linux-kernel - [PATCH 2/2][RESEND] tracing: Shrink the size of struct ftrace_event

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Date:	Thu, 25 Jul 2013 10:28:27 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 2/2][RESEND] tracing: Shrink the size of struct ftrace_event_field



Use bit fields, and the size of struct ftrace_event_field can be
shrunk from 48 bytes to 40 bytes on 64bit kernel.

slab_name active_obj nr_obj size obj_per_slab
---------------------------------------------
ftrace_event_field   1105   1105     48   85  (before)
ftrace_event_field   1224   1224     40  102  (after)

This saves a few Kbytes: (1224 * 40) - (1105 * 48) = 4080

Signed-off-by: Li Zefan <lizefan@...wei.com>
---
 kernel/trace/trace.h        | 8 ++++----
 kernel/trace/trace_events.c | 5 +++++
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 9e01458..a3365a4 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -916,10 +916,10 @@ struct ftrace_event_field {
 	struct list_head	link;
 	const char		*name;
 	const char		*type;
-	int			filter_type;
-	int			offset;
-	int			size;
-	int			is_signed;
+	unsigned int		filter_type:4;
+	unsigned int		offset:12;
+	unsigned int		size:12;
+	unsigned int		is_signed:1;
 };
 
 struct event_filter {
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 53582e9..67aee85 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -111,6 +111,11 @@ static int __trace_define_field(struct list_head *head, const char *type,
 	field->size = size;
 	field->is_signed = is_signed;
 
+	WARN_ON(offset >= (1 << 12));
+	WARN_ON(size >= (1 << 12));
+	WARN_ON(is_signed >= (1 << 1));
+	WARN_ON(field->filter_type >= (1 << 4));
+
 	list_add(&field->link, head);
 
 	return 0;
-- 1.8.0.2

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