Re: [PATCH v2 0/6] xen/PMU: PMU support for Xen PV guests

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 06/10/2014 09:57 AM, David Vrabel wrote:
> On 06/06/14 18:44, Boris Ostrovsky wrote:
>> (This is the second version of the series. The first one was posted long
>> time ago and there are too many changes to list.)
>>
>> This is the Linux side of Xen PMU support for PV(H) guests, including
>> dom0. Only kernel changes are here, toolstack patch will be provided
>> separately.
>>
>> Here is description from the hypervisor patch submission that applies
>> to this series as well:
>>
>> This version has following limitations:
>> * For accurate profiling of dom0/Xen dom0 VCPUs should be pinned.
>> * Hypervisor code is only profiled on processors that have running dom0 VCPUs
>> on them.
>> * No backtrace support.
> I think there needs to be a plausible plan for how to resolve these
> limitations.

Come think of it, pinning may not really be a requirement. I suspect a 
migrating vcpu will have effect similar to what happens when a process 
migrates to a different processor in bare-metal environment. We may need 
to add PCPU to report (and there is a reserved u32 filed in 
perf_sample_data that I may be able to use).

Backtraces should not be too big a deal (in theory). I in fact had a 
prototype at some point.

The second bullet is the most challenging, mostly because it may require 
changes to perf kernel code. I had a couple of ideas but they are rather 
half-baked.  One solution could be to profile only N PCPUs at a time 
(where N is the number of dom0's VCPUs) and rotate among all PCPUs.


-boris