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

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Message-ID: <CAFki+LmR-o+Fng21ggy48FUX7RhjjpjO87dn3Ld+L4BK2pSRZg@mail.gmail.com>
Date:   Mon, 17 May 2021 12:57:26 -0400
From:   Nitesh Lal <nilal@...hat.com>
To:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Robin Murphy <robin.murphy@....com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        intel-wired-lan@...ts.osuosl.org, jbrandeb@...nel.org,
        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>
Subject: Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when
 setting the hint

On Tue, May 4, 2021 at 12:25 PM Jesse Brandeburg
<jesse.brandeburg@...el.com> wrote:
>
> Robin Murphy wrote:
>
> > On 2021-05-01 03:18, Jesse Brandeburg wrote:
> > > It was pointed out by Nitesh that the original work I did in 2014
> > > to automatically set the interrupt affinity when requesting a
> > > mask is no longer necessary. The kernel has moved on and no
> > > longer has the original problem, BUT the original patch
> > > introduced a subtle bug when booting a system with reserved or
> > > excluded CPUs. Drivers calling this function with a mask value
> > > that included a CPU that was currently or in the future
> > > unavailable would generally not update the hint.
> > >
> > > I'm sure there are a million ways to solve this, but the simplest
> > > one is to just remove a little code that tries to force the
> > > affinity, as Nitesh has shown it fixes the bug and doesn't seem
> > > to introduce immediate side effects.
> >
> > Unfortunately, I think there are quite a few other drivers now relying
> > on this behaviour, since they are really using irq_set_affinity_hint()
> > as a proxy for irq_set_affinity(). Partly since the latter isn't
> > exported to modules, but also I have a vague memory of it being said
> > that it's nice to update the user-visible hint to match when the
> > affinity does have to be forced to something specific.
> >
> > Robin.
>
> Thanks for your feedback Robin, but there is definitely a bug here that
> is being exposed by this code. The fact that people are using this
> function means they're all exposed to this bug.
>
> Not sure if you saw, but this analysis from Nitesh explains what
> happened chronologically to the kernel w.r.t this code, it's a useful
> analysis! [1]
>
> I'd add in addition that irqbalance daemon *stopped* paying attention
> to hints quite a while ago, so I'm not quite sure what purpose they
> serve.
>
> [1]
> https://lore.kernel.org/lkml/CAFki+Lm0W_brLu31epqD3gAV+WNKOJfVDfX2M8ZM__aj3nv9uA@mail.gmail.com/
>

Wanted to follow up to see if there are any more objections or even
suggestions to take this forward?

-- 
Thanks
Nitesh