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

[He Ying]

在 2021/5/7 16:56, Marc Zyngier 写道:
> On Fri, 07 May 2021 08:30:06 +0100,
> He Ying <heying24@xxxxxxxxxx> wrote:
> > 在 2021/5/6 19:44, Marc Zyngier 写道:
> > > On Thu, 06 May 2021 08:50:42 +0100,
> > > He Ying <heying24@xxxxxxxxxx> wrote:
> > > > Hello Marc,
> > > >
> > > > We have faced a performance regression for handling ipis since this
> > > > commit. I think it's the same issue reported by Vincent.
> > > Can you share more details on what regression you have observed?
> > > What's the workload, the system, the performance drop?
> > OK. We have just calculated the pmu cycles from the entry of gic_handle_irq
> > to the entry of do_handle_ipi. Here is some more information about our test:
> >
> > CPU: Hisilicon hip05-d02
> >
> > Applying the patch series: 1115 cycles
> > Reverting the patch series: 599 cycles
> And? How is that meaningful? Interrupts are pretty rare compared to
> everything that happens in the system. How does it affect the
> behaviour of the system as a whole?
OK.
> > > > I found you pointed out the possible two causes:
> > > >
> > > > (1) irq_enter/exit on the rescheduling IPI means we reschedule much
> > > > more often.
> > > It turned out to be a red herring. We don't reschedule more often, but
> > > we instead suffer from the overhead of irq_enter()/irq_exit().
> > > However, this only matters for silly benchmarks, and no real-life
> > > workload showed any significant regression. Have you identified such
> > > realistic workload?
> > I'm afraid not. We just run some benchmarks and calculated pmu cycle
> > counters.  But we have observed running time from the entry of
> > gic_handle_irq to the entry of do_handle_ipi almost doubles. Doesn't
> > it affect realistic workload?
> Then I'm not that interested. Show me an actual regression in a real
> workload that affects people, and I'll be a bit more sympathetic to
> your complain. But quoting raw numbers do not help.
>
> There is a number of advantages to having IPI as IRQs, as it allows us
> to deal with proper allocation (other subsystem want to use IPIs), and
> eventually NMIs. There is a trade-off, and if that means wasting a few
> cycles, so be it.
OK. I see.
> > > > (2) irq_domain lookups add some overhead.
> > > While this is also a potential source of overhead, it turned out not
> > > to be the case.
> > OK.
> > > > But I don't see any following patches in mainline. So, are you still
> > > > working on this issue?  Looking forward to your reply.
> > > See [1]. However, there is probably better things to do than this
> > > low-level specialisation of IPIs, and Thomas outlined what needs to be
> > > done (see v1 of the patch series).
> > OK. I see the patch series. Would it be applied to the mainline
> > someday? I notice that more than 5 months have passed since you sent
> > the patch series.
> I have no plan to merge these patches any time soon, given that nobody
> has shown a measurable regression using something other than a trivial
> benchmark. If you come up with such an example, I will of course
> reconsider this position.

OK. Thanks a lot for all your reply. If I come up with a measurable 
regression

with a realistic workload, I'll contact you again.


Thanks.

> Thanks,
>
> 	M.