Re: [PATCH v2 00/11] KVM in-guest performance monitoring

[David Ahern <daahern@cisco.com>]

On 06/15/2011 07:22 AM, Avi Kivity wrote:
> On 06/15/2011 03:40 PM, David Ahern wrote:
>> On 06/15/2011 02:57 AM, Avi Kivity wrote:
>> >  Okay.  If you do anything interesting with it, please let us know.  I
>> >  only tested the watchdog, 'perf top', and 'perf stat'.
>> >
>>
>> For the following I was using the userspace command from latest
>> perf-core branch.
>>
>> cycles H/W event is not working for me, so perf-top did not do much
>> other than start.
> 
> Strange, IIRC it did for me.  I'll re-test.
> 
>> perf-stat -ddd shows a whole lot of 0's - which is interesting. It means
>> time enabled and time running are non-0, yet the counter value is 0.
>> cycles and instructions events also show as not counted
> 
> Most of those counters aren't supported by the emulated PMU. 

If the counter is unsupported perf-stat should show either <not counted>
or <not supported> (I submitted a patch for the latter which is in
perf-core branch). If you add -v to perf-stat you see the counters are
enabled and the time running is getting incremented. ie., something is
probably not implemented correctly.

> What does
> dmesg say about Perf?

[    0.050995] Performance Events: Nehalem events, core PMU driver.
[    0.051466] ... version:                1
[    0.052998] ... bit width:              40
[    0.053999] ... generic registers:      2
[    0.054998] ... value mask:             000000ffffffffff
[    0.055998] ... max period:             000000007fffffff
[    0.057997] ... fixed-purpose events:   0
[    0.058998] ... event mask:             0000000000000003

> 
>> Also, the numbers for branches and branch-misses just seem wrong
>> compared to the same command run in the host as well as running
>> perf-stat in the host on the vcpu thread running openssl (with the vcpu
>> pinned to a pcpu).
> 
> Could be due to the fact that the counter is running in host mode.  Will

You mean when perf is run in the guest?

> be fixed once the exclude_host/exclude_guest patch makes it in (and
> gains Intel support).
> 

How does exclude_{host,_guest} help if the guest-side counters are low
-- by orders of magnitude?

David