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

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Date:   Thu, 23 Sep 2021 14:19:05 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Mehta, Sohil" <sohil.mehta@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "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

On Tue, Sep 14, 2021 at 07:03:36PM +0000, Mehta, Sohil wrote:
> 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 |
> +---------------------+-------------------------+

Relative is just that, "relative".  If the real values are extremely
tiny, then relative is just "this goes a tiny tiny bit faster than what
you have today in eventfd", right?

So how about "absolute"?  What are we talking here?

And this is really only for the "one userspace task waking up another
userspace task" policies.  What real workload can actually use this?

thanks,

greg k-h