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

From: Marc Zyngier
Date: Tue Oct 27 2020 - 08:06:10 EST


On 2020-10-27 11:21, Vincent Guittot wrote:
On Tue, 27 Oct 2020 at 11:50, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:

On Tue, 27 Oct 2020 at 11:37, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On 2020-10-27 10:12, Vincent Guittot wrote:
> > HI Marc,
> >
> > On Mon, 19 Oct 2020 at 17:43, Vincent Guittot
> > <vincent.guittot@xxxxxxxxxx> wrote:
> >>
> >> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier <maz@xxxxxxxxxx> 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...

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