perf/core: Provide a kernel-internal interface to get to performance counters

There are reasons for kernel code to ask for, and use, performance
counters.
For example, in CPU freq governors this tends to be a good idea, but
there are other examples possible as well of course.

This patch adds the needed bits to do enable this functionality; they
have been tested in an experimental cpufreq driver that I'm working on,
and the changes are all that I needed to access counters properly.

[[email protected]: added pid to perf_event_create_kernel_counter so
that we can profile a particular task too

TODO: Have a better error reporting, don't just return NULL in fail
case.]

v2: Remove the wrong comment about the fact
    perf_event_create_kernel_counter must be called from a kernel
    thread.

Signed-off-by: Arjan van de Ven <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: "K.Prasad" <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Paul Mundt <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Avi Kivity <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

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