Re: [RFC][PATCH] Hypervisor profiling using GCOV

[George Dunlap <george.dunlap@eu.citrix.com>]

Tej wrote:
>> Does it get us significantly more than the existing xentrace and xenoprofile
>> tools? Especially the latter, which already does program-counter tracing and
>> generation of stats from that, via the oprofile tool.
>>     
>
> thanks for our feedback
> i agree xenoprofile is advanced proffiling tool in xen since ages, but
> GCOV could be useful tool to naive tester/programmer on XEN who really
> don't understand xenoprofile stats....
>   
Don't undersell gcov; its purpose is slightly different from either 
xentrace or xenoprofile.  Xentrace can be used to get information about 
what paths or reasons caused vmexits (as well as just seeing specific 
patterns that happen).  xenoprofile is a relatively low-overhead way of 
just profiling (which unfortuantely doesn't work properly in 32-on-64 
mode ATM).   gcov's main purpose is to tell you code coverage; profiling 
is just a side-effect.

I looked at the LTP page about lcov, and it looks like it was pretty 
useful for them.  It has graphical output with the number of times / 
percentage a given path was taken.  It probably is worth porting to Xen 
for the same reason -- to see how well given paths in Xen actually get 
exercised.
> e.g If i start with any hypervisor subsystem (scheduler), i will run
> xm test suite for scheduler and see  what all code is getting affected
> over a  period of time and proceed with it...
>   
It sounds like the main problem with xenoprofile is that it's hard to 
set up and use ATM: it could use some attention to the code, and some 
well-worded HOWTOs.  The fact that 32-on-64 doesn't work properly 
(truncates the long EIPs) doesn't help. :-)

So I think that it probably would be useful.  Unfortunately, I don't 
have time in the near future to look at either of these (new "xcov" 
functionality, or fixing xenoprofile).  Gianluca's been doing some 
interesting work with testing.  I'll ask him if he's interested in 
looking at it.

Thanks for your work though, Tej!

 -George