perf_counter: Allow for a wakeup watermark

Currently we wake the mmap() consumer once every PAGE_SIZE of data
and/or once event wakeup_events when specified.

For high speed sampling this results in too many wakeups wrt. the
buffer size, hence change this.

We move the default wakeup limit to 1/4-th the buffer size, and
provide for means to manually specify this limit.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 972f90d7a32f..6c1ef72ea501 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -199,10 +199,14 @@ struct perf_counter_attr {
                                 inherit_stat   :  1, /* per task counts       */
                                 enable_on_exec :  1, /* next exec enables     */
                                 task           :  1, /* trace fork/exit       */
+                                watermark      :  1, /* wakeup_watermark      */
 
-                                __reserved_1   : 50;
+                                __reserved_1   : 49;
 
-        __u32                   wakeup_events;  /* wakeup every n events */
+        union {
+                __u32           wakeup_events;    /* wakeup every n events */
+                __u32           wakeup_watermark; /* bytes before wakeup   */
+        };
         __u32                   __reserved_2;
 
         __u64                   __reserved_3;
@@ -521,6 +525,8 @@ struct perf_mmap_data {
         atomic_t                        wakeup;         /* needs a wakeup    */
         atomic_t                        lost;           /* nr records lost   */
 
+        long                            watermark;      /* wakeup watermark  */
+
         struct perf_counter_mmap_page   *user_page;
         void                            *data_pages[0];
 };