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SubjectRe: [patch 09/11] x86/vdso: Simplify the invalid vclock case
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> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx@linutronix.de> 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.
>
> @Andy: Welcome to the wonderful world of TSC.
>

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.
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