perf_counter: add ioctl(PERF_COUNTER_IOC_RESET)

Provide a way to reset an existing counter - this eases PAPI
libraries around perfcounters.

Similar to read() it doesn't collapse pending child counters.

[ Impact: new perfcounter fd ioctl method to reset counters ]
Suggested-by: NCorey Ashford <cjashfor@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090505155437.022272933@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -160,6 +160,7 @@ struct perf_counter_hw_event {
 #define PERF_COUNTER_IOC_ENABLE		_IO ('$', 0)
 #define PERF_COUNTER_IOC_DISABLE	_IO ('$', 1)
 #define PERF_COUNTER_IOC_REFRESH	_IOW('$', 2, u32)
+#define PERF_COUNTER_IOC_RESET		_IO ('$', 3)
 
 /*
  * Structure of the page that can be mapped via mmap

kernel/perf_counter.c

@@ -1288,6 +1288,11 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
 	return events;
 }
 
+static void perf_counter_reset(struct perf_counter *counter)
+{
+	atomic_set(&counter->count, 0);
+}
+
 static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct perf_counter *counter = file->private_data;
@@ -1303,6 +1308,9 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case PERF_COUNTER_IOC_REFRESH:
 		perf_counter_refresh(counter, arg);
 		break;
+	case PERF_COUNTER_IOC_RESET:
+		perf_counter_reset(counter);
+		break;
 	default:
 		err = -ENOTTY;
 	}