RE: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors

["Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx>]

> Yes, I can regularly generate messages by switching the governor
> from powersave (minimum frequency) to performance (maximum
> frequency).
> 
> > How far is time stepping backwards?
> > ns, us, or ms?
> 
> Rolf's message is typical:
> > Jan 19 00:11:09 server kernel: printk: 14 messages suppressed.
> > Jan 19 00:11:09 server kernel: Timer ISR/0: Time went backwards:
> > delta=-123862858 delta_cpu=-123862858 shadow=
> > 2993571302800 off=705014593 processed=2994400167600
> cpu_processed=2994400167600
>
> It seems to happen most reliably when there's a large jump in
> frequency (2.6 GHz to 800 MHz or vice versa) but I've seen it
> happen for smaller switches.  Usually there's 1 large backward
> movement, and then 2-4 smaller ones.  It doesn't always happen,
> especially if there are multiple shifts in a short period (ie,
> constant changes on the ondemand governor) and it seems to
> happen more consistently when increasing frequency.

Increasing the frequency requires (1) instructing the regulator to
increasing the voltage, (2) waiting for it to settle, (3) increasing the
multipler, (4) wait for the PLL to settle, (5) unpause the CPU.

Which of these steps are under software control on PowerNow?  

Since we probably don't know how long we had to wait between when the
multipler was increased and when the CPU was un-paused, it will be
necessary to recalibrate the CPU to the global time source (PIT/HPET).

Ian







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