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Date: Tue, 3 Jan 2017 13:59:09 -0500
From: Jason Baron <jbaron@...mai.com>
To: peterz@...radead.org, mingo@...nel.org
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
Joe Perches <joe@...ches.com>
Subject: [PATCH] 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 2 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 1 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 253 bytes, but reduced
the data + bss size by 19,136, for a net savings of 18,883 bytes.
text data bss dec hex filename
8315249 5061176 794624 14171049 d83ba9 vmlinux.pre
8315502 5046136 790528 14152166 d7f1e6 vmlinux.post
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Joe Perches <joe@...ches.com>
Signed-off-by: Jason Baron <jbaron@...mai.com>
---
Documentation/static-keys.txt | 4 +-
include/linux/jump_label.h | 17 +++++----
kernel/jump_label.c | 89 +++++++++++++++++++++++++++++++++----------
3 files changed, 82 insertions(+), 28 deletions(-)
diff --git a/Documentation/static-keys.txt b/Documentation/static-keys.txt
index ea8d7b4..32a25fa 100644
--- a/Documentation/static-keys.txt
+++ b/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
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index a0547c5..738d61e 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -89,11 +89,13 @@ extern bool static_key_initialized;
struct static_key {
atomic_t enabled;
-/* Set lsb bit to 1 if branch is default true, 0 ot */
+/*
+ * bit 1 => 1 if key is initially true
+ * 0 if initially false
+ * bit 2 => 1 if points to struct static_key_mod
+ * 0 if points to struct jump_entry
+ */
struct jump_entry *entries;
-#ifdef CONFIG_MODULES
- struct static_key_mod *next;
-#endif
};
#else
@@ -118,9 +120,10 @@ 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)
{
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 93ad6c1..2eb1ed5 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -234,7 +234,22 @@ static inline struct jump_entry *static_key_entries(struct static_key *key)
static inline bool static_key_type(struct static_key *key)
{
- return (unsigned long)key->entries & JUMP_TYPE_MASK;
+ return (unsigned long)key->entries & JUMP_TYPE_TRUE;
+}
+
+static inline bool static_key_linked(struct static_key *key)
+{
+ return (unsigned long)key->entries & JUMP_TYPE_LINKED;
+}
+
+static inline void static_key_clear_linked(struct static_key *key)
+{
+ *(unsigned long *)&key->entries &= ~JUMP_TYPE_LINKED;
+}
+
+static inline void static_key_set_linked(struct static_key *key)
+{
+ *(unsigned long *)&key->entries |= JUMP_TYPE_LINKED;
}
static inline struct static_key *jump_entry_key(struct jump_entry *entry)
@@ -307,12 +322,9 @@ void __init jump_label_init(void)
key = iterk;
/*
- * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
+ * Set key->entries to iter preserve JUMP_TYPE_MASK bits
*/
*((unsigned long *)&key->entries) += (unsigned long)iter;
-#ifdef CONFIG_MODULES
- key->next = NULL;
-#endif
}
static_key_initialized = true;
jump_label_unlock();
@@ -356,13 +368,20 @@ static int __jump_label_mod_text_reserved(void *start, void *end)
static void __jump_label_mod_update(struct static_key *key)
{
- struct static_key_mod *mod;
+ struct jump_entry *stop;
+ struct static_key_mod *mod =
+ (struct static_key_mod *)static_key_entries(key);
- for (mod = key->next; mod; mod = mod->next) {
+ while (mod) {
struct module *m = mod->mod;
- __jump_label_update(key, mod->entries,
- m->jump_entries + m->num_jump_entries);
+ if (!m)
+ stop = __stop___jump_table;
+ else
+ stop = m->jump_entries + m->num_jump_entries;
+ if (mod->entries)
+ __jump_label_update(key, mod->entries, stop);
+ mod = mod->next;
}
}
@@ -397,7 +416,7 @@ 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)
@@ -415,19 +434,36 @@ 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.
+ * Set key->entries to iter preserve JUMP_TYPE_MASK bits
*/
*((unsigned long *)&key->entries) += (unsigned long)iter;
- key->next = NULL;
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;
+ *(unsigned long *)&key->entries = (unsigned long)jlm2 |
+ static_key_type(key);
+ static_key_set_linked(key);
+ }
jlm->mod = mod;
jlm->entries = iter;
- jlm->next = key->next;
- key->next = jlm;
+ jlm->next = (struct static_key_mod *)static_key_entries(key);
+ *(unsigned long *)&key->entries = (unsigned long)jlm |
+ static_key_type(key);
+ 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 +490,28 @@ static void jump_label_del_module(struct module *mod)
if (within_module(iter->key, mod))
continue;
- prev = &key->next;
- jlm = key->next;
+ prev = (struct static_key_mod **)&key->entries;
+ jlm = (struct static_key_mod *)static_key_entries(key);
while (jlm && jlm->mod != mod) {
prev = &jlm->next;
jlm = jlm->next;
}
- if (jlm) {
- *prev = jlm->next;
+ if (!jlm)
+ continue;
+
+ *prev = (struct static_key_mod *)((unsigned long)jlm->next |
+ ((unsigned long)*prev & JUMP_TYPE_MASK));
+ kfree(jlm);
+
+ jlm = (struct static_key_mod *)static_key_entries(key);
+ /* if only one etry is left, fold it back into the static_key */
+ if (jlm->next == NULL) {
+ *(unsigned long *)&key->entries =
+ (unsigned long)jlm->entries |
+ static_key_type(key);
+ static_key_clear_linked(key);
kfree(jlm);
}
}
@@ -558,7 +606,8 @@ static void jump_label_update(struct static_key *key)
#ifdef CONFIG_MODULES
struct module *mod;
- __jump_label_mod_update(key);
+ if (static_key_linked(key))
+ __jump_label_mod_update(key);
preempt_disable();
mod = __module_address((unsigned long)key);
@@ -567,7 +616,7 @@ static void jump_label_update(struct static_key *key)
preempt_enable();
#endif
/* if there are no users, entry can be NULL */
- if (entry)
+ if (entry && !static_key_linked(key))
__jump_label_update(key, entry, stop);
}
--
2.6.1
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