jump label, locking/static_keys: Update docs
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>
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Jason Baron
Ingo Molnar
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@ -1,7 +1,22 @@
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Static Keys
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-----------
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By: Jason Baron <jbaron@redhat.com>
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DEPRECATED API:
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The use of 'struct static_key' directly, is now DEPRECATED. In addition
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static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
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struct static_key false = STATIC_KEY_INIT_FALSE;
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struct static_key true = STATIC_KEY_INIT_TRUE;
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static_key_true()
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static_key_false()
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The updated API replacements are:
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DEFINE_STATIC_KEY_TRUE(key);
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DEFINE_STATIC_KEY_FALSE(key);
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static_key_likely()
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statick_key_unlikely()
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0) Abstract
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@ -9,22 +24,22 @@ Static keys allows the inclusion of seldom used features in
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performance-sensitive fast-path kernel code, via a GCC feature and a code
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patching technique. A quick example:
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struct static_key key = STATIC_KEY_INIT_FALSE;
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DEFINE_STATIC_KEY_FALSE(key);
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...
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if (static_key_false(&key))
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if (static_branch_unlikely(&key))
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do unlikely code
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else
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do likely code
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...
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static_key_slow_inc();
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static_branch_enable(&key);
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...
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static_key_slow_inc();
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static_branch_disable(&key);
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...
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The static_key_false() branch will be generated into the code with as little
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The static_branch_unlikely() branch will be generated into the code with as little
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impact to the likely code path as possible.
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@ -56,7 +71,7 @@ the branch site to change the branch direction.
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For example, if we have a simple branch that is disabled by default:
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if (static_key_false(&key))
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if (static_branch_unlikely(&key))
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printk("I am the true branch\n");
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Thus, by default the 'printk' will not be emitted. And the code generated will
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@ -75,68 +90,55 @@ the basis for the static keys facility.
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In order to make use of this optimization you must first define a key:
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struct static_key key;
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Which is initialized as:
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struct static_key key = STATIC_KEY_INIT_TRUE;
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DEFINE_STATIC_KEY_TRUE(key);
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or:
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struct static_key key = STATIC_KEY_INIT_FALSE;
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DEFINE_STATIC_KEY_FALSE(key);
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If the key is not initialized, it is default false. The 'struct static_key',
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must be a 'global'. That is, it can't be allocated on the stack or dynamically
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The key must be global, that is, it can't be allocated on the stack or dynamically
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allocated at run-time.
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The key is then used in code as:
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if (static_key_false(&key))
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if (static_branch_unlikely(&key))
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do unlikely code
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else
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do likely code
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Or:
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if (static_key_true(&key))
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if (static_branch_likely(&key))
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do likely code
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else
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do unlikely code
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A key that is initialized via 'STATIC_KEY_INIT_FALSE', must be used in a
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'static_key_false()' construct. Likewise, a key initialized via
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'STATIC_KEY_INIT_TRUE' must be used in a 'static_key_true()' construct. A
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single key can be used in many branches, but all the branches must match the
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way that the key has been initialized.
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Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may
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be used in either static_branch_likely() or static_branch_unlikely()
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statemnts.
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The branch(es) can then be switched via:
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Branch(es) can be set true via:
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static_key_slow_inc(&key);
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static_branch_enable(&key);
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or false via:
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static_branch_disable(&key);
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The branch(es) can then be switched via reference counts:
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static_branch_inc(&key);
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...
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static_key_slow_dec(&key);
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static_branch_dec(&key);
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Thus, 'static_key_slow_inc()' means 'make the branch true', and
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'static_key_slow_dec()' means 'make the branch false' with appropriate
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Thus, 'static_branch_inc()' means 'make the branch true', and
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'static_branch_dec()' means 'make the branch false' with appropriate
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reference counting. For example, if the key is initialized true, a
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static_key_slow_dec(), will switch the branch to false. And a subsequent
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static_key_slow_inc(), will change the branch back to true. Likewise, if the
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key is initialized false, a 'static_key_slow_inc()', will change the branch to
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true. And then a 'static_key_slow_dec()', will again make the branch false.
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An example usage in the kernel is the implementation of tracepoints:
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static inline void trace_##name(proto) \
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{ \
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if (static_key_false(&__tracepoint_##name.key)) \
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__DO_TRACE(&__tracepoint_##name, \
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TP_PROTO(data_proto), \
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TP_ARGS(data_args), \
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TP_CONDITION(cond)); \
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}
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Tracepoints are disabled by default, and can be placed in performance critical
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pieces of the kernel. Thus, by using a static key, the tracepoints can have
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absolutely minimal impact when not in use.
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static_branch_dec(), will switch the branch to false. And a subsequent
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static_branch_inc(), will change the branch back to true. Likewise, if the
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key is initialized false, a 'static_branch_inc()', will change the branch to
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true. And then a 'static_branch_dec()', will again make the branch false.
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4) Architecture level code patching interface, 'jump labels'
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@ -150,9 +152,12 @@ simply fall back to a traditional, load, test, and jump sequence.
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* #define JUMP_LABEL_NOP_SIZE, see: arch/x86/include/asm/jump_label.h
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* __always_inline bool arch_static_branch(struct static_key *key), see:
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* __always_inline bool arch_static_branch(struct static_key *key, bool branch), see:
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arch/x86/include/asm/jump_label.h
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* __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch),
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see: arch/x86/include/asm/jump_label.h
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* void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type),
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see: arch/x86/kernel/jump_label.c
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@ -173,7 +178,7 @@ SYSCALL_DEFINE0(getppid)
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{
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int pid;
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+ if (static_key_false(&key))
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+ if (static_branch_unlikely(&key))
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+ printk("I am the true branch\n");
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rcu_read_lock();
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@ -7,17 +7,52 @@
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* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
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* Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
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*
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* DEPRECATED API:
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*
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* The use of 'struct static_key' directly, is now DEPRECATED. In addition
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* static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
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*
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* struct static_key false = STATIC_KEY_INIT_FALSE;
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* struct static_key true = STATIC_KEY_INIT_TRUE;
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* static_key_true()
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* static_key_false()
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*
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* The updated API replacements are:
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*
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* DEFINE_STATIC_KEY_TRUE(key);
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* DEFINE_STATIC_KEY_FALSE(key);
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* static_key_likely()
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* statick_key_unlikely()
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*
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* Jump labels provide an interface to generate dynamic branches using
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* self-modifying code. Assuming toolchain and architecture support, the result
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* of a "if (static_key_false(&key))" statement is an unconditional branch (which
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* defaults to false - and the true block is placed out of line).
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* self-modifying code. Assuming toolchain and architecture support, if we
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* define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
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* an "if (static_branch_unlikely(&key))" statement is an unconditional branch
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* (which defaults to false - and the true block is placed out of line).
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* Similarly, we can define an initially true key via
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* "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
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* "if (static_branch_unlikely(&key))", in which case we will generate an
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* unconditional branch to the out-of-line true branch. Keys that are
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* initially true or false can be using in both static_branch_unlikely()
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* and static_branch_likely() statements.
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*
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* However at runtime we can change the branch target using
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* static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
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* object, and for as long as there are references all branches referring to
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* that particular key will point to the (out of line) true block.
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* At runtime we can change the branch target by setting the key
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* to true via a call to static_branch_enable(), or false using
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* static_branch_disable(). If the direction of the branch is switched by
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* these calls then we run-time modify the branch target via a
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* no-op -> jump or jump -> no-op conversion. For example, for an
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* initially false key that is used in an "if (static_branch_unlikely(&key))"
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* statement, setting the key to true requires us to patch in a jump
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* to the out-of-line of true branch.
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*
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* Since this relies on modifying code, the static_key_slow_{inc,dec}() functions
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* In addtion to static_branch_{enable,disable}, we can also reference count
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* the key or branch direction via static_branch_{inc,dec}. Thus,
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* static_branch_inc() can be thought of as a 'make more true' and
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* static_branch_dec() as a 'make more false'. The inc()/dec()
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* interface is meant to be used exclusively from the inc()/dec() for a given
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* key.
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*
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* Since this relies on modifying code, the branch modifying functions
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* must be considered absolute slow paths (machine wide synchronization etc.).
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* OTOH, since the affected branches are unconditional, their runtime overhead
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* will be absolutely minimal, esp. in the default (off) case where the total
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@ -29,20 +64,10 @@
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* cause significant performance degradation. Struct static_key_deferred and
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* static_key_slow_dec_deferred() provide for this.
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*
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* Lacking toolchain and or architecture support, jump labels fall back to a simple
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* conditional branch.
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* Lacking toolchain and or architecture support, static keys fall back to a
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* simple conditional branch.
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*
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* struct static_key my_key = STATIC_KEY_INIT_TRUE;
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*
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* if (static_key_true(&my_key)) {
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* }
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*
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* will result in the true case being in-line and starts the key with a single
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* reference. Mixing static_key_true() and static_key_false() on the same key is not
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* allowed.
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*
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* Not initializing the key (static data is initialized to 0s anyway) is the
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* same as using STATIC_KEY_INIT_FALSE.
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* Additional babbling in: Documentation/static-keys.txt
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*/
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#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
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