perf_counter: add some comments

Add a few comments because I was forgetting what field what for what
functionality.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090408130409.036984214@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -344,10 +344,12 @@ struct file;
 
 struct perf_mmap_data {
 	struct rcu_head			rcu_head;
-	int				nr_pages;
-	atomic_t			wakeup;
-	atomic_t			head;
-	atomic_t			events;
+	int				nr_pages;	/* nr of data pages  */
+
+	atomic_t			wakeup;		/* POLL_ for wakeups */
+	atomic_t			head;		/* write position    */
+	atomic_t			events;		/* event limit       */
+
 	struct perf_counter_mmap_page   *user_page;
 	void 				*data_pages[0];
 };