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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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

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