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jump label, locking/static_keys: Update docs

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Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: benh@kernel.crashing.org
Cc: bp@alien8.de
Cc: davem@davemloft.net
Cc: ddaney@caviumnetworks.com
Cc: heiko.carstens@de.ibm.com
Cc: linux-kernel@vger.kernel.org
Cc: liuj97@gmail.com
Cc: luto@amacapital.net
Cc: michael@ellerman.id.au
Cc: rabin@rab.in
Cc: ralf@linux-mips.org
Cc: rostedt@goodmis.org
Cc: vbabka@suse.cz
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/6b50f2f6423a2244f37f4b1d2d6c211b9dcdf4f8.1438227999.git.jbaron@akamai.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Jason Baron 2015-07-30 03:59:48 +00:00 committed by Ingo Molnar
parent 2bf9e0ab08
commit 412758cb26
2 changed files with 99 additions and 69 deletions
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101
Documentation/static-keys.txt
View file

@ -1,7 +1,22 @@
Static Keys
-----------
By: Jason Baron <jbaron@redhat.com>
DEPRECATED API:
The use of 'struct static_key' directly, is now DEPRECATED. In addition
static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
struct static_key false = STATIC_KEY_INIT_FALSE;
struct static_key true = STATIC_KEY_INIT_TRUE;
static_key_true()
static_key_false()
The updated API replacements are:
DEFINE_STATIC_KEY_TRUE(key);
DEFINE_STATIC_KEY_FALSE(key);
static_key_likely()
statick_key_unlikely()
0) Abstract
@ -9,22 +24,22 @@ Static keys allows the inclusion of seldom used features in
performance-sensitive fast-path kernel code, via a GCC feature and a code
patching technique. A quick example:
struct static_key key = STATIC_KEY_INIT_FALSE;
DEFINE_STATIC_KEY_FALSE(key);
...
if (static_key_false(&key))
if (static_branch_unlikely(&key))
do unlikely code
else
do likely code
...
static_key_slow_inc();
static_branch_enable(&key);
...
static_key_slow_inc();
static_branch_disable(&key);
...
The static_key_false() branch will be generated into the code with as little
The static_branch_unlikely() branch will be generated into the code with as little
impact to the likely code path as possible.
@ -56,7 +71,7 @@ the branch site to change the branch direction.
For example, if we have a simple branch that is disabled by default:
if (static_key_false(&key))
if (static_branch_unlikely(&key))
printk("I am the true branch\n");
Thus, by default the 'printk' will not be emitted. And the code generated will
@ -75,68 +90,55 @@ the basis for the static keys facility.
In order to make use of this optimization you must first define a key:
struct static_key key;
Which is initialized as:
struct static_key key = STATIC_KEY_INIT_TRUE;
DEFINE_STATIC_KEY_TRUE(key);
or:
struct static_key key = STATIC_KEY_INIT_FALSE;
DEFINE_STATIC_KEY_FALSE(key);
If the key is not initialized, it is default false. The 'struct static_key',
must be a 'global'. That is, it can't be allocated on the stack or dynamically
The key must be global, that is, it can't be allocated on the stack or dynamically
allocated at run-time.
The key is then used in code as:
if (static_key_false(&key))
if (static_branch_unlikely(&key))
do unlikely code
else
do likely code
Or:
if (static_key_true(&key))
if (static_branch_likely(&key))
do likely code
else
do unlikely code
A key that is initialized via 'STATIC_KEY_INIT_FALSE', must be used in a
'static_key_false()' construct. Likewise, a key initialized via
'STATIC_KEY_INIT_TRUE' must be used in a 'static_key_true()' construct. A
single key can be used in many branches, but all the branches must match the
way that the key has been initialized.
Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may
be used in either static_branch_likely() or static_branch_unlikely()
statemnts.
The branch(es) can then be switched via:
Branch(es) can be set true via:
static_key_slow_inc(&key);
static_branch_enable(&key);
or false via:
static_branch_disable(&key);
The branch(es) can then be switched via reference counts:
static_branch_inc(&key);
...
static_key_slow_dec(&key);
static_branch_dec(&key);
Thus, 'static_key_slow_inc()' means 'make the branch true', and
'static_key_slow_dec()' means 'make the branch false' with appropriate
Thus, 'static_branch_inc()' means 'make the branch true', and
'static_branch_dec()' means 'make the branch false' with appropriate
reference counting. For example, if the key is initialized true, a
static_key_slow_dec(), will switch the branch to false. And a subsequent
static_key_slow_inc(), will change the branch back to true. Likewise, if the
key is initialized false, a 'static_key_slow_inc()', will change the branch to
true. And then a 'static_key_slow_dec()', will again make the branch false.
An example usage in the kernel is the implementation of tracepoints:
static inline void trace_##name(proto) \
{ \
if (static_key_false(&__tracepoint_##name.key)) \
__DO_TRACE(&__tracepoint_##name, \
TP_PROTO(data_proto), \
TP_ARGS(data_args), \
TP_CONDITION(cond)); \
}
Tracepoints are disabled by default, and can be placed in performance critical
pieces of the kernel. Thus, by using a static key, the tracepoints can have
absolutely minimal impact when not in use.
static_branch_dec(), will switch the branch to false. And a subsequent
static_branch_inc(), will change the branch back to true. Likewise, if the
key is initialized false, a 'static_branch_inc()', will change the branch to
true. And then a 'static_branch_dec()', will again make the branch false.
4) Architecture level code patching interface, 'jump labels'
@ -150,9 +152,12 @@ simply fall back to a traditional, load, test, and jump sequence.
* #define JUMP_LABEL_NOP_SIZE, see: arch/x86/include/asm/jump_label.h
* __always_inline bool arch_static_branch(struct static_key *key), see:
* __always_inline bool arch_static_branch(struct static_key *key, bool branch), see:
arch/x86/include/asm/jump_label.h
* __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch),
see: arch/x86/include/asm/jump_label.h
* void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type),
see: arch/x86/kernel/jump_label.c
@ -173,7 +178,7 @@ SYSCALL_DEFINE0(getppid)
{
int pid;
+ if (static_key_false(&key))
+ if (static_branch_unlikely(&key))
+ printk("I am the true branch\n");
rcu_read_lock();

67
include/linux/jump_label.h
View file

@ -7,17 +7,52 @@
* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
* Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
*
* DEPRECATED API:
*
* The use of 'struct static_key' directly, is now DEPRECATED. In addition
* static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
*
* struct static_key false = STATIC_KEY_INIT_FALSE;
* struct static_key true = STATIC_KEY_INIT_TRUE;
* static_key_true()
* static_key_false()
*
* The updated API replacements are:
*
* DEFINE_STATIC_KEY_TRUE(key);
* DEFINE_STATIC_KEY_FALSE(key);
* static_key_likely()
* statick_key_unlikely()
*
* Jump labels provide an interface to generate dynamic branches using
* self-modifying code. Assuming toolchain and architecture support, the result
* of a "if (static_key_false(&key))" statement is an unconditional branch (which
* defaults to false - and the true block is placed out of line).
* self-modifying code. Assuming toolchain and architecture support, if we
* define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
* an "if (static_branch_unlikely(&key))" statement is an unconditional branch
* (which defaults to false - and the true block is placed out of line).
* Similarly, we can define an initially true key via
* "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
* "if (static_branch_unlikely(&key))", in which case we will generate an
* unconditional branch to the out-of-line true branch. Keys that are
* initially true or false can be using in both static_branch_unlikely()
* and static_branch_likely() statements.
*
* 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.
* At runtime we can change the branch target by setting the key
* to true via a call to static_branch_enable(), or false using
* static_branch_disable(). If the direction of the branch is switched by
* these calls then we run-time modify the branch target via a
* no-op -> jump or jump -> no-op conversion. For example, for an
* initially false key that is used in an "if (static_branch_unlikely(&key))"
* statement, setting the key to true requires us to patch in a jump
* to the out-of-line of true branch.
*
* Since this relies on modifying code, the static_key_slow_{inc,dec}() functions
* In addtion to static_branch_{enable,disable}, we can also reference count
* the key or branch direction via static_branch_{inc,dec}. Thus,
* static_branch_inc() can be thought of as a 'make more true' and
* static_branch_dec() as a 'make more false'. The inc()/dec()
* interface is meant to be used exclusively from the inc()/dec() for a given
* key.
*
* Since this relies on modifying code, the branch modifying 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
@ -29,20 +64,10 @@
* cause significant performance degradation. Struct static_key_deferred and
* static_key_slow_dec_deferred() provide for this.
*
* Lacking toolchain and or architecture support, jump labels fall back to a simple
* conditional branch.
* Lacking toolchain and or architecture support, static keys fall 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.
* Additional babbling in: Documentation/static-keys.txt
*/
#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)