Re: [Xen-devel] [PATCH v1 00/13] x86/PMU: Xen PMU PV support

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 11/09/13 19:22, Boris Ostrovsky wrote:
> On 09/11/2013 01:01 PM, George Dunlap wrote:
> > On 10/09/13 16:47, Boris Ostrovsky wrote:
> > > On 09/10/2013 11:34 AM, Jan Beulich wrote:
> > > > > > > On 10.09.13 at 17:20, Boris Ostrovsky 
> > > > > > > <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > > > 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.
> > > > With that I assume this is an RFC rather than full-fledged submission?
> > > I was thinking that this would be something like stage 1 
> > > implementation (and
> > > probably should have mentioned this in the cover letter).
> > >
> > > For this stage I wanted to confine all changes on Linux side to xen 
> > > subtrees.
> > > Properly addressing the above limitation would likely require 
> > > changes in non-xen
> > > sources (change in perf file format, remote MSR access etc.).
> > I think having the vpmu stuff for PV guests is a great idea, and from 
> > a quick skim through I don't have any problems with the general 
> > approach.  (Obviously some more detailed review will be needed.)
> > However, I'm not a fan of this method of collecting perf stuff for 
> > Xen and other VMs together in the cpu buffers for dom0.  I think it's 
> > ugly, fragile, and non-scalable, and I would prefer to see if we 
> > could implement the same feature (allowing perf to analyze Xen and 
> > other vcpus) some other way.  And I would rather not use it as a 
> > "stage 1", for fear that it would become entrenched.
> I can see how collecting samples for other domains may be questionable 
> now (DOM0_PRIV mode) since at this stage there is no way to 
> distinguish between samples for non-priviledged domains.
> But why do you think that getting data for both dom0 and Xen is 
> problematic? Someone has to process Xen's samples and who would do 
> this if not dom0? We could store samples in separate files (e.g. 
> perf.data.dom0 and perf.data.xen) but that's toolstack's job.
It's not so much about dom0 collecting the samples and passing them on 
to the analysis tools; this is already what xenalyze does, in essence.  
It's about the requirement of having the dom0 vcpus pinned 1-1 to 
physical cpus: both limiting the flexibility for scheduling, and 
limiting the configuration flexibility wrt having dom0 vcpus < pcpus.  
That is what seems an ugly hack to me -- having dom0 sort of try to do 
something that requires hypervisor-level privileges and making a bit of 
a mess of it.
I'm unfortunately not familiar enough with the perf system to know 
exactly what it is that Linux needs to do (why, for example, you think 
it would need remote MSR access if dom0 weren't pinned), and how hard 
would be for Xen just to do that work, and provide an "adapter" that 
would translate Xen-specific stuff into something perf could consume.  
Would it be possible, for example, for dom0 to specify what needed to be 
collected, for Xen to generate the samples in a Xen-specific format, and 
then have something in dom0 that would separate the samples into one 
file per domain that look similar enough to a trace file that the perf 
system could consume it?
 -George

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