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

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Message-Id: <c8d69bc1-55eb-44c3-bf97-c5c4c475fded@www.fastmail.com>
Date:   Thu, 30 Sep 2021 10:26:55 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Sohil Mehta" <sohil.mehta@...el.com>,
        "Stefan Hajnoczi" <stefanha@...hat.com>
Cc:     "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 Thu, Sep 30, 2021, at 10:24 AM, 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.

Thanks!

>
>
>> (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, can you please provide some more context/details on this? Is this 
> regarding the blocking syscalls discussion (in patch 11) or something else?
>

Yes, and I'll follow up there.  I hereby upgrade my opinion of SENDUIPI wakeups to "probably doable but maybe not in a nice way."