perf: Fix wakeup storm for RO mmap()s

RO mmap()s don't update the tail pointer, so
comparing against it for determining the written data
size doesn't really do any good.

Keep track of when we last did a wakeup, and compare
against that.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20100521090710.684479310@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/linux/perf_event.h

@@ -599,7 +599,7 @@ struct perf_mmap_data {
 	local_t				head;		/* write position    */
 	local_t				nest;		/* nested writers    */
 	local_t				events;		/* event limit       */
-	local_t				wakeup;		/* needs a wakeup    */
+	local_t				wakeup;		/* wakeup stamp      */
 	local_t				lost;		/* nr records lost   */
 
 	long				watermark;	/* wakeup watermark  */

kernel/perf_event.c

@@ -3056,8 +3056,8 @@ int perf_output_begin(struct perf_output_handle *handle,
 	handle->offset	= offset;
 	handle->head	= head;
 
-	if (head - tail > data->watermark)
-		local_inc(&data->wakeup);
+	if (head - local_read(&data->wakeup) > data->watermark)
+		local_add(data->watermark, &data->wakeup);
 
 	if (have_lost) {
 		lost_event.header.type = PERF_RECORD_LOST;