Re: [RFC][PATCH 3/5] perf: Add hrtimer code for PMI-less hardware counters

[Matt Fleming <matt@console-pimps.org>]

On Mon, Aug 30, 2010 at 02:55:02PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-08-30 at 13:13 +0100, Matt Fleming wrote:
> > Currently, it's impossible to periodically sample hardware counters that
> > lack performance monitoring interrupt (PMI) support. In order to sample
> > these counters we can create an event group which is backed by a
> > hrtimer, thereby simulating a PMI.
> > 
> > When the hrtimer goes off we sample the values in the hardware
> > counters. Because we obviously can't rely on the hrtimer going off at
> > exactly the sample period (say, every 1000 cache misses) the values need
> > to be weighted by the variable period since the last hrtimer went
> > off. This is so that we can compensate for the variability in hrtimer
> > period.
> > 
> > If perf record tries to create a sampling counter and the hardware
> > doesn't support it then we'll fall back to creating an event group with
> > a hrtimer.
> 
> 
> Why is this changing kernel code?
> 
> You can create those groups in userspace..

I'm lost. Is it possible to do this patch entirely in userspace? How do
we periodically sample the counters if it's not being done in the
kernel?