding CPU
> number since they're using their own spreading scheme. Now, phasing out
> irq_set_affinity_hint(), and thus relying on request_irq() to spread the
> load instead, there don't seem to be a easy way to get the CPU number.
>
For drivers that don't want to rely on request_irq for spreading and want
to force their own affinity mask can use irq_set_affinity() which is an
exported interface now [1] and clearly indicates the purpose of the usage.
As Thomas suggested we are still keeping irq_set_affinity_hint() as a
wrapper until we make appropriate changes in individual drivers that use
this API for different reasons. Please feel free to send out a patch
for this driver once the changes are merged.
[1] https://lkml.org/lkml/2021/5/18/271
--
Thanks
Nitesh
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