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static keys: Introduce 'struct static_key', static_key_true()/false() and static_key_slow_[inc|dec]()

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So here's a boot tested patch on top of Jason's series that does
all the cleanups I talked about and turns jump labels into a
more intuitive to use facility. It should also address the
various misconceptions and confusions that surround jump labels.

Typical usage scenarios:

        #include <linux/static_key.h>

        struct static_key key = STATIC_KEY_INIT_TRUE;

        if (static_key_false(&key))
                do unlikely code
        else
                do likely code

Or:

        if (static_key_true(&key))
                do likely code
        else
                do unlikely code

The static key is modified via:

        static_key_slow_inc(&key);
        ...
        static_key_slow_dec(&key);

The 'slow' prefix makes it abundantly clear that this is an
expensive operation.

I've updated all in-kernel code to use this everywhere. Note
that I (intentionally) have not pushed through the rename
blindly through to the lowest levels: the actual jump-label
patching arch facility should be named like that, so we want to
decouple jump labels from the static-key facility a bit.

On non-jump-label enabled architectures static keys default to
likely()/unlikely() branches.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Jason Baron <jbaron@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: a.p.zijlstra@chello.nl
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20120222085809.GA26397@elte.hu
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Ingo Molnar
2012-02-24 08:31:31 +01:00
parent 1cfa60dc7d
commit c5905afb0e
31 changed files with 298 additions and 205 deletions
Download Patch File Download Diff File Expand all files Collapse all files

139
include/linux/jump_label.h
View File

@@ -9,15 +9,15 @@
*
* Jump labels provide an interface to generate dynamic branches using
* self-modifying code. Assuming toolchain and architecture support the result
* of a "if (static_branch(&key))" statement is a unconditional branch (which
* of a "if (static_key_false(&key))" statement is a unconditional branch (which
* defaults to false - and the true block is placed out of line).
*
* However at runtime we can change the 'static' branch target using
* jump_label_{inc,dec}(). These function as a 'reference' count on the key
* However at runtime we can change the branch target using
* static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
* object and for as long as there are references all branches referring to
* that particular key will point to the (out of line) true block.
*
* Since this relies on modifying code the jump_label_{inc,dec}() functions
* Since this relies on modifying code the static_key_slow_{inc,dec}() functions
* must be considered absolute slow paths (machine wide synchronization etc.).
* OTOH, since the affected branches are unconditional their runtime overhead
* will be absolutely minimal, esp. in the default (off) case where the total
@@ -26,12 +26,26 @@
*
* When the control is directly exposed to userspace it is prudent to delay the
* decrement to avoid high frequency code modifications which can (and do)
* cause significant performance degradation. Struct jump_label_key_deferred and
* jump_label_dec_deferred() provide for this.
* cause significant performance degradation. Struct static_key_deferred and
* static_key_slow_dec_deferred() provide for this.
*
* Lacking toolchain and or architecture support, it falls back to a simple
* conditional branch.
*/
*
* struct static_key my_key = STATIC_KEY_INIT_TRUE;
*
* if (static_key_true(&my_key)) {
* }
*
* will result in the true case being in-line and starts the key with a single
* reference. Mixing static_key_true() and static_key_false() on the same key is not
* allowed.
*
* Not initializing the key (static data is initialized to 0s anyway) is the
* same as using STATIC_KEY_INIT_FALSE and static_key_false() is
* equivalent with static_branch().
*
*/
#include <linux/types.h>
#include <linux/compiler.h>
@@ -39,16 +53,17 @@
#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
struct jump_label_key {
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 jump_label_mod *next;
struct static_key_mod *next;
#endif
};
struct jump_label_key_deferred {
struct jump_label_key key;
struct static_key_deferred {
struct static_key key;
unsigned long timeout;
struct delayed_work work;
};
@@ -66,13 +81,34 @@ struct module;
#ifdef HAVE_JUMP_LABEL
#ifdef CONFIG_MODULES
#define JUMP_LABEL_INIT {ATOMIC_INIT(0), NULL, NULL}
#else
#define JUMP_LABEL_INIT {ATOMIC_INIT(0), NULL}
#endif
#define JUMP_LABEL_TRUE_BRANCH 1UL
static __always_inline bool static_branch(struct jump_label_key *key)
static
inline struct jump_entry *jump_label_get_entries(struct static_key *key)
{
return (struct jump_entry *)((unsigned long)key->entries
& ~JUMP_LABEL_TRUE_BRANCH);
}
static inline bool jump_label_get_branch_default(struct static_key *key)
{
if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
return true;
return false;
}
static __always_inline bool static_key_false(struct static_key *key)
{
return arch_static_branch(key);
}
static __always_inline bool static_key_true(struct static_key *key)
{
return !static_key_false(key);
}
/* Deprecated. Please use 'static_key_false() instead. */
static __always_inline bool static_branch(struct static_key *key)
{
return arch_static_branch(key);
}
@@ -88,21 +124,24 @@ extern void arch_jump_label_transform(struct jump_entry *entry,
extern void arch_jump_label_transform_static(struct jump_entry *entry,
enum jump_label_type type);
extern int jump_label_text_reserved(void *start, void *end);
extern void jump_label_inc(struct jump_label_key *key);
extern void jump_label_dec(struct jump_label_key *key);
extern void jump_label_dec_deferred(struct jump_label_key_deferred *key);
extern bool jump_label_enabled(struct jump_label_key *key);
extern void static_key_slow_inc(struct static_key *key);
extern void static_key_slow_dec(struct static_key *key);
extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
extern bool static_key_enabled(struct static_key *key);
extern void jump_label_apply_nops(struct module *mod);
extern void jump_label_rate_limit(struct jump_label_key_deferred *key,
unsigned long rl);
extern void
jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
#define STATIC_KEY_INIT_TRUE ((struct static_key) \
{ .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
#define STATIC_KEY_INIT_FALSE ((struct static_key) \
{ .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
#else /* !HAVE_JUMP_LABEL */
#include <linux/atomic.h>
#define JUMP_LABEL_INIT {ATOMIC_INIT(0)}
struct jump_label_key {
struct static_key {
atomic_t enabled;
};
@@ -110,30 +149,45 @@ static __always_inline void jump_label_init(void)
{
}
struct jump_label_key_deferred {
struct jump_label_key key;
struct static_key_deferred {
struct static_key key;
};
static __always_inline bool static_branch(struct jump_label_key *key)
static __always_inline bool static_key_false(struct static_key *key)
{
if (unlikely(atomic_read(&key->enabled)))
if (unlikely(atomic_read(&key->enabled)) > 0)
return true;
return false;
}
static inline void jump_label_inc(struct jump_label_key *key)
static __always_inline bool static_key_true(struct static_key *key)
{
if (likely(atomic_read(&key->enabled)) > 0)
return true;
return false;
}
/* Deprecated. Please use 'static_key_false() instead. */
static __always_inline bool static_branch(struct static_key *key)
{
if (unlikely(atomic_read(&key->enabled)) > 0)
return true;
return false;
}
static inline void static_key_slow_inc(struct static_key *key)
{
atomic_inc(&key->enabled);
}
static inline void jump_label_dec(struct jump_label_key *key)
static inline void static_key_slow_dec(struct static_key *key)
{
atomic_dec(&key->enabled);
}
static inline void jump_label_dec_deferred(struct jump_label_key_deferred *key)
static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
{
jump_label_dec(&key->key);
static_key_slow_dec(&key->key);
}
static inline int jump_label_text_reserved(void *start, void *end)
@@ -144,9 +198,9 @@ static inline int jump_label_text_reserved(void *start, void *end)
static inline void jump_label_lock(void) {}
static inline void jump_label_unlock(void) {}
static inline bool jump_label_enabled(struct jump_label_key *key)
static inline bool static_key_enabled(struct static_key *key)
{
return !!atomic_read(&key->enabled);
return (atomic_read(&key->enabled) > 0);
}
static inline int jump_label_apply_nops(struct module *mod)
@@ -154,13 +208,20 @@ static inline int jump_label_apply_nops(struct module *mod)
return 0;
}
static inline void jump_label_rate_limit(struct jump_label_key_deferred *key,
static inline void
jump_label_rate_limit(struct static_key_deferred *key,
unsigned long rl)
{
}
#define STATIC_KEY_INIT_TRUE ((struct static_key) \
{ .enabled = ATOMIC_INIT(1) })
#define STATIC_KEY_INIT_FALSE ((struct static_key) \
{ .enabled = ATOMIC_INIT(0) })
#endif /* HAVE_JUMP_LABEL */
#define jump_label_key_enabled ((struct jump_label_key){ .enabled = ATOMIC_INIT(1), })
#define jump_label_key_disabled ((struct jump_label_key){ .enabled = ATOMIC_INIT(0), })
#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
#define jump_label_enabled static_key_enabled
#endif /* _LINUX_JUMP_LABEL_H */