Messages in this thread | | | Date | Thu, 20 Oct 2005 10:01:07 +0200 | From | Ingo Molnar <> | Subject | Re: Ktimer / -rt9 (+custom) monotonic_clock going backwards. |
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* Steven Rostedt <rostedt@goodmis.org> wrote:
> > i could imagine the following hardware effects to cause time warps: > > > > - the TSC is in fact the 'read counter' method of the local APIC timer > > hardware. So there can be interactions in theory: programming the APIC > > timer could impact the TSC and vice versa. There have been CPU > > erratums in this area in the past. > > Could this cause a 2 second drop backwards?
i dont think so.
> > - the TSC itself could have short, temporary warps. I had a box that > > showed such effects. > > Can this be a 2 second warp?
the ones i saw were in the 1000-cycles range.
> My older code first used jiffies as a timer, then I switched to TSC > and then to APIC timer, and then finally ktimer. ktimer was the first > to show a backwards get_time.
another thing: the monotonicity check is only in get_ktime_mono(), while there are other places where a monotonic clock is used, which this check might miss.
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