Re: [PATCH v2] irqchip/gic-v3-its: Balance initial LPI affinity across CPUs

[John Garry <john.garry@huawei.com>]

Hi Marc,

>>> When mapping a LPI, the ITS driver picks the first possible
>>> affinity, which is in most cases CPU0, assuming that if
>>> that's not suitable, someone will come and set the affinity
>>> to something more interesting.
>>>
>>> It apparently isn't the case, and people complain of poor
>>> performance when many interrupts are glued to the same CPU.
>>> So let's place the interrupts by finding the "least loaded"
>>> CPU (that is, the one that has the fewer LPIs mapped to it).
>>> So called 'managed' interrupts are an interesting case where
>>> the affinity is actually dictated by the kernel itself, and
>>> we should honor this.
>>>
>>> Reported-by: John Garry <john.garry@huawei.com>
>>> Link: 
>>> https://lore.kernel.org/r/1575642904-58295-1-git-send-email-john.garry@huawei.com 
>>>
>>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>>> Cc: John Garry <john.garry@huawei.com>
>>> Cc: Ming Lei <ming.lei@redhat.com>
>>> ---
>>> Reviving this at John's request.
>>
>> Thanks very much. I may request a colleague test this due to possible
>> precautionary office closure.
> 
> Huh. Not great... :-(
> 
>>
>>  The major change is that the
>>> affinity follows the x86 model, as described by Thomas.
>>
>> There seems to be a subtle difference between this implementation and
>> what Thomas described for managed interrupts handling on x86. That
>> being, managed interrupt loading is counted separately to total
>> interrupts per CPU for x86. That seems quite important so that we
>> spread managed interrupts evenly.
> 
> Hmmm. Yes. That'd require a separate per-CPU counter. Nothing too invasive
> though. I'll roll that in soon. I still wonder about interaction of 
> collocated
> managed and non-managed interrupts, but we can cross that bridge later.

Great. And I think I may have mentioned this before (or I did and it was 
not a good idea), it now seems that we may be able to just leverage the 
generic matrix irq code here.

Cheers,
John