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

[Marc Zyngier]

Hi Vincent,

On 2020-10-19 13:42, Vincent Guittot wrote:
> Hi Marc,
>
> On Tue, 1 Sep 2020 at 16:44, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > In order to deal with IPIs as normal interrupts, let's add
> > a new way to register them with the architecture code.
> >
> > set_smp_ipi_range() takes a range of interrupts, and allows
> > the arch code to request them as if the were normal interrupts.
> > A standard handler is then called by the core IRQ code to deal
> > with the IPI.
> >
> > This means that we don't need to call irq_enter/irq_exit, and
> > that we don't need to deal with set_irq_regs either. So let's
> > move the dispatcher into its own function, and leave handle_IPI()
> > as a compatibility function.
> >
> > On the sending side, let's make use of ipi_send_mask, which
> > already exists for this purpose.
> >
> > 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?

Thanks,

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