[Intel-wired-lan] [PATCH] genirq: Provide new interfaces for affinity hints
Shung-Hsi Yu
shung-hsi.yu at suse.com
Fri May 28 07:20:41 UTC 2021
On Thu, May 27, 2021 at 09:06:04AM -0400, Nitesh Lal wrote:
> 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()
I *do* want the driver to rely on request_irq() for spreading.
It is retrieving effective affinity after request_irq() call that I can't
seem to figure out.
Thanks,
Shung-Hsi
> 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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