linux-kernel - [tip:perf/core] perf: Optimize perf_output

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Date:	Wed, 6 Nov 2013 05:18:47 -0800
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, anton@...ba.org,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, mingo@...nel.org,
	michael@...erman.id.au, peterz@...radead.org, vince@...ter.net,
	paulmck@...ux.vnet.ibm.com, benh@...nel.crashing.org,
	fweisbec@...il.com, oleg@...hat.com, VICTORK@...ibm.com,
	tglx@...utronix.de, mikey@...ling.org
Subject: [tip:perf/core] perf: Optimize perf_output_begin()

Commit-ID:  85f59edf9684603026c64c902791748116d29478
Gitweb:     http://git.kernel.org/tip/85f59edf9684603026c64c902791748116d29478
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Thu, 31 Oct 2013 17:25:38 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
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@...radead.org>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc: Michael Ellerman <michael@...erman.id.au>
Cc: Michael Neuling <mikey@...ling.org>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: james.hogan@...tec.com
Cc: Vince Weaver <vince@...ter.net>
Cc: Victor Kaplansky <VICTORK@...ibm.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Anton Blanchard <anton@...ba.org>
Link: http://lkml.kernel.org/n/tip-c19u6egfldyx86tpyc3zgkw9@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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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