Re: Ktimer / -rt9 (+custom) monotonic_clock going backwards.

[Ingo Molnar <mingo@elte.hu>]

* 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.

	Ingo
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