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

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Message-ID: <YVXmGTo5Uzp44QQq@stefanha-x1.localdomain>
Date:   Thu, 30 Sep 2021 17:30:17 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Sohil Mehta <sohil.mehta@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Raj Ashok <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support

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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