Re: [RFC][PATCH] perf_events, x86: PEBS support

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-02-03 at 15:30 +0100, Stephane Eranian wrote:
> On Wed, Feb 3, 2010 at 3:19 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Wed, 2010-02-03 at 15:07 +0100, Stephane Eranian wrote:
> >> >> The only improvement that PEBS provides is that you get an IP and the
> >> >> machine state at retirement of an instruction that caused the event to
> >> >> increment. Thus, the IP points to the next dynamic instruction. The instruction
> >> >> is not the one that cause the P-th occurence of the event, if you set the
> >> >> period to P. It is at P+N, where N cannot be predicted and varies depending
> >> >> on the event and executed code. This introduces some bias in the samples..
> >> >
> >> > I'm not sure I follow, it records the next event after overflow, doesn't
> >> > that make it P+1?
> >> >
> >> That is not what I wrote. I did not say if records at P+1. I said it records
> >> at P+N, where N varies from sample to sample and cannot be predicted.
> >> N is expressed in the unit of the sampling event.
> >
> > OK, so I'm confused.
> >
> > The manual says it arms the PEBS assist on overflow, and the PEBS thing
> > will then record the next event. Which to me reads like P+1.
> >
> you are assuming arming is instantaneous.

Yes I was, ok that stinks.

If only they would reset the counter on overflow instead of on record,
that would solve quite a few issues I imagine.

Then add IP to the actual instruction and you've got yourself a useful
tool :-)