SystemTap vs. FC5 Xen kernels (was: 03-23-2006 Meeting minutes)

[William Cohen]

Stone, Joshua I wrote:
> William Cohen wrote:
> 
>>Sometimes the options are turned off on purpose. Do the Xen kernels
>>build with "CONFIG_KPROBES=y"?
> 
> 
> yes
>  
> 
>>If kernels are built, do the resulting kernels actually have kprobe
>>support? It use to be that xen kernels copy and pasted code from other
>>arches. What additional patches are needed to bring the kprobe support
>>in the xen kernel in line with the various architectures?
> 
> 
> 
> In order to get the debug info working, I removed the change to
> lib/Kconfig.debug (mentioned in my other email), but left the change in
> arch/x86_64/kernel/entry-xen.S.  I suspect that more will need to be
> done here before CONFIG_DEBUG_INFO really works completely, as there's
> probably some reason it was disabled, but this is good enough for
> testing SystemTap.
> 
> On both xen0 and xenU, all pass-5 tests succeed, except those that use
> timer.profile.  (systemtap.base/timers.stp and
> systemtap.maps/pmap_agg_overflow.stp).  It seems that the timer.profile
> thinks it is registered successfully, but the callback is never
> triggered.  I will investigate this further...
> 
> 
> Josh

Josh,

The pass-5 tests working on xen is very good news.

On the earlier s390 kernels the regular timer interrupt was eliminated. 
This was done for efficiency. The s390 has had virtualization for years. 
Having the Linux kernels do the timer interrupt was unwanted overhead. I 
wonder if the xen kernel is doing the same, only have timer interrupt 
when something really needs to get run at a certain time.

http://lwn.net/Articles/138969/

Do we need variations on the timer probe,  wall clock time and virtual time?

-Will