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

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Date:   Tue, 14 Sep 2021 19:03:36 +0000
From:   "Mehta, Sohil" <sohil.mehta@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Luck, Tony" <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Lutomirski, Andy" <luto@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <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>,
        "Kammela, Gayatri" <gayatri.kammela@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Witt, Randy E" <randy.e.witt@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Thomas, Ramesh" <ramesh.thomas@...el.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support

Resending.. There were some email delivery issues.

On 9/13/2021 1:27 PM, Dave Hansen wrote:
>	User Interrupts directly deliver events to user space and are
>	10x faster than the closest alternative.

Thanks Dave. This is definitely more attention-grabbing than the
previous intro. I'll include this next time.

One thing to note, the 10x gain is only applicable for User IPIs.
For other source of User Interrupts (like kernel-to-user
notifications and other external sources), we don't have the data
yet.

I realized the User IPI data in the cover also needs some
clarification. The 10x gain is only seen when the receiver is
spinning in User space - waiting for interrupts.

If the receiver were to block (wait) in the kernel, the performance
would drop as expected. However, User IPI (blocked) would still be
10% faster than Eventfd and 40% faster than signals.

Here is the updated table:
+---------------------+-------------------------+
| IPC type            |   Relative Latency      |
|                     |(normalized to User IPI) |
+---------------------+-------------------------+
| User IPI            |                     1.0 |
| User IPI (blocked)  |                     8.9 |
| Signal              |                    14.8 |
| Eventfd             |                     9.7 |
| Pipe                |                    16.3 |
| Domain              |                    17.3 |
+---------------------+-------------------------+

--Sohil