linux-kernel - Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK

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Date:   Wed, 12 Sep 2018 10:11:42 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO



> On Sep 12, 2018, at 7:29 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Wed, 12 Sep 2018, Florian Weimer wrote:
>>> On 09/12/2018 04:17 PM, Thomas Gleixner wrote:
>>>> On Wed, 12 Sep 2018, Florian Weimer wrote:
>>>> Does this mean glibc can keep using a single vDSO entrypoint, the one we
>>>> have today?
>>> 
>>> We have no intention to change that.
>> 
>> Okay, I was wondering because Andy seemed to have proposed just that.
>> 
>>> But we surely could provide separate entry points as an extra to avoid a
>>> bunch of conditionals.
>> 
>> We could adjust to that, but the benefit would be long-term because it's an
>> ABI change for glibc, and they tend to take a long time to propagate.
>> 
>> But I must say that clock_gettime is an odd place to start.  I would have
>> expected any of the type-polymorphic multiplexer interfaces (fcntl, ioctl,
>> ptrace, futex) to be a more natural starting point. 8-)
> 
> Well, the starting point of this was to provide clock_tai support in the
> vdso. clock_gettime() in the vdso vs. the real syscall is a factor of 10 in
> speed. clock_gettime() is a pretty hot function in some workloads.
> 
> Andy then noticed that some conditionals could be avoided entirely by using
> a different entry point and offered one along with a 10% speedup. We don't
> have to go there, we can.
> 
> The multiplexer interfaces need much more surgery and talking about futex,
> we'd need to sit down with quite some people and identify the things they
> actually care about before just splitting it up and keeping the existing
> overloaded trainwreck the same.
> 

There’s also the issue of how much the speedup matters. For futex, maybe a better interface saves 3ns, but a futex syscall is hundreds of ns. clock_gettime() is called at high frequency and can be ~25ns. Saving a few ns is a bigger deal.