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

[Andy Lutomirski]

> On Sep 18, 2018, at 12:52 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Mon, 17 Sep 2018, John Stultz wrote:
>>> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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 weâd have to go back several hundred ms before we cross zero.