linux-kernel - Re: [PATCH] genirq: Provide new interfaces for affinity hints

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Date:   Thu, 27 May 2021 09:06:04 -0400
From:   Nitesh Lal <nilal@...hat.com>
To:     Shung-Hsi Yu <shung-hsi.yu@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        intel-wired-lan@...ts.osuosl.org, jbrandeb@...nel.org,
        "frederic@...nel.org" <frederic@...nel.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "rppt@...ux.vnet.ibm.com" <rppt@...ux.vnet.ibm.com>,
        "jinyuqi@...wei.com" <jinyuqi@...wei.com>,
        "zhangshaokun@...ilicon.com" <zhangshaokun@...ilicon.com>,
        netdev@...r.kernel.org, chris.friesen@...driver.com,
        Marc Zyngier <maz@...nel.org>,
        Neil Horman <nhorman@...driver.com>, pjwaskiewicz@...il.com
Subject: Re: [PATCH] genirq: Provide new interfaces for affinity hints

On Thu, May 27, 2021 at 6:03 AM Shung-Hsi Yu <shung-hsi.yu@...e.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