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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 11/03/13 14:53, Konrad Rzeszutek Wilk wrote:
> On Mon, Mar 11, 2013 at 11:11:02AM +0000, George Dunlap wrote:
> > 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.
> And by default it won't work as for Intel you need these flags:
>
> cpuid=['0xa:eax=0x07300403,ebx=0x00000004,ecx=0x00000000,edx=0x00000603' ]
>
> What we get right now when booting PVHVM under Intel is:
>
> [    0.160989] Performance Events: unsupported p6 CPU model 45 no PMU driver, 
> software events only.
> [    0.168098] NMI watchdog disabled (cpu0): hardware events not enabled
>
> Unless said above CPUID flag is provided.
> > 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?
> The other thing is that there is an Xen watchdog. The one that Jan Beulich
> wrote which should also work under PVHVM:
>
> drivers/watchdog/xen_wdt.c
But my main question is: If the Linux perf system successfully detects a 
vpmu, will it use the Xen watchdog, or will it try to use the vpmu?  Do 
we need to do anything to make sure that when running under Xen, Linux 
will *not* try to use the vpmu for the watchdog?
 -George

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