Re: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression?

Michael S. Tsirkin
2013-11-04T20:31:09Z

On Mon, Nov 04, 2013 at 10:13:39PM +0200, Gleb Natapov wrote:
> On Mon, Nov 04, 2013 at 10:11:33PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Oct 31, 2013 at 09:48:08AM +0200, Gleb Natapov wrote:
> > > On Thu, Oct 31, 2013 at 02:21:46AM +0200, Michael S. Tsirkin wrote:
> > > > commit 8bf00a529967dafbbb210b377c38a15834d1e979:
> > > > " KVM: VMX: add support for switching of PERF_GLOBAL_CTRL " was
> > > > as far as I can tell supposed to bring about performance improvement
> > > > on hardware that supports it?
> > > No, it (and commits after it) supposed to fix a bug which it did.
> > >
> > > > Instead it seems to make the typical case (not running guest
> > > > under perf) a bit slower than it used to be.
> > > > the cost of VMexit goes up by about 50 cycles
> > > > on sandy bridge where the optimization in question
> > > > actually is activated.
> > > >
> > > You seams to be confused. 8bf00a529967dafbbb210 adds support for special
> > > PERF_GLOBAL_CTRL switching, but does not add code to switch anything,
> > > so the commit itself is a nop.
> >
> > It does add code to add_atomic_switch_msr.
> >
> So what? You do not read what I wrote.

It's simple: if I revert 8bf00a529967dafbbb210 then exit latency
is reduced.
You seem to tell me it should be a nop, but in practice it isn't.

> > > Next commit d7cd97964ba6d70c5
> > > uses add_atomic_switch_msr()/clear_atomic_switch_msr()
> > > to switch PERF_GLOBAL_CTRL, but it does not depend on
> > > VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL support which previous
> > > patch added, if the support is not there the switching will use
> > > another mechanism which is even slower. So MSR is switched no matter
> > > if PERF_GLOBAL_CTRL is enabled or not. If you saying that using
> > > VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is slower than using generic
> > > vmentry MSR switching then I pretty much doubt it since the only purpose
> > > of special VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is to be faster
> > > then general mechanism.
> > >
> > > --
> > > Gleb.
> > > --
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>
> --
> Gleb.