Re: [Xen-devel] [PATCH] Always save/restore performance counters when HVM guest switching VCPU

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 08/03/13 15:11, Boris Ostrovsky wrote:
> ----- george.dunlap@xxxxxxxxxxxxx wrote:
>
> > On 08/03/13 14:50, Boris Ostrovsky wrote:
> > > ----- JBeulich@xxxxxxxx wrote:
> > >
> > > > > > > On 04.03.13 at 13:42, George Dunlap
> > <George.Dunlap@xxxxxxxxxxxxx>
> > > > wrote:
> > > > > On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@xxxxxxx>
> > > > wrote:
> > > > > > From: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > > > > >
> > > > > > Currently, the performance counter registers are saved/restores
> > > > > > when the HVM guest switchs VCPUs only if they are running.
> > > > > > However, PERF has one check where it writes the MSR and read
> > back
> > > > > > the value to check if the MSR is working.  This has shown to
> > fails
> > > > > > the check if the VCPU is moved in between rdmsr and wrmsr and
> > > > > > resulting in the values are different.
> > > > > Many moons ago (circa 2005) when I used performance counters, I
> > > > found
> > > > > that adding them to the save/restore path added a non-neligible
> > > > > overhead -- something like 5% slow-down.  Do you have any reason
> > to
> > > > > believe this is no longer the case?  Have you done any benchmarks
> > > > > before and after?
> > > I was doing some VPMU tracing a couple of weeks ago and by looking
> > at
> > > trace timestamps I think I saw about 4000 cycles on VPMU save and
> > > ~9000 cycles on restore. Don't remember what it was percentage-wise
> > of
> > > a whole context switch.
> > >
> > > This was on Intel.
> > That's a really hefty expense to make all users pay on every context
> > switch, on behalf of a random check in a piece of software that only a
> > handful of people are going to be actually using.
> I believe Linux uses perf infrastructure to implement the watchdog.
Hmm -- well if it is the case that adding performance counters to the 
vcpu context switch path will add a measurable overhead, then we 
probably don't want them enabled for typical guests anyway.  If people 
are actually using the performance counters to measure performance, that 
makes sense; but for watchdogs it seems like Xen should be able to 
provide something that is useful for a watchdog without the extra 
overhead of saving and restoring performance counters.
Konrad, any thoughts?

 -George

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