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>

1 files changed, 2 insertions, 2 deletions

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;