Re: [Xen-devel] Questions about Using Perf at Dom0

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 08/29/2016 05:08 PM, Sanghyun Hong wrote:
>> Yes, this will allow the hypervisor to collect samples from multiple
>> guests. However, the tool (perf) probably won't be able to properly
>> process these samples. But you can try.
>
> I understand, thus, I applied the patches and set
> the /pmu_mode/ to *all*. However, I’m really curious what you mean by
> the tool (*perf*) probably won’t be able to properly process these
> samples. Is there any things I have to have in mind while collecting
> the counters, or will it have incorrect values of the counters?

The hypervisor will provide dom0 with a raw sample (guest's, dom0's or
hypervisor's) and then it's the job of dom0 kernel to properly tag and
format it and make it available to the userland (i.e. perf itself). The
tool will then look at the tag and display the event as belonging to
host (dom0, really) or guest. This is supported for KVM, with 'perf kvm'
commands.

So if you can run perf kvm commands then you may be able to
differentiate dom0's and guest's samples (you will also see hypervisor
samples but they won't get resolved to symbols). I am just not sure per
kvm will run on a non-KVM host. I had a private copy where it did but
this was based on a fairly old version of Linux.

(Also, stack profiling is not supported at all).

>
>> You will want to run dom0 on all physical processors (i.e. no
>> dom0_max_vcpus boot option) and pin all VCPUs for both dom0 and the
>> guest.
>
> Sure, thanks! My machine has four physical cores, and I’ve set
> the /dom0_max_vcpus=4/. I think it will have the same effect in
> removing the /dom0_max_vcpus/ option.

Yes.

-boris

>
> Best,
> Sanghyun.
>
>
>> On Aug 29, 2016, at 4:59 PM, Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx <mailto:boris.ostrovsky@xxxxxxxxxx>> wrote:
>>
>> On 08/29/2016 02:42 PM, Sanghyun Hong wrote:
>>> Hi Boris,
>>>
>>> I’ve found the
>>> documentations(https://github.com/torvalds/linux/blob/master/Documentation/ABI/testing/sysfs-hypervisor-pmu)
>>> in the kernel source code, and it seems like if we change the mode
>>> from *self* to *all* then we can collect counters for all the domains,
>>> right?
>>
>> Yes, this will allow the hypervisor to collect samples from multiple
>> guests. However, the tool (perf) probably won't be able to properly
>> process these samples. But you can try.
>>
>> You will want to run dom0 on all physical processors (i.e. no
>> dom0_max_vcpus boot option) and pin all VCPUs for both dom0 and the
>> guest.
>>
>>
>> -boris
>>
>>>
>>> All the best,
>>> Sanghyun.
>>>
>>>
>>>> On Aug 29, 2016, at 12:36 PM, Boris Ostrovsky
>>>> <boris.ostrovsky@xxxxxxxxxx <mailto:boris.ostrovsky@xxxxxxxxxx>
>>>> <mailto:boris.ostrovsky@xxxxxxxxxx>> wrote:
>>>>
>>>> On 08/29/2016 09:18 AM, Sanghyun Hong wrote:
>>>>> Dear Xen-Devel Community:
>>>>>
>>>>> I’m a grad student working on measuring performance counters at the
>>>>> Xen domains. I read this
>>>>> thread(https://wiki.xenproject.org/wiki/Xen_Profiling:_oprofile_and_perf)
>>>>> in
>>>>> web, and it says using Linux perf command will let us collecting
>>>>> performance counters in both dom0 and domU. Does it mean that we can
>>>>> collect both of them at once if we run perf command on the dom0? (If
>>>>> not, does it mean we can collect counters for each domain separately
>>>>> once we run the perf command in each domain?
>>>>
>>>> Profiling both guest and dom0 (and the hypervisor) requires changes to
>>>> perf and those are not there yet.
>>>>
>>>> But you can run perf in each guest (including dom0) separately. Make
>>>> sure you have vpmu=true boot option.
>>>>
>>>> -boris
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>



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