jump_label: Reduce the size of struct static_key

The static_key->next field goes mostly unused. The field is used for
associating module uses with a static key. Most uses of struct static_key
define a static key in the core kernel and make use of it entirely within
the core kernel, or define the static key in a module and make use of it
only from within that module. In fact, of the ~3,000 static keys defined,
I found only about 5 or so that did not fit this pattern.

Thus, we can remove the static_key->next field entirely and overload
the static_key->entries field. That is, when all the static_key uses
are contained within the same module, static_key->entries continues
to point to those uses. However, if the static_key uses are not contained
within the module where the static_key is defined, then we allocate a
struct static_key_mod, store a pointer to the uses within that
struct static_key_mod, and have the static key point at the static_key_mod.
This does incur some extra memory usage when a static_key is used in a
module that does not define it, but since there are only a handful of such
cases there is a net savings.

In order to identify if the static_key->entries pointer contains a
struct static_key_mod or a struct jump_entry pointer, bit 1 of
static_key->entries is set to 1 if it points to a struct static_key_mod and
is 0 if it points to a struct jump_entry. We were already using bit 0 in a
similar way to store the initial value of the static_key. This does mean
that allocations of struct static_key_mod and that the struct jump_entry
tables need to be at least 4-byte aligned in memory. As far as I can tell
all arches meet this criteria.

For my .config, the patch increased the text by 778 bytes, but reduced
the data + bss size by 14912, for a net savings of 14,134 bytes.

   text	   data	    bss	    dec	    hex	filename
8092427	5016512	 790528	13899467	 d416cb	vmlinux.pre
8093205	5001600	 790528	13885333	 d3df95	vmlinux.post

Link: http://lkml.kernel.org/r/1486154544-4321-1-git-send-email-jbaron@akamai.com

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/static-keys.txt

@@ -155,7 +155,9 @@ or:
 
 There are a few functions and macros that architectures must implement in order
 to take advantage of this optimization. If there is no architecture support, we
-simply fall back to a traditional, load, test, and jump sequence.
+simply fall back to a traditional, load, test, and jump sequence. Also, the
+struct jump_entry table must be at least 4-byte aligned because the
+static_key->entry field makes use of the two least significant bits.
 
 * select HAVE_ARCH_JUMP_LABEL, see: arch/x86/Kconfig
 

include/linux/jump_label.h

@@ -89,11 +89,17 @@ extern bool static_key_initialized;
 
 struct static_key {
 	atomic_t enabled;
-/* Set lsb bit to 1 if branch is default true, 0 ot */
-	struct jump_entry *entries;
-#ifdef CONFIG_MODULES
-	struct static_key_mod *next;
-#endif
+/*
+ * bit 0 => 1 if key is initially true
+ *	    0 if initially false
+ * bit 1 => 1 if points to struct static_key_mod
+ *	    0 if points to struct jump_entry
+ */
+	union {
+		unsigned long type;
+		struct jump_entry *entries;
+		struct static_key_mod *next;
+	};
 };
 
 #else
@@ -118,9 +124,10 @@ struct module;
 
 #ifdef HAVE_JUMP_LABEL
 
-#define JUMP_TYPE_FALSE	0UL
-#define JUMP_TYPE_TRUE	1UL
-#define JUMP_TYPE_MASK	1UL
+#define JUMP_TYPE_FALSE		0UL
+#define JUMP_TYPE_TRUE		1UL
+#define JUMP_TYPE_LINKED	2UL
+#define JUMP_TYPE_MASK		3UL
 
 static __always_inline bool static_key_false(struct static_key *key)
 {

kernel/jump_label.c

@@ -229,12 +229,28 @@ void __weak __init_or_module arch_jump_label_transform_static(struct jump_entry
 
 static inline struct jump_entry *static_key_entries(struct static_key *key)
 {
-	return (struct jump_entry *)((unsigned long)key->entries & ~JUMP_TYPE_MASK);
+	WARN_ON_ONCE(key->type & JUMP_TYPE_LINKED);
+	return (struct jump_entry *)(key->type & ~JUMP_TYPE_MASK);
 }
 
 static inline bool static_key_type(struct static_key *key)
 {
-	return (unsigned long)key->entries & JUMP_TYPE_MASK;
+	return key->type & JUMP_TYPE_TRUE;
+}
+
+static inline bool static_key_linked(struct static_key *key)
+{
+	return key->type & JUMP_TYPE_LINKED;
+}
+
+static inline void static_key_clear_linked(struct static_key *key)
+{
+	key->type &= ~JUMP_TYPE_LINKED;
+}
+
+static inline void static_key_set_linked(struct static_key *key)
+{
+	key->type |= JUMP_TYPE_LINKED;
 }
 
 static inline struct static_key *jump_entry_key(struct jump_entry *entry)
@@ -247,6 +263,26 @@ static bool jump_entry_branch(struct jump_entry *entry)
 	return (unsigned long)entry->key & 1UL;
 }
 
+/***
+ * A 'struct static_key' uses a union such that it either points directly
+ * to a table of 'struct jump_entry' or to a linked list of modules which in
+ * turn point to 'struct jump_entry' tables.
+ *
+ * The two lower bits of the pointer are used to keep track of which pointer
+ * type is in use and to store the initial branch direction, we use an access
+ * function which preserves these bits.
+ */
+static void static_key_set_entries(struct static_key *key,
+				   struct jump_entry *entries)
+{
+	unsigned long type;
+
+	WARN_ON_ONCE((unsigned long)entries & JUMP_TYPE_MASK);
+	type = key->type & JUMP_TYPE_MASK;
+	key->entries = entries;
+	key->type |= type;
+}
+
 static enum jump_label_type jump_label_type(struct jump_entry *entry)
 {
 	struct static_key *key = jump_entry_key(entry);
@@ -306,13 +342,7 @@ void __init jump_label_init(void)
 			continue;
 
 		key = iterk;
-		/*
-		 * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
-		 */
-		*((unsigned long *)&key->entries) += (unsigned long)iter;
-#ifdef CONFIG_MODULES
-		key->next = NULL;
-#endif
+		static_key_set_entries(key, iter);
 	}
 	static_key_initialized = true;
 	jump_label_unlock();
@@ -336,6 +366,29 @@ struct static_key_mod {
 	struct module *mod;
 };
 
+static inline struct static_key_mod *static_key_mod(struct static_key *key)
+{
+	WARN_ON_ONCE(!(key->type & JUMP_TYPE_LINKED));
+	return (struct static_key_mod *)(key->type & ~JUMP_TYPE_MASK);
+}
+
+/***
+ * key->type and key->next are the same via union.
+ * This sets key->next and preserves the type bits.
+ *
+ * See additional comments above static_key_set_entries().
+ */
+static void static_key_set_mod(struct static_key *key,
+			       struct static_key_mod *mod)
+{
+	unsigned long type;
+
+	WARN_ON_ONCE((unsigned long)mod & JUMP_TYPE_MASK);
+	type = key->type & JUMP_TYPE_MASK;
+	key->next = mod;
+	key->type |= type;
+}
+
 static int __jump_label_mod_text_reserved(void *start, void *end)
 {
 	struct module *mod;
@@ -358,11 +411,23 @@ static void __jump_label_mod_update(struct static_key *key)
 {
 	struct static_key_mod *mod;
 
-	for (mod = key->next; mod; mod = mod->next) {
-		struct module *m = mod->mod;
+	for (mod = static_key_mod(key); mod; mod = mod->next) {
+		struct jump_entry *stop;
+		struct module *m;
+
+		/*
+		 * NULL if the static_key is defined in a module
+		 * that does not use it
+		 */
+		if (!mod->entries)
+			continue;
 
-		__jump_label_update(key, mod->entries,
-				    m->jump_entries + m->num_jump_entries);
+		m = mod->mod;
+		if (!m)
+			stop = __stop___jump_table;
+		else
+			stop = m->jump_entries + m->num_jump_entries;
+		__jump_label_update(key, mod->entries, stop);
 	}
 }
 
@@ -397,7 +462,7 @@ static int jump_label_add_module(struct module *mod)
 	struct jump_entry *iter_stop = iter_start + mod->num_jump_entries;
 	struct jump_entry *iter;
 	struct static_key *key = NULL;
-	struct static_key_mod *jlm;
+	struct static_key_mod *jlm, *jlm2;
 
 	/* if the module doesn't have jump label entries, just return */
 	if (iter_start == iter_stop)
@@ -414,20 +479,32 @@ static int jump_label_add_module(struct module *mod)
 
 		key = iterk;
 		if (within_module(iter->key, mod)) {
-			/*
-			 * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
-			 */
-			*((unsigned long *)&key->entries) += (unsigned long)iter;
-			key->next = NULL;
+			static_key_set_entries(key, iter);
 			continue;
 		}
 		jlm = kzalloc(sizeof(struct static_key_mod), GFP_KERNEL);
 		if (!jlm)
 			return -ENOMEM;
+		if (!static_key_linked(key)) {
+			jlm2 = kzalloc(sizeof(struct static_key_mod),
+				       GFP_KERNEL);
+			if (!jlm2) {
+				kfree(jlm);
+				return -ENOMEM;
+			}
+			preempt_disable();
+			jlm2->mod = __module_address((unsigned long)key);
+			preempt_enable();
+			jlm2->entries = static_key_entries(key);
+			jlm2->next = NULL;
+			static_key_set_mod(key, jlm2);
+			static_key_set_linked(key);
+		}
 		jlm->mod = mod;
 		jlm->entries = iter;
-		jlm->next = key->next;
-		key->next = jlm;
+		jlm->next = static_key_mod(key);
+		static_key_set_mod(key, jlm);
+		static_key_set_linked(key);
 
 		/* Only update if we've changed from our initial state */
 		if (jump_label_type(iter) != jump_label_init_type(iter))
@@ -454,16 +531,34 @@ static void jump_label_del_module(struct module *mod)
 		if (within_module(iter->key, mod))
 			continue;
 
+		/* No memory during module load */
+		if (WARN_ON(!static_key_linked(key)))
+			continue;
+
 		prev = &key->next;
-		jlm = key->next;
+		jlm = static_key_mod(key);
 
 		while (jlm && jlm->mod != mod) {
 			prev = &jlm->next;
 			jlm = jlm->next;
 		}
 
-		if (jlm) {
+		/* No memory during module load */
+		if (WARN_ON(!jlm))
+			continue;
+
+		if (prev == &key->next)
+			static_key_set_mod(key, jlm->next);
+		else
 			*prev = jlm->next;
+
+		kfree(jlm);
+
+		jlm = static_key_mod(key);
+		/* if only one etry is left, fold it back into the static_key */
+		if (jlm->next == NULL) {
+			static_key_set_entries(key, jlm->entries);
+			static_key_clear_linked(key);
 			kfree(jlm);
 		}
 	}
@@ -492,8 +587,10 @@ jump_label_module_notify(struct notifier_block *self, unsigned long val,
 	case MODULE_STATE_COMING:
 		jump_label_lock();
 		ret = jump_label_add_module(mod);
-		if (ret)
+		if (ret) {
+			WARN(1, "Failed to allocatote memory: jump_label may not work properly.\n");
 			jump_label_del_module(mod);
+		}
 		jump_label_unlock();
 		break;
 	case MODULE_STATE_GOING:
@@ -554,11 +651,14 @@ int jump_label_text_reserved(void *start, void *end)
 static void jump_label_update(struct static_key *key)
 {
 	struct jump_entry *stop = __stop___jump_table;
-	struct jump_entry *entry = static_key_entries(key);
+	struct jump_entry *entry;
 #ifdef CONFIG_MODULES
 	struct module *mod;
 
-	__jump_label_mod_update(key);
+	if (static_key_linked(key)) {
+		__jump_label_mod_update(key);
+		return;
+	}
 
 	preempt_disable();
 	mod = __module_address((unsigned long)key);
@@ -566,6 +666,7 @@ static void jump_label_update(struct static_key *key)
 		stop = mod->jump_entries + mod->num_jump_entries;
 	preempt_enable();
 #endif
+	entry = static_key_entries(key);
 	/* if there are no users, entry can be NULL */
 	if (entry)
 		__jump_label_update(key, entry, stop);