linux-kernel - Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

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Message-ID: <alpine.DEB.2.21.1809271630470.8118@nanos.tec.linutronix.de>
Date:   Thu, 27 Sep 2018 16:36:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

On Wed, 19 Sep 2018, Thomas Gleixner wrote:
> On Tue, 18 Sep 2018, Andy Lutomirski wrote:
> > > On Sep 18, 2018, at 3:46 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > > On Tue, 18 Sep 2018, Andy Lutomirski wrote:
> > >> Do we do better if we use signed arithmetic for the whole calculation?
> > >> Then a small backwards movement would result in a small backwards result.
> > >> Or we could offset everything so that we’d have to go back several
> > >> hundred ms before we cross zero.
> > > 
> > > That would be probably the better solution as signed math would be
> > > problematic when the resulting ns value becomes negative. As the delta is
> > > really small, otherwise the TSC sync check would have caught it, the caller
> > > should never be able to observe time going backwards.
> > > 
> > > I'll have a look into that. It needs some thought vs. the fractional part
> > > of the base time, but it should be not rocket science to get that
> > > correct. Famous last words...
> > > 
> > 
> > It’s also fiddly to tune. If you offset it too much, then the fancy
> > divide-by-repeated-subtraction loop will hurt more than the comparison to
> > last.
> 
> Not really. It's sufficient to offset it by at max. 1000 cycles or so. That
> won't hurt the magic loop, but it will definitely cover that slight offset
> case.

I got it working, but first of all the gain is close to 0.

There is this other subtle issue that we've seen TSCs slowly drifting apart
which is caught by the TSC watchdog eventually, but if it exeeds the offset
_before_ the watchdog triggers, we're back to square one.

So I rather stay on the safe side and just accept that we have to deal with
that. Sigh.

Thanks,

	tglx