Commit-ID: c7f2e3cd6c1f4932ccc4135d050eae3f7c7aef63
Gitweb: http://git.kernel.org/tip/c7f2e3cd6c1f4932ccc4135d050eae3f7c7aef63
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Mon, 25 Nov 2013 11:49:10 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 11 Dec 2013 15:53:22 +0100
perf: Optimize ring-buffer write by depending on control dependencies
Remove a full barrier from the ring-buffer write path by relying on
a control dependency to order a LOAD -> STORE scenario.
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-8alv40z6ikk57jzbaobnxrjl@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/events/ring_buffer.c | 42 ++++++++++++++++++++++++++----------------
1 file changed, 26 insertions(+), 16 deletions(-)
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index e8b168a..146a579 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -61,19 +61,20 @@ again:
*
* kernel user
*
- * READ ->data_tail READ ->data_head
- * smp_mb() (A) smp_rmb() (C)
- * WRITE $data READ $data
- * smp_wmb() (B) smp_mb() (D)
- * STORE ->data_head WRITE ->data_tail
+ * if (LOAD ->data_tail) { LOAD ->data_head
+ * (A) smp_rmb() (C)
+ * STORE $data LOAD $data
+ * smp_wmb() (B) smp_mb() (D)
+ * STORE ->data_head STORE ->data_tail
+ * }
*
* Where A pairs with D, and B pairs with C.
*
- * I don't think A needs to be a full barrier because we won't in fact
- * write data until we see the store from userspace. So we simply don't
- * issue the data WRITE until we observe it. Be conservative for now.
+ * In our case (A) is a control dependency that separates the load of
+ * the ->data_tail and the stores of $data. In case ->data_tail
+ * indicates there is no room in the buffer to store $data we do not.
*
- * OTOH, D needs to be a full barrier since it separates the data READ
+ * D needs to be a full barrier since it separates the data READ
* from the tail WRITE.
*
* For B a WMB is sufficient since it separates two WRITEs, and for C
@@ -81,7 +82,7 @@ again:
*
* See perf_output_begin().
*/
- smp_wmb();
+ smp_wmb(); /* B, matches C */
rb->user_page->data_head = head;
/*
@@ -144,17 +145,26 @@ int perf_output_begin(struct perf_output_handle *handle,
if (!rb->overwrite &&
unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
goto fail;
+
+ /*
+ * The above forms a control dependency barrier separating the
+ * @tail load above from the data stores below. Since the @tail
+ * load is required to compute the branch to fail below.
+ *
+ * A, matches D; the full memory barrier userspace SHOULD issue
+ * after reading the data and before storing the new tail
+ * position.
+ *
+ * See perf_output_put_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().
+ * We rely on the implied barrier() by local_cmpxchg() to ensure
+ * none of the data stores below can be lifted up by the compiler.
*/
- smp_mb();
if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
local_add(rb->watermark, &rb->wakeup);
--
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