Re: [RFC] perf_events: support for uncore a.k.a. nest units

[Ingo Molnar <mingo@elte.hu>]

* Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:

> I really think we need some sort of data structure which is passed from the 
> kernel to user space to represent the topology of the system, and give 
> useful information to be able to identify each PMU node.  Whether this is 
> done with a sysfs-style tree, a table in a file, XML, etc... it doesn't 
> really matter much, but it needs to be something that can be parsed 
> relatively easily and *contains just enough information* for the user to be 
> able to correctly choose PMUs, and for the kernel to be able to relate that 
> back to actual PMU hardware.

The right way would be to extend the current event description under 
/debug/tracing/events with hardware descriptors and (maybe) to formalise this 
into a separate /proc/events/ or into a separate filesystem.

The advantage of this is that in the grand scheme of things we _really_ dont 
want to limit performance events to 'hardware' hierarchies, or to 
devices/sysfs, some existing /proc scheme, or any other arbitrary (and 
fundamentally limiting) object enumeration.

We want a unified, logical enumeration of all events and objects that we care 
about from a performance monitoring and analysis point of view, shaped for the 
purpose of and parsed by perf user-space. And since the current event 
descriptors are already rather rich as they enumerate all sorts of things: 

 - tracepoints
 - hw-breakpoints
 - dynamic probes
 
etc., and are well used by tooling we should expand those with real hardware 
structure.

Thanks,

	Ingo