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 14:17:10 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Marc Zyngier <maz@...nel.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 Tue, 27 Oct 2020 at 13:06, Marc Zyngier <maz@...nel.org> wrote:
>
> On 2020-10-27 11:21, Vincent Guittot wrote:
> > On Tue, 27 Oct 2020 at 11:50, Vincent Guittot
> > <vincent.guittot@...aro.org> wrote:
> >>
> >> On Tue, 27 Oct 2020 at 11:37, Marc Zyngier <maz@...nel.org> wrote:
> >> >
> >> > 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.
> >>
> >> Ok, I'm going to test this series to see if it fixes the perf
> >> regression
> >
> > You have spotted the root cause of the regression. We are back to ~1%
> > performance diff on the hikey
>
> Yeah. Only thing is that I can't look at this hack without vomiting...

At least, we know the root cause and the impact of irq_enter/exit
>
>          M.
> --
> Jazz is not dead. It just smells funny...