Messages in this thread |  | | Date | Tue, 18 Sep 2018 09:52:26 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case |
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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.
Thanks,
tglx
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