Date:   Tue, 10 Dec 2019 10:28:12 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     Ming Lei <ming.lei@...hat.com>, <tglx@...utronix.de>,
        <chenxiang66@...ilicon.com>, <bigeasy@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <hare@...e.com>, <hch@....de>,
        <axboe@...nel.dk>, <bvanassche@....org>, <peterz@...radead.org>,
        <mingo@...hat.com>
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity  for managed interrupt

On 2019-12-10 09:45, John Garry wrote:
> On 10/12/2019 01:43, Ming Lei wrote:
>> On Mon, Dec 09, 2019 at 02:30:59PM +0000, John Garry wrote:
>>> On 07/12/2019 08:03, Ming Lei wrote:
>>>> On Fri, Dec 06, 2019 at 10:35:04PM +0800, John Garry wrote:
>>>>> Currently the cpu allowed mask for the threaded part of a 
>>>>> threaded irq
>>>>> handler will be set to the effective affinity of the hard irq.
>>>>>
>>>>> Typically the effective affinity of the hard irq will be for a 
>>>>> single cpu. As such,
>>>>> the threaded handler would always run on the same cpu as the hard 
>>>>> irq.
>>>>>
>>>>> We have seen scenarios in high data-rate throughput testing that 
>>>>> the cpu
>>>>> handling the interrupt can be totally saturated handling both the 
>>>>> hard
>>>>> interrupt and threaded handler parts, limiting throughput.
>>>>
>
> Hi Ming,
>
>>>> Frankly speaking, I never observed that single CPU is saturated by 
>>>> one storage
>>>> completion queue's interrupt load. Because CPU is still much 
>>>> quicker than
>>>> current storage device.
>>>>
>>>> If there are more drives, one CPU won't handle more than one 
>>>> queue(drive)'s
>>>> interrupt if (nr_drive * nr_hw_queues) < nr_cpu_cores.
>>>
>>> Are things this simple? I mean, can you guarantee that fio 
>>> processes are
>>> evenly distributed as such?
>> That is why I ask you for the details of your test.
>> If you mean hisilicon SAS,
>
> Yes, it is.
>
>  the interrupt load should have been distributed
>> well given the device has multiple reply queues for distributing 
>> interrupt
>> load.
>>
>>>
>>>>
>>>> So could you describe your case in a bit detail? Then we can 
>>>> confirm
>>>> if this change is really needed.
>>>
>>> The issue is that the CPU is saturated in servicing the hard and 
>>> threaded
>>> part of the interrupt together - here's the sort of thing which we 
>>> saw
>>> previously:
>>> Before:
>>> CPU	%usr	%sys	%irq	%soft	%idle
>>> all	2.9	13.1	1.2	4.6	78.2
>>> 0	0.0	29.3	10.1	58.6	2.0
>>> 1	18.2	39.4	0.0	1.0	41.4
>>> 2	0.0	2.0	0.0	0.0	98.0
>>>
>>> CPU0 has no effectively no idle.
>> The result just shows the saturation, we need to root cause it 
>> instead
>> of workaround it via random changes.
>>
>>>
>>> Then, by allowing the threaded part to roam:
>>> After:
>>> CPU	%usr	%sys	%irq	%soft	%idle
>>> all	3.5	18.4	2.7	6.8	68.6
>>> 0	0.0	20.6	29.9	29.9	19.6
>>> 1	0.0	39.8	0.0	50.0	10.2
>>>
>>> Note: I think that I may be able to reduce the irq hard part load 
>>> in the
>>> endpoint driver, but not that much such that we see still this 
>>> issue.
>>>
>>>>
>>>>>
>>>>> For when the interrupt is managed, allow the threaded part to run 
>>>>> on all
>>>>> cpus in the irq affinity mask.
>>>>
>>>> I remembered that performance drop is observed by this approach in 
>>>> some
>>>> test.
>>>
>>>  From checking the thread about the NVMe interrupt swamp, just 
>>> switching to
>>> threaded handler alone degrades performance. I didn't see any 
>>> specific
>>> results for this change from Long Li - 
>>> https://lkml.org/lkml/2019/8/21/128
>> I am pretty clear the reason for Azure, which is caused by 
>> aggressive interrupt
>> coalescing, and this behavior shouldn't be very common, and it can 
>> be
>> addressed by the following patch:
>> 
>> http://lists.infradead.org/pipermail/linux-nvme/2019-November/028008.html
>> Then please share your lockup story, such as, which HBA/drivers, 
>> test steps,
>> if you complete IOs from multiple disks(LUNs) on single CPU, if you 
>> have
>> multiple queues, how many active LUNs involved in the test, ...
>
> 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?

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.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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