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

Marc Zyngier
2020-10-27T12:06:10Z

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