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

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Date:   Tue, 27 Oct 2020 10:37:35 +0000
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

On 2020-10-27 10:12, Vincent Guittot wrote:
> HI Marc,
> 
> On Mon, 19 Oct 2020 at 17:43, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>> 
>> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier <maz@...nel.org> wrote:
>> >
> 
> ...
> 
>> > >>
>> > >> 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?
> 
> Any idea about the root cause of the regression ?
> I have faced it on more arm64 platforms in the meantime

two possible causes:

(1) irq_enter/exit on the rescheduling IPI means we reschedule much more 
often
(2) irq_domain lookups add some overhead.

For (1), I have this series[1] which is ugly as sin and needs much more 
testing.

For (2), I have some ideas which need more work (let the irq domain 
resolve to
an irq_desc instead of an interrupt number, avoiding another radix-tree 
lookup).

         M.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/ipi-fixes
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