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

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.

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?


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