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

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Date:   Mon, 17 May 2021 19:50:35 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Nitesh Lal <nilal@...hat.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.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 2021-05-17 19:08, Thomas Gleixner wrote:
> On Mon, May 17 2021 at 18:26, Robin Murphy wrote:
>> On 2021-05-17 17:57, Nitesh Lal wrote:
>> I'm not implying that there isn't a bug, or that this code ever made
>> sense in the first place, just that fixing it will unfortunately be a
>> bit more involved than a simple revert. This patch as-is *will* subtly
>> break at least the system PMU drivers currently using
> 
> s/using/abusing/
> 
>> irq_set_affinity_hint() - those I know require the IRQ affinity to
>> follow whichever CPU the PMU context is bound to, in order to meet perf
>> core's assumptions about mutual exclusion.
> 
> Which driver is that?

Right now, any driver which wants to control an IRQ's affinity and also 
build as a module, for one thing. I'm familiar with drivers/perf/ where 
a basic pattern has been widely copied; some of the callers in other 
subsystems appear to *expect* it to set the underlying affinity as well, 
but whether any of those added within the last 6 years represent a 
functional dependency rather than just a performance concern I don't know.

Robin.