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References: <20210501021832.743094-1-jesse.brandeburg@intel.com>
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From:   Nitesh Lal <nilal@redhat.com>
Date:   Mon, 17 May 2021 12:57:26 -0400
Message-ID: <CAFki+LmR-o+Fng21ggy48FUX7RhjjpjO87dn3Ld+L4BK2pSRZg@mail.gmail.com>
Subject: Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when
 setting the hint
To:     Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Robin Murphy <robin.murphy@arm.com>,
        "frederic@kernel.org" <frederic@kernel.org>,
        "juri.lelli@redhat.com" <juri.lelli@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        intel-wired-lan@lists.osuosl.org, jbrandeb@kernel.org,
        Alex Belits <abelits@marvell.com>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "bhelgaas@google.com" <bhelgaas@google.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "sfr@canb.auug.org.au" <sfr@canb.auug.org.au>,
        "stephen@networkplumber.org" <stephen@networkplumber.org>,
        "rppt@linux.vnet.ibm.com" <rppt@linux.vnet.ibm.com>,
        "jinyuqi@huawei.com" <jinyuqi@huawei.com>,
        "zhangshaokun@hisilicon.com" <zhangshaokun@hisilicon.com>,
        netdev@vger.kernel.org, chris.friesen@windriver.com,
        Marc Zyngier <maz@kernel.org>
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On Tue, May 4, 2021 at 12:25 PM Jesse Brandeburg
<jesse.brandeburg@intel.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