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

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

[Stefan Hajnoczi]

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On Thu, Sep 30, 2021 at 10:24:24AM -0700, 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.

One possible use case came to mind in QEMU's TCG just-in-time compiler:

QEMU's TCG threads execute translated code. There are events that
require interrupting these threads. Today a check is performed at the
start of every translated block. Most of the time the check is false and
it's a waste of CPU.

User interrupts can eliminate the need for checks by interrupting TCG
threads when events occur.

I don't know whether this will improve performance or how feasible it is
to implement, but I've added people who might have ideas. (For a summary
of user interrupts, see
https://lwn.net/SubscriberLink/871113/60652640e11fc5df/.)

Stefan

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

[Richard Henderson]

On 10/1/21 12:35 PM, Stefan Hajnoczi wrote:
> QEMU's TCG threads execute translated code. There are events that
> require interrupting these threads. Today a check is performed at the
> start of every translated block. Most of the time the check is false and
> it's a waste of CPU.
> 
> User interrupts can eliminate the need for checks by interrupting TCG
> threads when events occur.

We used to use interrupts, and stopped because we need to wait until the guest is in a 
stable state.  The guest is always in a stable state at the beginning of each TB.

See 378df4b2375.


r~