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Date:   Tue, 18 Sep 2018 10:52:58 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     John Stultz <john.stultz@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case
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References: <20180914125006.349747096@linutronix.de> <20180914125118.909646643@linutronix.de> <CALCETrXh70oFHfT0Z-JXcnAumhAgLqSea5UiPY3WXu=v2GcKTg@mail.gmail.com> <CALAqxLUv9Rd_eA7u9gZNFvTNrT1Z67RpHLozdeNC0W2yZm=Heg@mail.gmail.com>
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On Tue, 18 Sep 2018, Peter Zijlstra wrote:
> On Tue, Sep 18, 2018 at 09:52:26AM +0200, Thomas Gleixner wrote:
> > On Mon, 17 Sep 2018, John Stultz wrote:
> > > On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@kernel.org> wrote:
> > > > Also, I'm not entirely convinced that this "last" thing is needed at
> > > > all.  John, what's the scenario under which we need it?
> > > 
> > > So my memory is probably a bit foggy, but I recall that as we
> > > accelerated gettimeofday, we found that even on systems that claimed
> > > to have synced TSCs, they were actually just slightly out of sync.
> > > Enough that right after cycles_last had been updated, a read on
> > > another cpu could come in just behind cycles_last, resulting in a
> > > negative interval causing lots of havoc.
> > > 
> > > So the sanity check is needed to avoid that case.
> > 
> > Your memory serves you right. That's indeed observable on CPUs which
> > lack TSC_ADJUST.
> 
> But, if the gtod code can observe this, then why doesn't the code that
> checks the sync?

Because it depends where the involved CPUs are in the topology. The sync
code might just run on the same package an simply not see it. Yes, w/o
TSC_ADJUST the TSC sync code can just fail to see the havoc.

Thanks,

	tglx