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: Peter 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: Ingo Molnar <mingo@elte.hu>

2 files changed, 3 insertions, 3 deletions

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 7cd7b356447d..7098ebbb3b3a 100644
--- a/include/linux/perf_event.h
+++ b/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  */
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 7a932526946f..1531e0b409a5 100644
--- a/kernel/perf_event.c
+++ b/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;