Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default

Andi Kleen
2005-11-29T21:52:10Z

On Tue, Nov 29, 2005 at 01:43:11PM -0800, Nicholas Miell wrote:
> On Tue, 2005-11-29 at 19:13 +0100, Andi Kleen wrote:
> > > Where did you see that PMC0 (PERSEL0/PERFCTR0) can only be programmed
> > > to count cpu cycles (i.e. cpu_clk_unhalted)? As far as I can tell from
> > > the documentation, the 4 counters are symetrical and can measure
> > > any event that the processor offers.
> >
> > Linux NMI watchdog does that.
> >
> > All other perfctr users are supposed to keep their fingers away
> > from the watchdog (it looks like oprofile doesn't but not for much
> > longer ...)
>
> Why? Hardcoding PMC 0 to be a cycle counter seems to be a waste of a
> perfectly usable performance counter. What if I want to profile four
> things, none of them requiring a cycle count?

You won't then anymore. Providing a full replacement for RDTSC is
more important.

-Andi