perf_counter: per event wakeups

By request, provide a way to request a wakeup every 'n' events instead
of every page of output.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Orig-LKML-Reference: <20090402091319.323309784@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -145,7 +145,7 @@ struct perf_counter_hw_event {
 				__reserved_1   : 53;
 
 	__u32			extra_config_len;
-	__u32			__reserved_4;
+	__u32			wakeup_events;	/* wakeup every n events */
 
 	__u64			__reserved_2;
 	__u64			__reserved_3;
@@ -321,6 +321,7 @@ struct perf_mmap_data {
 	int				nr_pages;
 	atomic_t			wakeup;
 	atomic_t			head;
+	atomic_t			events;
 	struct perf_counter_mmap_page   *user_page;
 	void 				*data_pages[0];
 };

kernel/perf_counter.c

@@ -1760,7 +1760,15 @@ static void perf_output_copy(struct perf_output_handle *handle,
 
 static void perf_output_end(struct perf_output_handle *handle)
 {
-	if (handle->wakeup)
+	int wakeup_events = handle->counter->hw_event.wakeup_events;
+
+	if (wakeup_events) {
+		int events = atomic_inc_return(&handle->data->events);
+		if (events >= wakeup_events) {
+			atomic_sub(wakeup_events, &handle->data->events);
+			__perf_output_wakeup(handle);
+		}
+	} else if (handle->wakeup)
 		__perf_output_wakeup(handle);
 	rcu_read_unlock();
 }