Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs

[Jacob Keller <jacob.e.keller@intel.com>]

On 10/21/2020 5:02 PM, Jakub Kicinski wrote:
> On Wed, 21 Oct 2020 22:25:48 +0200 Thomas Gleixner wrote:
>> On Tue, Oct 20 2020 at 20:07, Thomas Gleixner wrote:
>>> On Tue, Oct 20 2020 at 12:18, Nitesh Narayan Lal wrote:  
>>>> However, IMHO we would still need a logic to prevent the devices from
>>>> creating excess vectors.  
>>>
>>> Managed interrupts are preventing exactly that by pinning the interrupts
>>> and queues to one or a set of CPUs, which prevents vector exhaustion on
>>> CPU hotplug.
>>>
>>> Non-managed, yes that is and always was a problem. One of the reasons
>>> why managed interrupts exist.  
>>
>> But why is this only a problem for isolation? The very same problem
>> exists vs. CPU hotplug and therefore hibernation.
>>
>> On x86 we have at max. 204 vectors available for device interrupts per
>> CPU. So assumed the only device interrupt in use is networking then any
>> machine which has more than 204 network interrupts (queues, aux ...)
>> active will prevent the machine from hibernation.
>>
>> Aside of that it's silly to have multiple queues targeted at a single
>> CPU in case of hotplug. And that's not a theoretical problem.  Some
>> power management schemes shut down sockets when the utilization of a
>> system is low enough, e.g. outside of working hours.
>>
>> The whole point of multi-queue is to have locality so that traffic from
>> a CPU goes through the CPU local queue. What's the point of having two
>> or more queues on a CPU in case of hotplug?
>>
>> The right answer to this is to utilize managed interrupts and have
>> according logic in your network driver to handle CPU hotplug. When a CPU
>> goes down, then the queue which is associated to that CPU is quiesced
>> and the interrupt core shuts down the relevant interrupt instead of
>> moving it to an online CPU (which causes the whole vector exhaustion
>> problem on x86). When the CPU comes online again, then the interrupt is
>> reenabled in the core and the driver reactivates the queue.
> 
> I think Mellanox folks made some forays into managed irqs, but I don't
> remember/can't find the details now.
> 

I remember looking into this a few years ago, and not getting very far
either.

> For networking the locality / queue per core does not always work,
> since the incoming traffic is usually spread based on a hash. Many
> applications perform better when network processing is done on a small
> subset of CPUs, and application doesn't get interrupted every 100us. 
> So we do need extra user control here.
> 
> We have a bit of a uAPI problem since people had grown to depend on
> IRQ == queue == NAPI to configure their systems. "The right way" out
> would be a proper API which allows associating queues with CPUs rather
> than IRQs, then we can use managed IRQs and solve many other problems.
> 

I think we (Intel) hit some of the same issues you mention.

I know I personally would like to see something that lets a lot of the
current driver-specific policy be moved out. I think it should be
possible to significantly simplify the abstraction used by the drivers.

> Such new API has been in the works / discussions for a while now.
> 
> (Magnus keep me honest here, if you disagree the queue API solves this.)
>