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

Marc Zyngier
2021-05-06T11:44:45Z

On Thu, 06 May 2021 08:50:42 +0100,
He Ying <heying24@huawei.com> 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?

> 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?

> (2) irq_domain lookups add some overhead.

While this is also a potential source of overhead, it turned out not
to be the case.

> 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).

Thanks,

M.

[1] https://lore.kernel.org/lkml/20201124141449.572446-1-maz@kernel.org/

--
Without deviation from the norm, progress is not possible.