On Tue, 18 Sep 2018, Peter Zijlstra wrote: > On Tue, Sep 18, 2018 at 09:52:26AM +0200, Thomas Gleixner 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. > > But, if the gtod code can observe this, then why doesn't the code that > checks the sync? Because it depends where the involved CPUs are in the topology. The sync code might just run on the same package an simply not see it. Yes, w/o TSC_ADJUST the TSC sync code can just fail to see the havoc. Thanks, tglx _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
