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

Vincent Guittot
2020-10-27T10:50:55Z

On Tue, 27 Oct 2020 at 11:37, Marc Zyngier <maz@kernel.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@linaro.org> wrote:
> >>
> >> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier <maz@kernel.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

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