linux-kernel - [tip:perf/core] perf: Add unlikely() to the ring-buffer code

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Date:	Wed, 6 Nov 2013 05:18:38 -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: Add unlikely() to the ring-buffer code

Commit-ID:  c72b42a3dde487132da80202756c101b371b2add
Gitweb:     http://git.kernel.org/tip/c72b42a3dde487132da80202756c101b371b2add
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Thu, 31 Oct 2013 17:20:25 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 6 Nov 2013 12:34:19 +0100

perf: Add unlikely() to the ring-buffer code

Add unlikely() annotations to 'slow' paths:

When having a sampling event but no output buffer; you have bigger
issues -- also the bail is still faster than actually doing the work.

When having a sampling event but a control page only buffer, you have
bigger issues -- again the bail is still faster than actually doing
work.

Optimize for the case where you're not loosing events -- again, not
doing the work is still faster but make sure that when you have to
actually do work its as fast as possible.

The typical watermark is 1/2 the buffer size, so most events will not
take this path.

Shrinks perf_output_begin() by 16 bytes on x86_64-defconfig.

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-wlg3jew3qnutm8opd0hyeuwn@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/events/ring_buffer.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 6929c58..383cde4 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -121,17 +121,17 @@ int perf_output_begin(struct perf_output_handle *handle,
 		event = event->parent;
 
 	rb = rcu_dereference(event->rb);
-	if (!rb)
+	if (unlikely(!rb))
 		goto out;
 
-	handle->rb	= rb;
-	handle->event	= event;
-
-	if (!rb->nr_pages)
+	if (unlikely(!rb->nr_pages))
 		goto out;
 
+	handle->rb    = rb;
+	handle->event = event;
+
 	have_lost = local_read(&rb->lost);
-	if (have_lost) {
+	if (unlikely(have_lost)) {
 		lost_event.header.size = sizeof(lost_event);
 		perf_event_header__init_id(&lost_event.header, &sample_data,
 					   event);
@@ -157,7 +157,7 @@ int perf_output_begin(struct perf_output_handle *handle,
 		head += size;
 	} while (local_cmpxchg(&rb->head, offset, head) != offset);
 
-	if (head - local_read(&rb->wakeup) > rb->watermark)
+	if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
 		local_add(rb->watermark, &rb->wakeup);
 
 	handle->page = offset >> (PAGE_SHIFT + page_order(rb));
@@ -167,7 +167,7 @@ int perf_output_begin(struct perf_output_handle *handle,
 	handle->addr += handle->size;
 	handle->size = (PAGE_SIZE << page_order(rb)) - handle->size;
 
-	if (have_lost) {
+	if (unlikely(have_lost)) {
 		lost_event.header.type = PERF_RECORD_LOST;
 		lost_event.header.misc = 0;
 		lost_event.id          = event->id;
--
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