> 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 we’d have to go back several hundred ms before we cross zero. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
