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Date:   Tue, 21 Apr 2020 15:28:23 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yury Norov <yury.norov@gmail.com>,
        Paul Turner <pjt@google.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Josh Don <joshdon@google.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-kernel@vger.kernel.org,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH 0/4] sched/rt: Distribute tasks in find_lowest_rq()
In-Reply-To: <20200421142243.lea26mnmxnjpynlf@e107158-lin.cambridge.arm.com>
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On 2020-04-21 15:22, Qais Yousef wrote:
> On 04/21/20 15:09, Marc Zyngier wrote:
>> On 2020-04-21 14:18, Valentin Schneider wrote:
>> > On 21/04/20 13:13, Qais Yousef wrote:
>> 
>> [...]
>> 
>> > > I CCed Marc who's the maintainer of this file who can clarify better
>> > > if this
>> > > really breaks anything.
>> > >
>> > > If any interrupt expects to be affined to a specific CPU then this
>> > > must be
>> > > described in DT/driver. I think the GIC controller is free to
>> > > distribute them
>> > > to any cpu otherwise if !force. Which is usually done by
>> > > irq_balancer anyway
>> > > in userspace, IIUC.
>> > >
>> > > I don't see how cpumask_any_and() break anything here too. I
>> > > actually think it
>> > > improves on things by better distribute the irqs on the system by
>> > > default.
>> 
>> That's a pretty bold statement. Unfortunately, it isn't universally 
>> true.
>> Some workload will be very happy with interrupts spread all over the 
>> map,
>> and some others will suffer from it because, well, it interrupts 
>> userspace.
>> 
>> > As you say, if someone wants smarter IRQ affinity they can do
>> > irq_balancer
>> > and whatnot. The default kernel policy for now has been to shove
>> > everything
>> > on the lowest-numbered CPU, and I see no valid reason to change that.
>> 
>> Exactly. I would like to keep the kernel policy as simple as possible 
>> for
>> non-managed interrupts (managed interrupts are another kettle of fish
>> entirely).
>> Userpace is in control to place things "intelligently", so let's not 
>> try and
>> make the kernel smarter than it strictly needs to be.
> 
> Fair enough. But why is it asking for cpumask_any() in the first place?

Implementation detail. Turn it into cpumask_first_and() if you want.

         M.
-- 
Jazz is not dead. It just smells funny...