dynamic_debug: add jump label support

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

Link: http://lkml.kernel.org/r/d165b465e8c89bc582d973758d40be44c33f018b.1467837322.git.jbaron@akamai.com
Signed-off-by: Jason Baron <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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)));
 
 
@@ -60,37 +70,75 @@ 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,			\
 		.function = __func__,				\
 		.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)
 
 #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)
 
 #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);		\

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,