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

[Corey Ashford]

On 1/20/2010 11:21 PM, Ingo Molnar wrote:
> * Corey Ashford<cjashfor@xxxxxxxxxxxxxxxxxx>  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.

This is an intriguing idea; I like the idea of generalizing all of this info 
into one structure.

So you think that this structure should contain event info as well?  If these 
structures are created by the kernel, I think that would necessitate placing 
large event tables into the kernel, which is something I think we'd prefer to 
avoid because of the amount of memory it would take.  Keep in mind that we need 
not only event names, but event descriptions, encodings, attributes (e.g. unit 
masks), attribute descriptions, etc.  I suppose the kernel could read a file 
from the file system, and then add this info to the tree, but that just seems 
bad.  Are there existing places in the kernel where it reads a user space file 
to create a user space pseudo filesystem?

I think keeping event naming in user space, and PMU naming in kernel space might 
be a better idea: the kernel exposes the available PMUs to user space via some 
structure, and a user space library tries to recognize the exposed PMUs and 
provide event lists and other needed info.  The perf tool would use this library 
to be able to list available events to users.

--
Regards,

- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR
503-578-3507
cjashfor@xxxxxxxxxx

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