[PATCH 3.16 16/86] jump label, locking/static_keys: Update docs

Ben Hutchings
2019-05-16T16:02:01Z

From: Jason Baron <jbaron@akamai.com>

commit 412758cb26704e5087ca2976ec3b28fb2bdbfad4 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
Documentation/static-keys.txt | 99 ++++++++++++++++++-----------------
include/linux/jump_label.h | 67 ++++++++++++++++--------
2 files changed, 98 insertions(+), 68 deletions(-)

--- a/Documentation/static-keys.txt
+++ b/Documentation/static-keys.txt
@@ -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 seld
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 dir

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,

* #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();
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -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).
- *
- * 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.
+ * 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.
+ *
+ * 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.
+ *
+ * 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 static_key_slow_{inc,dec}() functions
+ * 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.
- *
- * 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.
+ * Lacking toolchain and or architecture support, static keys fall back to a
+ * simple conditional branch.
*
- * 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)