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

Marc Zyngier
2020-10-27T10:38:03Z

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.

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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