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

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Date:   Wed, 12 Sep 2018 16:20:15 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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 09/12/2018 04:17 PM, Thomas Gleixner wrote:
> On Wed, 12 Sep 2018, Florian Weimer wrote:
>> On 09/09/2018 10:05 PM, Thomas Gleixner wrote:
>>> See the patch below. It's integrating TAI without slowing down everything
>>> and it definitely does not result in indirect calls.
>>>
>>> On a HSW it slows down clock_gettime() by ~0.5ns. On a SKL I get a speedup
>>> by ~0.5ns. On a AMD Epyc server it's 1.2ns speedup. So it somehow depends
>>> on the uarch and I also observed compiler version dependend variations.
>>
>> 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-)

Thanks,
Florian