[PATCH 06/10] perf: Fix wakeup storm for RO mmap()s

[Peter Zijlstra]

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: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 include/linux/perf_event.h |    2 +-
 kernel/perf_event.c        |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

Index: linux-2.6/include/linux/perf_event.h
===================================================================
--- linux-2.6.orig/include/linux/perf_event.h
+++ linux-2.6/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  */
Index: linux-2.6/kernel/perf_event.c
===================================================================
--- linux-2.6.orig/kernel/perf_event.c
+++ linux-2.6/kernel/perf_event.c
@@ -3055,8 +3055,8 @@ int perf_output_begin(struct perf_output
 	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;


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