[PATCH v2 4/4] 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 7000+ bytes in the kernel image
below. It does increase 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
8194852	4879776	 925696	14000324	 d5a0c4	vmlinux.pre
8187337	4960224	 925696	14073257	 d6bda9	vmlinux.post

Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
---
 include/linux/dynamic_debug.h | 60 ++++++++++++++++++++++++++++++++++++++-----
 lib/dynamic_debug.c           |  7 +++++
 2 files changed, 61 insertions(+), 6 deletions(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 4f1bbc6..546d680 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -1,6 +1,10 @@
 #ifndef _DYNAMIC_DEBUG_H
 #define _DYNAMIC_DEBUG_H
 
+#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
+#include <linux/jump_label.h>
+#endif
+
 /*
  * An instance of this structure is created in a special
  * ELF section at every dynamic debug callsite.  At runtime,
@@ -33,6 +37,12 @@ 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 +70,7 @@ 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 +78,51 @@ 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 DYNAMIC_DEBUG_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 DYNAMIC_DEBUG_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, 0, 0)
+
+#ifdef DEBUG
+#define DYNAMIC_DEBUG_BRANCH(descriptor) \
+	likely(descriptor.flags & _DPRINTK_FLAGS_PRINT)
+#else
+#define DYNAMIC_DEBUG_BRANCH(descriptor) \
+	unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT)
+#endif
+
+#endif
+
 #define dynamic_pr_debug(fmt, ...)				\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DYNAMIC_DEBUG_BRANCH(descriptor))			\
 		__dynamic_pr_debug(&descriptor, pr_fmt(fmt),	\
 				   ##__VA_ARGS__);		\
 } while (0)
@@ -82,7 +130,7 @@ do {								\
 #define dynamic_dev_dbg(dev, fmt, ...)				\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DYNAMIC_DEBUG_BRANCH(descriptor))			\
 		__dynamic_dev_dbg(&descriptor, dev, fmt,	\
 				  ##__VA_ARGS__);		\
 } while (0)
@@ -90,7 +138,7 @@ do {								\
 #define dynamic_netdev_dbg(dev, fmt, ...)			\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);		\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DYNAMIC_DEBUG_BRANCH(descriptor))			\
 		__dynamic_netdev_dbg(&descriptor, dev, fmt,	\
 				     ##__VA_ARGS__);		\
 } while (0)
@@ -100,7 +148,7 @@ do {								\
 do {								\
 	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor,		\
 		__builtin_constant_p(prefix_str) ? prefix_str : "hexdump");\
-	if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))	\
+	if (DYNAMIC_DEBUG_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