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

[Marc Zyngier]

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...