Re: [patch 0/3] [Announcement] Performance Counters for Linux

[Paul Mundt <lethal@linux-sh.org>]

On Fri, Dec 05, 2008 at 12:08:14PM -0800, David Miller wrote:
> From: Andi Kleen <andi@firstfloor.org>
> Date: Fri, 05 Dec 2008 13:39:43 +0100
> 
> > Given these are more obscure features, but not being able to fit
> > them easily into your model from the start isn't a very promising sign
> > for the long term extensibility of the design.
> 
> Another thing I'm interested in is if this new stuff will work with
> performance counters that lack an overflow interrupt.
> 
> We have several chips that are like this, and perfmon supported that
> on the kernel side, and also provided overflow emulation for such
> hardware in userspace (where such complexity belongs).

There doesn't seem to have been any reply to this point unfortunately,
and it is something I am also wondering about.

The sh perf counters were not designed with overflowing in mind, they are
split in to a pair of 48-bit or 64-bit counters that simply keep running.
Any write simply clears the value and the counter starts over. They are
simply counters only, and generate no events whatsoever.

Oprofile has been a pretty bad fit for them, and while I'm slightly more
optimistic about perfmon, I'm rather less enthusiastic about yet another
peformance counter implementation that I am unable to make any use of.