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Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity
 for managed interrupt
To:     Marc Zyngier <maz@kernel.org>
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From:   John Garry <john.garry@huawei.com>
Message-ID: <06d1e2ff-9ec7-2262-25a0-4503cb204b0b@huawei.com>
Date:   Tue, 10 Dec 2019 12:05:20 +0000
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On 10/12/2019 11:36, Marc Zyngier wrote:
> On 2019-12-10 10:59, John Garry wrote:
>>>>
>>>> There is no lockup, just a potential performance boost in this change.
>>>>
>>>> My colleague Xiang Chen can provide specifics of the test, as he is
>>>> the one running it.
>>>>
>>>> But one key bit of info - which I did not think most relevant before
>>>> - that is we have 2x SAS controllers running the throughput test on
>>>> the same host.
>>>>
>>>> As such, the completion queue interrupts would be spread identically
>>>> over the CPUs for each controller. I notice that ARM GICv3 ITS
>>>> interrupt controller (which we use) does not use the generic irq
>>>> matrix allocator, which I think would really help with this.
>>>>
>>>> Hi Marc,
>>>>
>>>> Is there any reason for which we couldn't utilise of the generic irq
>>>> matrix allocator for GICv3?
>>>
>>
>> Hi Marc,
>>
>>> For a start, the ITS code predates the matrix allocator by about three
>>> years. Also, my understanding of this allocator is that it allows
>>> x86 to cope with a very small number of possible interrupt vectors
>>> per CPU. The ITS doesn't have such issue, as:
>>> 1) the namespace is global, and not per CPU
>>> 2) the namespace is *huge*
>>> Now, what property of the matrix allocator is the ITS code missing?
>>> I'd be more than happy to improve it.
>>
>> I think specifically the property that the matrix allocator will try
>> to find a CPU for irq affinity which "has the lowest number of managed
>> IRQs allocated" - I'm quoting the comment on 
>> matrix_find_best_cpu_managed().
> 
> But that decision is due to allocation constraints. You can have at most
> 256 interrupts per CPU, so the allocator tries to balance it.
> 
> On the contrary, the ITS does care about how many interrupt target any
> given CPU. The whole 2^24 interrupt namespace can be thrown at a single
> CPU.
> 
>> The ITS code will make the lowest online CPU in the affinity mask the
>> target CPU for the interrupt, which may result in some CPUs handling
>> so many interrupts.
> 
> If what you want is for the *default* affinity to be spread around,
> that should be achieved pretty easily. Let me have a think about how
> to do that.

Cool, I anticipate that it should help my case.

I can also seek out some NVMe cards to see how it would help a more 
"generic" scenario.

Cheers,
John