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

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Message-ID: <alpine.DEB.2.21.1809121616040.9957@nanos.tec.linutronix.de>
Date:   Wed, 12 Sep 2018 16:17:26 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Florian Weimer <fweimer@...hat.com>
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 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.

But we surely could provide separate entry points as an extra to avoid a
bunch of conditionals.

Thanks,

	tglx