[patch 09/11] x86/vdso: Simplify the invalid vclock case

Andy Lutomirski
Tue, 18 Sep 2018 14:01:12 -0000

> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx at linutronix.de>
wrote:
>
>> On Mon, 17 Sep 2018, John Stultz wrote:
>>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto at
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.