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Date:   Thu, 30 Sep 2021 17:30:17 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Sohil Mehta <sohil.mehta@intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tony Luck <tony.luck@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
        Christian Brauner <christian@brauner.io>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Shuah Khan <shuah@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Raj Ashok <ashok.raj@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Gayatri Kammela <gayatri.kammela@intel.com>,
        Zeng Guang <guang.zeng@intel.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        Randy E Witt <randy.e.witt@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        Ramesh Thomas <ramesh.thomas@intel.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arch@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-kselftest@vger.kernel.org
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support
Message-ID: <YVXmGTo5Uzp44QQq@stefanha-x1.localdomain>
References: <20210913200132.3396598-1-sohil.mehta@intel.com>
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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
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
> >
> > 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 call=
s and
> > hardware interrupt based notifications. User interrupts provide the fou=
ndation
> > for more efficient (low latency and low CPU utilization) versions of th=
ese
> > common operations by avoiding transitions through the kernel.
> >
>=20
> ...
>=20
> I spent some time reviewing the docs (ISE) and contemplating how this all=
 fits together, and I have a high level question:
>=20
> Can someone give an example of a realistic workload that would benefit fr=
om SENDUIPI and precisely how it would use SENDUIPI?  Or an example of a re=
alistic workload that would benefit from hypothetical device-initiated user=
 interrupts and how it would use them?  I'm having trouble imagining someth=
ing 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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