Commit-ID: 85f59edf9684603026c64c902791748116d29478
Gitweb: http://git.kernel.org/tip/85f59edf9684603026c64c902791748116d29478
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 31 Oct 2013 17:25:38 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 6 Nov 2013 12:34:20 +0100
perf: Optimize perf_output_begin()
There's no point in re-doing the memory-barrier when we fail the
cmpxchg(). Also placing it after the space reservation loop makes it
clearer it only separates the userpage->tail read from the data
stores.
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Cc: Michael Ellerman <michael@xxxxxxxxxxxxxx>
Cc: Michael Neuling <mikey@xxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: james.hogan@xxxxxxxxxx
Cc: Vince Weaver <vince@xxxxxxxxxx>
Cc: Victor Kaplansky <VICTORK@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Anton Blanchard <anton@xxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-c19u6egfldyx86tpyc3zgkw9@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/events/ring_buffer.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 383cde4..6ed16ec 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -141,15 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle,
perf_output_get_handle(handle);
do {
- /*
- * Userspace could choose to issue a mb() before updating the
- * tail pointer. So that all reads will be completed before the
- * write is issued.
- *
- * See perf_output_put_handle().
- */
tail = ACCESS_ONCE(rb->user_page->data_tail);
- smp_mb();
offset = head = local_read(&rb->head);
if (!rb->overwrite &&
unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
@@ -157,6 +149,15 @@ int perf_output_begin(struct perf_output_handle *handle,
head += size;
} while (local_cmpxchg(&rb->head, offset, head) != offset);
+ /*
+ * Separate the userpage->tail read from the data stores below.
+ * Matches the MB userspace SHOULD issue after reading the data
+ * and before storing the new tail position.
+ *
+ * See perf_output_put_handle().
+ */
+ smp_mb();
+
if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
local_add(rb->watermark, &rb->wakeup);
--
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