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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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


> 
>  -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel