Re: [RFC PATCH 00/13] x86 User Interrupts support

[Andy Lutomirski]

On Thu, Sep 30, 2021, at 10:24 AM, Sohil Mehta wrote:
> On 9/30/2021 9:30 AM, Stefan Hajnoczi wrote:
>> On Tue, Sep 28, 2021 at 09:31:34PM -0700, Andy Lutomirski wrote:
>>>
>>> I spent some time reviewing the docs (ISE) and contemplating how this all fits together, and I have a high level question:
>>>
>>> Can someone give an example of a realistic workload that would benefit from SENDUIPI and precisely how it would use SENDUIPI?  Or an example of a realistic workload that would benefit from hypothetical device-initiated user interrupts and how it would use them?  I'm having trouble imagining something that wouldn't work as well or better by simply polling, at least on DMA-coherent architectures like x86.
>> I was wondering the same thing. One thing came to mind:
>>
>> An application that wants to be *interrupted* from what it's doing
>> rather than waiting until the next polling point. For example,
>> applications that are CPU-intensive and have green threads. I can't name
>> a real application like this though :P.
>
> Thank you Stefan and Andy for giving this some thought.
>
> We are consolidating the information internally on where and how exactly 
> we expect to see benefits with real workloads for the various sources of 
> User Interrupts. It will take a few days to get back on this one.

Thanks!

>
>
>> (I can imagine some benefit to a hypothetical improved SENDUIPI with idential user semantics but that supported a proper interaction with the scheduler and blocking syscalls.  But that's not what's documented in the ISE...)
>
> Andy, can you please provide some more context/details on this? Is this 
> regarding the blocking syscalls discussion (in patch 11) or something else?
>

Yes, and I'll follow up there.  I hereby upgrade my opinion of SENDUIPI wakeups to "probably doable but maybe not in a nice way."