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

[Stefan Hajnoczi]

On Tue, Sep 28, 2021 at 09:31:34PM -0700, Andy Lutomirski wrote:
> On Mon, Sep 13, 2021, at 1:01 PM, Sohil Mehta wrote:
> > User Interrupts Introduction
> > ============================
> >
> > User Interrupts (Uintr) is a hardware technology that enables delivering
> > interrupts directly to user space.
> >
> > Today, virtually all communication across privilege boundaries happens by going
> > through the kernel. These include signals, pipes, remote procedure calls and
> > hardware interrupt based notifications. User interrupts provide the foundation
> > for more efficient (low latency and low CPU utilization) versions of these
> > common operations by avoiding transitions through the kernel.
> >
> 
> ...
> 
> 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.

Stefan

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