[Xen-devel] RE: pv_ops (2.6.31) and CONFIG_NO_HZ

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> On 11/02/09 13:45, Dan Magenheimer wrote:
> > Has anyone looked at using CONFIG_NO_HZ with an upstream
> > pv_ops kernel?  This seems like an intelligent combination...
> > since pvclock is a good timesource, it seems like there
> > should be no reason to have a regular tick, and one
> > could pack more idle domains into one physical machine.  
> 
> Yes, I've always recommeded NO_HZ.  The pvops kernel disables 
> the 100Hz
> periodic timer and always uses the high-precision timers for its
> clockevents.  The core kernel still asks for periodic interrupts on
> occasion, so it isn't completely tickless, but that's not really
> something we should solve at the Xen level.

OK thanks.  I'll look into this a bit further for HVMs.

> > I tried enabling CONFIG_NO_HZ from an otherwise working .config
> > file for 2.6.31 and the new kernel (running as a PV domU) 
> panics early
> > in boot ("Kernel panic -- not syncing: Attempted to kill init!"
> > with a call trace of syscall_call->sys_exit_group->do_group_exit->
> > do_exit->panic).  So I'm wondering if I did something stupid
> > or it is known to not work.

> No, that's strange.  However, someone else reported something similar
> which turns out to be triggered by the pvclock vsyscall stuff.   I
> haven't worked out what's failing yet.  (Or why NO_HZ would depend on
> it, though it might be related to the highres timers options.)

This was a kernel.org 2.6.31 kernel, not from your git tree
so no pvclock vsyscall stuff would be in it.
(Sorry I guess the subject was confusing.)

> Can you post the full crash message?

No, most of it scrolls away in the VNC window.  But
since I now know it is supposed to work, I'll look into it
further.

Thanks,
Dan

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