perfmon2 minimal v2: introduction

The following patches implement a minimal perfmon2 subsystem which provides access to the hardware performance counters of modern processors.

This series supports ONLY per-thread counting of PMU events (no sampling) for AMD64 and Intel Core processors in both 32 and 64 bit modes.

This is version 2 of the series following the feedback from LKML. The series is against 2.6.26-rc6. Compared to previous version, this new series addresses the comments I received from Aran, Greg KH and others:

- some small patches were integrated with other bigger patches which they closely related to.

- sysfs code has been largely simplified

Please review.

First, a meta-complaint - Stephane, you're not the only person who does the following, and in fact, you're not the worst offender. I've seen a *lot* of even less clear patchsets on lkml - I actually only bothered to chase *this* one down because I happened to know that a co-worker might be interested.

Would it be asking too much to have a paragraph or so on *why* we want this patchset? Does it add functionality, or fix a bug, or reduce memory footprint for machines that don't need sampling, or? Give us a few sentences of why we should be excited this code is landing - otherwise it's likely to be just another new option that people will hit 'N' when they come across it in 'make oldconfig'. Tell us why we as testers/bleeding edgers want to hit 'Y'.

I finally grovelled around in patch 2, and found this in a comment in perfmon_attach.c:

+ * More information about perfmon available at:
+ * http://www.hpl.hp.com/research/linux/perfmon

However, said page merely says "The project is now completely hosted <here> http://perfmon2.sf.net/"

Eventually, after chasing a few links, I find myself at:

http://sourceforge.net/projects/perfmon2

"The goal of the project is to design and implement, on all major architectures, a standard Linux kernel interface, to access the hardware performance counters of modern processors. The project also developed a user library,libpfm, and a tool, pfmon."

Several *more* links and I find http://perfmon2.sourceforge.net/perfmon2-20080124.pdf

which finally tells me *something* about what this does and why I would want to be interested in it...

"Oh. So this *could* be used to tell Professor J Random that his code runs like a sedated sloth because things that should be cache-hot are actually cache-cold and we're spending 75% of our time waiting for cache line loads..."

*NOW* I'm interested. :)

Second comment - I haven't done a thorough check, but the Kconfig entry shows up near patch 7 or 8 of 19. Traditionally, that's done in the *last* patch so everything added is intentionally dead code, to make it easier to bisect through. It looks like possible trouble if bisecting between 2 other points (say 2.6.27-rc1 and 2.6.27-rc2 or whenever this merges) happens to land near patch 13 of 19.