linux-kernel - Re: [PATCH 6/7] x86/vdso: Add vDSO functions for user wait instructions

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Date:   Wed, 1 Aug 2018 10:55:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...hat.com>,
        H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 6/7] x86/vdso: Add vDSO functions for user wait
 instructions

On Tue, 31 Jul 2018, Andy Lutomirski wrote:
> On Tue, Jul 31, 2018 at 2:22 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> u64 start = __vdso_read_art_ns();
> >
> > Errm. No. You can't read ART. ART is only used by decives to which it is
> > distributed. You can only read TSC here and convert that to nsecs.
> 
> Bah.
> 
> But my point remains -- I think that the user (non-vDSO) code should
> think in nanoseconds, not TSC ticks.  That we have have a much better
> chance of getting migration right.

Agreed. And we should not create new interfaces for that. We already have
clock_gettime() which is the right thing to use.

> > A possible solution would be to use CLOCK_MOTONIC_RAW which is not affected
> > by NTP/PTP adjustments. But that still has the issue of TSC not being the
> > timekeeping clocksource. Bah, the whole TSC deadline mode sucks. I have no
> > idea what's wrong with simple down counters. They Just Work.
> 
> I think it's not totally crazy to declare UMWAIT on a system with a
> non-TSC clocksource to be unsupported.

Sure, the information is already available in the VDSO gtod data.

Thanks,

	tglx