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

[Andy Lutomirski]

> On Sep 27, 2018, at 7:36 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Wed, 19 Sep 2018, Thomas Gleixner wrote:
>> On Tue, 18 Sep 2018, Andy Lutomirski wrote:
>>>> On Sep 18, 2018, at 3:46 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>>>> On Tue, 18 Sep 2018, Andy Lutomirski wrote:
>>>>> 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.
>>>> 
>>>> That would be probably the better solution as signed math would be
>>>> problematic when the resulting ns value becomes negative. As the delta is
>>>> really small, otherwise the TSC sync check would have caught it, the caller
>>>> should never be able to observe time going backwards.
>>>> 
>>>> I'll have a look into that. It needs some thought vs. the fractional part
>>>> of the base time, but it should be not rocket science to get that
>>>> correct. Famous last words...
>>>> 
>>> 
>>> Itâs also fiddly to tune. If you offset it too much, then the fancy
>>> divide-by-repeated-subtraction loop will hurt more than the comparison to
>>> last.
>> 
>> Not really. It's sufficient to offset it by at max. 1000 cycles or so. That
>> won't hurt the magic loop, but it will definitely cover that slight offset
>> case.
> 
> I got it working, but first of all the gain is close to 0.
> 
> There is this other subtle issue that we've seen TSCs slowly drifting apart
> which is caught by the TSC watchdog eventually, but if it exeeds the offset
> _before_ the watchdog triggers, we're back to square one.
> 
> So I rather stay on the safe side and just accept that we have to deal with
> that. Sigh.
> 
> 

Seems okay to me. Oh well.