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

On 03/11/2013 10:59 AM, George Dunlap wrote:
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
On Fri, Mar 1, 2013 at 8:49 PM,  <suravee.suthikulpanit@xxxxxxx>
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
the value to check if the MSR is working.  This has shown to
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
that adding them to the save/restore path added a non-neligible
overhead -- something like 5% slow-down.  Do you have any reason
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
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
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

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:


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?

It looks to me that both watchdogs are running. Perf's counter 0 (which is what watchdog uses) is definitely enabled.


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