[PATCH 3/3] dynamic_debug: add jump label support

[Jason Baron]

Although dynamic debug is often only used for debug builds, sometimes its
enabled for production builds as well. Minimize its impact by using jump
labels. This reduces the text section by 6000+ bytes in the kernel image
below. It does increae data, but this should only be referenced when
changing the direction of the branches, and hence usually not in cache.

    text     data     bss      dec    hex filename
10369421  2033336  917504 13320261 cb4045 vmlinux.pre
10363054  2103736  917504 13384294 cc3a66 vmlinux.post

Cc: Joe Perches <joe@xxxxxxxxxxx>
Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
---
 include/linux/dynamic_debug.h | 54 ++++++++++++++++++++++++++++++++++++++-----
 lib/dynamic_debug.c           |  7 ++++++
 2 files changed, 55 insertions(+), 6 deletions(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 4f1bbc6..9f3144d 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -1,6 +1,8 @@
 #ifndef _DYNAMIC_DEBUG_H
 #define _DYNAMIC_DEBUG_H
 
+#include <linux/jump_label_branch.h>
+
 /*
  * An instance of this structure is created in a special
  * ELF section at every dynamic debug callsite.  At runtime,
@@ -33,6 +35,12 @@ struct _ddebug {
 #define _DPRINTK_FLAGS_DEFAULT 0
 #endif
 	unsigned int flags:8;
+#ifdef HAVE_JUMP_LABEL
+	union {
+		struct static_key_true dd_key_true;
+		struct static_key_false dd_key_false;
+	} key;
+#endif
 } __attribute__((aligned(8)));
 
 
@@ -60,7 +68,7 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
 			  const struct net_device *dev,
 			  const char *fmt, ...);
 
-#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt)		\
+#define DEFINE_DYNAMIC_DEBUG_METADATA_KEY(name, fmt, key, init)	\
 	static struct _ddebug  __aligned(8)			\
 	__attribute__((section("__verbose"))) name = {		\
 		.modname = KBUILD_MODNAME,			\
@@ -68,13 +76,47 @@ void __dynamic_netdev_dbg(struct _ddebug *descriptor,
 		.filename = __FILE__,				\
 		.format = (fmt),				\
 		.lineno = __LINE__,				\
-		.flags =  _DPRINTK_FLAGS_DEFAULT,		\
+		.flags = _DPRINTK_FLAGS_DEFAULT,		\
+		dd_key_init(key, init)				\
 	}
 
+#ifdef HAVE_JUMP_LABEL
+
+#define dd_key_init(key, init) key = (init)
+
+#ifdef DEBUG
+#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
+	DEFINE_DYNAMIC_DEBUG_METADATA_KEY(name, fmt, .key.dd_key_true, \
+					  (STATIC_KEY_TRUE_INIT))
+
+#define DDEBUG_BRANCH(descriptor) \
+	static_branch_likely(&descriptor.key.dd_key_true)
+#else
+#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
+	DEFINE_DYNAMIC_DEBUG_METADATA_KEY(name, fmt, .key.dd_key_false, \
+					  (STATIC_KEY_FALSE_INIT))
+
+#define DDEBUG_BRANCH(descriptor) \
+	static_branch_unlikely(&descriptor.key.dd_key_false)
+#endif
+
+#else
+
+#define dd_key_init(key, init)
+
+#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
+	DEFINE_DYNAMIC_DEBUG_METADATA_KEY(name, fmt, .key.dd_key_false, \
+					  (STATIC_KEY_FALSE_INIT))
+
+#define DDEBUG_BRANCH(descriptor) \
+	unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT)
+
+#endif
+
 #define dynamic_pr_debug(fmt, ...)				\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DDEBUG_BRANCH(descriptor))				\
 		__dynamic_pr_debug(&descriptor, pr_fmt(fmt),	\
 				   ##__VA_ARGS__);		\
 } while (0)
@@ -82,7 +124,7 @@ do {								\
 #define dynamic_dev_dbg(dev, fmt, ...)				\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DDEBUG_BRANCH(descriptor))				\
 		__dynamic_dev_dbg(&descriptor, dev, fmt,	\
 				  ##__VA_ARGS__);		\
 } while (0)
@@ -90,7 +132,7 @@ do {								\
 #define dynamic_netdev_dbg(dev, fmt, ...)			\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DDEBUG_BRANCH(descriptor))				\
 		__dynamic_netdev_dbg(&descriptor, dev, fmt,	\
 				     ##__VA_ARGS__);		\
 } while (0)
@@ -100,7 +142,7 @@ do {								\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor,		\
 		__builtin_constant_p(prefix_str) ? prefix_str : "hexdump");\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DDEBUG_BRANCH(descriptor))				\
 		print_hex_dump(KERN_DEBUG, prefix_str,		\
 			       prefix_type, rowsize, groupsize,	\
 			       buf, len, ascii);		\
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index fe42b6e..da796e2 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -188,6 +188,13 @@ static int ddebug_change(const struct ddebug_query *query,
 			newflags = (dp->flags & mask) | flags;
 			if (newflags == dp->flags)
 				continue;
+#ifdef HAVE_JUMP_LABEL
+			if (dp->flags & _DPRINTK_FLAGS_PRINT) {
+				if (!(flags & _DPRINTK_FLAGS_PRINT))
+					static_branch_disable(&dp->key.dd_key_true);
+			} else if (flags & _DPRINTK_FLAGS_PRINT)
+				static_branch_enable(&dp->key.dd_key_true);
+#endif
 			dp->flags = newflags;
 			vpr_info("changed %s:%d [%s]%s =%s\n",
 				 trim_prefix(dp->filename), dp->lineno,
-- 
2.6.1