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

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Message-Id: <456bf9cf-87b8-4c3d-ac0c-7e392bcf26de@www.fastmail.com>
Date:   Tue, 28 Sep 2021 21:31:34 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Sohil Mehta" <sohil.mehta@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>
Cc:     "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 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 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