Re: [PATCH 5/5] perf, x86: Prefer RDPMC over RDMSR for reading counters

Peter Zijlstra
2012-06-06T14:21:24Z

On Wed, 2012-06-06 at 07:16 -0700, Andi Kleen wrote:
> On Wed, Jun 06, 2012 at 12:46:19PM +0200, Peter Zijlstra wrote:
> > On Tue, 2012-06-05 at 17:56 -0700, Andi Kleen wrote:
> > > From: Andi Kleen <ak@linux.intel.com>
> > >
> > > RDPMC is much faster than RDMSR for reading performance counters,
> > > since it's not serializing. Use it if possible in the perf handler.
> > >
> > > Only tested on Sandy Bridge, so I only enabled it there so far.
> >
> > That's just stupid.. I took Vince's patch from a while back.
>
> What do you mean? It's significantly faster to read the counters this
> way, because it avoids serialization and other overhead.

What I'm saying is you only enabling it for snb and being too lazy to
test anything else. Nor do I think its worth the conditional, all chips
we have PMU support for have rdpmc instructions.

> Vince's patch only enabled it for user space I believe, This is for lowering
> the kernel PMI handler overhead.

No, his patch did the kernel thing. Furthermore he actually tested it on
a bunch of machines.