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

Thomas Gleixner
Tue, 18 Sep 2018 07:52:26 -0000

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.

Thanks,

	tglx