Re: [RFC][PATCH 3/9] perf: export registerred pmus via sysfs

[Lin Ming <ming.m.lin@intel.com>]

On Tue, 2010-05-11 at 17:18 +0800, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > The new PMUs will use a dynamic range that starts at PERF_TYPE_MAX.
> 
> I dont think we should use a dynamic range of event sources - it's a 
> completely useless indirection that has no meaning to humans.
> 
> As far as machine interfaces go a much cleaner approach would be to allow an 
> open fd to a sysfs file to be passed to sys_perf_event_open() - this would 
> identify the event source. This needs a small extension of the ABI but we 
> could thus get rid of the 'type' enumeration altogether and express _all_ 
> event sources via fds to sysfs files.

I still don't understand this sys_fd -> pmu lookup, would you please
explain it more detail?

struct pmu {
	kobject kobj;
	...
};

What I can imagine is,

1. In userspace, sys_fd =
open("/sys/devices/system/cpu/event_source", ..), then sys_fd is passed
to sys_perf_event_open()

2. In kernel, sys_file = <find the sys file structure with sys_fd>

3. kobject = <retrieve the kobject from sys_file>

4. pmu = container_of(kobject, struct pmu, kobj)

If my understanding is correct, then step 3 above seems strange. It's
not the typical usage of sys file.

Thanks,
Lin Ming