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

["Stone, Joshua I" <joshua.i.stone@intel.com>]

William Cohen wrote:
> 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/

One of the comments after this article specifically mentions that
"similar patches" are in xen as well.  I find it odd that the
timer.ms/jiffies probes still work fine, but timer.profile gets no
probes at all.  It may just be that the dynamic-ticks patch changes the
timer path, so the softIRQ timers still happen, but the hard IRQ timers
used for timer.profile are sidestepped.

It may turn out that timer.profile will have to be emulated using PMU
sampling on clockticks...

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

Possibly - do you have a suggestion for how to accomplish this?  The
current timer probes are using the {add,mod,del}_timer interface, which
is based purely on jiffies.


Josh