On Mon, Feb 10, 2014 at 11:49:29AM +0000, Will Deacon wrote:
> On Mon, Feb 10, 2014 at 11:48:13AM +0000, Peter Zijlstra wrote:
> > On Fri, Feb 07, 2014 at 10:02:16AM -0800, Paul E. McKenney wrote:
> > > As near as I can tell, compiler writers hate the idea of prohibiting
> > > speculative-store optimizations because it requires them to introduce
> > > both control and data dependency tracking into their compilers. Many of
> > > them seem to hate dependency tracking with a purple passion. At least,
> > > such a hatred would go a long way towards explaining the incomplete
> > > and high-overhead implementations of memory_order_consume, the long
> > > and successful use of idioms based on the memory_order_consume pattern
> > > notwithstanding [*]. ;-)
> >
> > Just tell them that because the hardware provides control dependencies
> > we actually use and rely on them.
>
> s/control/address/ ?
Nope, control.
Since stores cannot be speculated and thus require linear control flow
history we can use it to order LOAD -> STORE when the LOAD is required
for the control flow decision and the STORE depends on the control flow
path.
Also see commit 18c03c61444a211237f3d4782353cb38dba795df to
Documentation/memory-barriers.txt
---
commit c7f2e3cd6c1f4932ccc4135d050eae3f7c7aef63
Author: Peter Zijlstra <pet...@infradead.org>
Date: Mon Nov 25 11:49:10 2013 +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" <paul...@us.ibm.com>
Signed-off-by: Peter Zijlstra <pet...@infradead.org>
Link: http://lkml.kernel.org/n/tip-8alv40z6ikk57jzbaobnx...@git.kernel.org
Signed-off-by: Ingo Molnar <mi...@kernel.org>
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index e8b168af135b..146a5792b1d2 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -61,19 +61,20 @@ static void perf_output_put_handle(struct
perf_output_handle *handle)
*
* 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 @@ static void perf_output_put_handle(struct perf_output_handle
*handle)
*
* 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);
