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



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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