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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: pv_ops (2.6.31) and CONFIG_NO_HZ
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 02 Nov 2009 14:42:13 -0800
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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.

> 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.)

Can you post the full crash message?


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