linux-kernel - Re: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal interrupts

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Date:   Mon, 19 Oct 2020 14:04:19 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Sumit Garg <sumit.garg@...aro.org>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Saravana Kannan <saravanak@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal
 interrupts

Hi Vincent,

On 2020-10-19 13:42, Vincent Guittot wrote:
> Hi Marc,
> 
> On Tue, 1 Sep 2020 at 16:44, Marc Zyngier <maz@...nel.org> wrote:
>> 
>> In order to deal with IPIs as normal interrupts, let's add
>> a new way to register them with the architecture code.
>> 
>> set_smp_ipi_range() takes a range of interrupts, and allows
>> the arch code to request them as if the were normal interrupts.
>> A standard handler is then called by the core IRQ code to deal
>> with the IPI.
>> 
>> This means that we don't need to call irq_enter/irq_exit, and
>> that we don't need to deal with set_irq_regs either. So let's
>> move the dispatcher into its own function, and leave handle_IPI()
>> as a compatibility function.
>> 
>> On the sending side, let's make use of ipi_send_mask, which
>> already exists for this purpose.
>> 
>> One of the major difference is that we end up, in some cases
>> (such as when performing IRQ time accounting on the scheduler
>> IPI), end up with nested irq_enter()/irq_exit() pairs.
>> Other than the (relatively small) overhead, there should be
>> no consequences to it (these pairs are designed to nest
>> correctly, and the accounting shouldn't be off).
> 
> While rebasing on mainline, I have faced a performance regression for
> the benchmark:
> perf bench sched pipe
> on my arm64 dual quad core (hikey) and my 2 nodes x 112 CPUS (thx2)
> 
> The regression comes from:
> commit: d3afc7f12987 ("arm64: Allow IPIs to be handled as normal 
> interrupts")

That's interesting, as this patch doesn't really change anything (most
of the potential overhead comes in later). The only potential overhead
I can see is that the scheduler_ipi() call is now wrapped around
irq_enter()/irq_exit().

> 
>           v5.9              + this patch
> hikey :   48818(+/- 0.31)   37503(+/- 0.15%)  -23.2%
> thx2  :  132410(+/- 1.72)  122646(+/- 1.92%)   -7.4%
> 
> By + this patch,  I mean merging branch from this patch. Whereas
> merging the previous:
> commit: 83cfac95c018 ("genirq: Allow interrupts to be excluded from
> /proc/interrupts")
>  It doesn't show any regression

Since you are running perf, can you spot where the overhead occurs?

Thanks,

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