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Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1809180948570.3558@nanos.tec.linutronix.de>
Date:   Tue, 18 Sep 2018 07:01:12 -0700
Cc:     John Stultz <john.stultz@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.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>
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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> <alpine.DEB.2.21.1809180948570.3558@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
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> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Mon, 17 Sep 2018, John Stultz wrote:
>>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@kernel.org> wrot=
e:
>>> Also, I'm not entirely convinced that this "last" thing is needed at
>>> all.  John, what's the scenario under which we need it?
>>=20
>> 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.
>>=20
>> So the sanity check is needed to avoid that case.
>=20
> Your memory serves you right. That's indeed observable on CPUs which
> lack TSC_ADJUST.
>=20
> @Andy: Welcome to the wonderful world of TSC.
>=20

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 c=
ould offset everything so that we=E2=80=99d have to go back several hundred m=
s before we cross zero.=