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From:   Nitesh Lal <nilal@redhat.com>
Date:   Thu, 27 May 2021 09:06:04 -0400
Message-ID: <CAFki+LnuharcwJy=x4Z27ixCMK1u2s2cvHh9=Bcf90rO06osEw@mail.gmail.com>
Subject: Re: [PATCH] genirq: Provide new interfaces for affinity hints
To:     Shung-Hsi Yu <shung-hsi.yu@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        intel-wired-lan@lists.osuosl.org, jbrandeb@kernel.org,
        "frederic@kernel.org" <frederic@kernel.org>,
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        Alex Belits <abelits@marvell.com>,
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        Neil Horman <nhorman@tuxdriver.com>, pjwaskiewicz@gmail.com
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On Thu, May 27, 2021 at 6:03 AM Shung-Hsi Yu <shung-hsi.yu@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