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

[Marc Zyngier]

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.

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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