[mirror_ubuntu-artful-kernel.git] / Documentation / static-keys.txt

===========
Static Keys
===========

.. warning::

   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);
	DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
	DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
	static_branch_likely()
	static_branch_unlikely()

Abstract
========

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::

	DEFINE_STATIC_KEY_FALSE(key);

	...

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

	...
	static_branch_enable(&key);
	...
	static_branch_disable(&key);
	...

The static_branch_unlikely() branch will be generated into the code with as little
impact to the likely code path as possible.


Motivation
==========


Currently, tracepoints are implemented using a conditional branch. The
conditional check requires checking a global variable for each tracepoint.
Although the overhead of this check is small, it increases when the memory
cache comes under pressure (memory cache lines for these global variables may
be shared with other memory accesses). As we increase the number of tracepoints
in the kernel this overhead may become more of an issue. In addition,
tracepoints are often dormant (disabled) and provide no direct kernel
functionality. Thus, it is highly desirable to reduce their impact as much as
possible. Although tracepoints are the original motivation for this work, other
kernel code paths should be able to make use of the static keys facility.


Solution
========


gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label:

http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html

Using the 'asm goto', we can create branches that are either taken or not taken
by default, without the need to check memory. Then, at run-time, we can patch
the branch site to change the branch direction.

For example, if we have a simple branch that is disabled by default::

	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
consist of a single atomic 'no-op' instruction (5 bytes on x86), in the
straight-line code path. When the branch is 'flipped', we will patch the
'no-op' in the straight-line codepath with a 'jump' instruction to the
out-of-line true branch. Thus, changing branch direction is expensive but
branch selection is basically 'free'. That is the basic tradeoff of this
optimization.

This lowlevel patching mechanism is called 'jump label patching', and it gives
the basis for the static keys facility.

Static key label API, usage and examples
========================================


In order to make use of this optimization you must first define a key::

	DEFINE_STATIC_KEY_TRUE(key);

or::

	DEFINE_STATIC_KEY_FALSE(key);


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_branch_unlikely(&key))
                do unlikely code
        else
                do likely code

Or::

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

Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may
be used in either static_branch_likely() or static_branch_unlikely()
statements.

Branch(es) can be set true via::

	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_branch_dec(&key);

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_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.

Where an array of keys is required, it can be defined as::

	DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);

or::

	DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);

4) Architecture level code patching interface, 'jump labels'


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. 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

* ``#define JUMP_LABEL_NOP_SIZE``,
    see: arch/x86/include/asm/jump_label.h

* ``__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

* ``__init_or_module void arch_jump_label_transform_static(struct jump_entry *entry, enum jump_label_type type)``,
    see: arch/x86/kernel/jump_label.c

* ``struct jump_entry``,
    see: arch/x86/include/asm/jump_label.h


5) Static keys / jump label analysis, results (x86_64):


As an example, let's add the following branch to 'getppid()', such that the
system call now looks like::

  SYSCALL_DEFINE0(getppid)
  {
        int pid;

  +     if (static_branch_unlikely(&key))
  +             printk("I am the true branch\n");

        rcu_read_lock();
        pid = task_tgid_vnr(rcu_dereference(current->real_parent));
        rcu_read_unlock();

        return pid;
  }

The resulting instructions with jump labels generated by GCC is::

  ffffffff81044290 <sys_getppid>:
  ffffffff81044290:       55                      push   %rbp
  ffffffff81044291:       48 89 e5                mov    %rsp,%rbp
  ffffffff81044294:       e9 00 00 00 00          jmpq   ffffffff81044299 <sys_getppid+0x9>
  ffffffff81044299:       65 48 8b 04 25 c0 b6    mov    %gs:0xb6c0,%rax
  ffffffff810442a0:       00 00
  ffffffff810442a2:       48 8b 80 80 02 00 00    mov    0x280(%rax),%rax
  ffffffff810442a9:       48 8b 80 b0 02 00 00    mov    0x2b0(%rax),%rax
  ffffffff810442b0:       48 8b b8 e8 02 00 00    mov    0x2e8(%rax),%rdi
  ffffffff810442b7:       e8 f4 d9 00 00          callq  ffffffff81051cb0 <pid_vnr>
  ffffffff810442bc:       5d                      pop    %rbp
  ffffffff810442bd:       48 98                   cltq
  ffffffff810442bf:       c3                      retq
  ffffffff810442c0:       48 c7 c7 e3 54 98 81    mov    $0xffffffff819854e3,%rdi
  ffffffff810442c7:       31 c0                   xor    %eax,%eax
  ffffffff810442c9:       e8 71 13 6d 00          callq  ffffffff8171563f <printk>
  ffffffff810442ce:       eb c9                   jmp    ffffffff81044299 <sys_getppid+0x9>

Without the jump label optimization it looks like::

  ffffffff810441f0 <sys_getppid>:
  ffffffff810441f0:       8b 05 8a 52 d8 00       mov    0xd8528a(%rip),%eax        # ffffffff81dc9480 <key>
  ffffffff810441f6:       55                      push   %rbp
  ffffffff810441f7:       48 89 e5                mov    %rsp,%rbp
  ffffffff810441fa:       85 c0                   test   %eax,%eax
  ffffffff810441fc:       75 27                   jne    ffffffff81044225 <sys_getppid+0x35>
  ffffffff810441fe:       65 48 8b 04 25 c0 b6    mov    %gs:0xb6c0,%rax
  ffffffff81044205:       00 00
  ffffffff81044207:       48 8b 80 80 02 00 00    mov    0x280(%rax),%rax
  ffffffff8104420e:       48 8b 80 b0 02 00 00    mov    0x2b0(%rax),%rax
  ffffffff81044215:       48 8b b8 e8 02 00 00    mov    0x2e8(%rax),%rdi
  ffffffff8104421c:       e8 2f da 00 00          callq  ffffffff81051c50 <pid_vnr>
  ffffffff81044221:       5d                      pop    %rbp
  ffffffff81044222:       48 98                   cltq
  ffffffff81044224:       c3                      retq
  ffffffff81044225:       48 c7 c7 13 53 98 81    mov    $0xffffffff81985313,%rdi
  ffffffff8104422c:       31 c0                   xor    %eax,%eax
  ffffffff8104422e:       e8 60 0f 6d 00          callq  ffffffff81715193 <printk>
  ffffffff81044233:       eb c9                   jmp    ffffffff810441fe <sys_getppid+0xe>
  ffffffff81044235:       66 66 2e 0f 1f 84 00    data32 nopw %cs:0x0(%rax,%rax,1)
  ffffffff8104423c:       00 00 00 00

Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction
vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched
to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump
label case adds::

  6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.

If we then include the padding bytes, the jump label code saves, 16 total bytes
of instruction memory for this small function. In this case the non-jump label
function is 80 bytes long. Thus, we have saved 20% of the instruction
footprint. We can in fact improve this even further, since the 5-byte no-op
really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp.
However, we have not yet implemented optimal no-op sizes (they are currently
hard-coded).

Since there are a number of static key API uses in the scheduler paths,
'pipe-test' (also known as 'perf bench sched pipe') can be used to show the
performance improvement. Testing done on 3.3.0-rc2:

jump label disabled::

 Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):

        855.700314 task-clock                #    0.534 CPUs utilized            ( +-  0.11% )
           200,003 context-switches          #    0.234 M/sec                    ( +-  0.00% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +- 39.58% )
               487 page-faults               #    0.001 M/sec                    ( +-  0.02% )
     1,474,374,262 cycles                    #    1.723 GHz                      ( +-  0.17% )
   <not supported> stalled-cycles-frontend
   <not supported> stalled-cycles-backend
     1,178,049,567 instructions              #    0.80  insns per cycle          ( +-  0.06% )
       208,368,926 branches                  #  243.507 M/sec                    ( +-  0.06% )
         5,569,188 branch-misses             #    2.67% of all branches          ( +-  0.54% )

       1.601607384 seconds time elapsed                                          ( +-  0.07% )

jump label enabled::

 Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):

        841.043185 task-clock                #    0.533 CPUs utilized            ( +-  0.12% )
           200,004 context-switches          #    0.238 M/sec                    ( +-  0.00% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +- 40.87% )
               487 page-faults               #    0.001 M/sec                    ( +-  0.05% )
     1,432,559,428 cycles                    #    1.703 GHz                      ( +-  0.18% )
   <not supported> stalled-cycles-frontend
   <not supported> stalled-cycles-backend
     1,175,363,994 instructions              #    0.82  insns per cycle          ( +-  0.04% )
       206,859,359 branches                  #  245.956 M/sec                    ( +-  0.04% )
         4,884,119 branch-misses             #    2.36% of all branches          ( +-  0.85% )

       1.579384366 seconds time elapsed

The percentage of saved branches is .7%, and we've saved 12% on
'branch-misses'. This is where we would expect to get the most savings, since
this optimization is about reducing the number of branches. In addition, we've
saved .2% on instructions, and 2.8% on cycles and 1.4% on elapsed time.
Commit	Line	Data
603699bb MCC	1	===========
	2	Static Keys
	3	===========
1cfa60dc	4
603699bb	5	.. warning::
412758cb	6
603699bb	7	DEPRECATED API:
412758cb	8
603699bb MCC	9	The use of 'struct static_key' directly, is now DEPRECATED. In addition
603699bb MCC	10	static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following::
412758cb	11
603699bb MCC	12	struct static_key false = STATIC_KEY_INIT_FALSE;
	13	struct static_key true = STATIC_KEY_INIT_TRUE;
	14	static_key_true()
	15	static_key_false()
412758cb	16
603699bb	17	The updated API replacements are::
1cfa60dc	18
603699bb MCC	19	DEFINE_STATIC_KEY_TRUE(key);
	20	DEFINE_STATIC_KEY_FALSE(key);
	21	DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
	22	DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
	23	static_branch_likely()
	24	static_branch_unlikely()
	25
	26	Abstract
	27	========
1cfa60dc JB	28
	29	Static keys allows the inclusion of seldom used features in
	30	performance-sensitive fast-path kernel code, via a GCC feature and a code
603699bb	31	patching technique. A quick example::
1cfa60dc	32
412758cb	33	DEFINE_STATIC_KEY_FALSE(key);
1cfa60dc JB	34
	35	...
	36
412758cb	37	if (static_branch_unlikely(&key))
1cfa60dc JB	38	do unlikely code
	39	else
	40	do likely code
	41
	42	...
412758cb	43	static_branch_enable(&key);
1cfa60dc	44	...
412758cb	45	static_branch_disable(&key);
1cfa60dc JB	46	...
1cfa60dc JB	47
412758cb	48	The static_branch_unlikely() branch will be generated into the code with as little
1cfa60dc JB	49	impact to the likely code path as possible.
	50
	51
603699bb MCC	52	Motivation
603699bb MCC	53	==========
1cfa60dc JB	54
	55
	56	Currently, tracepoints are implemented using a conditional branch. The
	57	conditional check requires checking a global variable for each tracepoint.
	58	Although the overhead of this check is small, it increases when the memory
	59	cache comes under pressure (memory cache lines for these global variables may
	60	be shared with other memory accesses). As we increase the number of tracepoints
	61	in the kernel this overhead may become more of an issue. In addition,
	62	tracepoints are often dormant (disabled) and provide no direct kernel
	63	functionality. Thus, it is highly desirable to reduce their impact as much as
	64	possible. Although tracepoints are the original motivation for this work, other
	65	kernel code paths should be able to make use of the static keys facility.
	66
	67
603699bb MCC	68	Solution
603699bb MCC	69	========
1cfa60dc JB	70
	71
	72	gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label:
	73
	74	http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
	75
	76	Using the 'asm goto', we can create branches that are either taken or not taken
	77	by default, without the need to check memory. Then, at run-time, we can patch
	78	the branch site to change the branch direction.
	79
603699bb	80	For example, if we have a simple branch that is disabled by default::
1cfa60dc	81
412758cb	82	if (static_branch_unlikely(&key))
1cfa60dc JB	83	printk("I am the true branch\n");
	84
	85	Thus, by default the 'printk' will not be emitted. And the code generated will
	86	consist of a single atomic 'no-op' instruction (5 bytes on x86), in the
	87	straight-line code path. When the branch is 'flipped', we will patch the
	88	'no-op' in the straight-line codepath with a 'jump' instruction to the
	89	out-of-line true branch. Thus, changing branch direction is expensive but
	90	branch selection is basically 'free'. That is the basic tradeoff of this
	91	optimization.
	92
	93	This lowlevel patching mechanism is called 'jump label patching', and it gives
	94	the basis for the static keys facility.
	95
603699bb MCC	96	Static key label API, usage and examples
603699bb MCC	97	========================================
1cfa60dc JB	98
1cfa60dc JB	99
603699bb	100	In order to make use of this optimization you must first define a key::
1cfa60dc	101
412758cb	102	DEFINE_STATIC_KEY_TRUE(key);
1cfa60dc	103
603699bb	104	or::
1cfa60dc	105
412758cb JB	106	DEFINE_STATIC_KEY_FALSE(key);
412758cb JB	107
1cfa60dc	108
412758cb	109	The key must be global, that is, it can't be allocated on the stack or dynamically
1cfa60dc JB	110	allocated at run-time.
1cfa60dc JB	111
603699bb	112	The key is then used in code as::
1cfa60dc	113
412758cb	114	if (static_branch_unlikely(&key))
1cfa60dc JB	115	do unlikely code
	116	else
	117	do likely code
	118
603699bb	119	Or::
1cfa60dc	120
412758cb	121	if (static_branch_likely(&key))
1cfa60dc JB	122	do likely code
	123	else
	124	do unlikely code
	125
412758cb JB	126	Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may
412758cb JB	127	be used in either static_branch_likely() or static_branch_unlikely()
9bb0e9cb	128	statements.
1cfa60dc	129
603699bb	130	Branch(es) can be set true via::
1cfa60dc	131
603699bb	132	static_branch_enable(&key);
1cfa60dc	133
603699bb	134	or false via::
412758cb	135
603699bb	136	static_branch_disable(&key);
1cfa60dc	137
603699bb	138	The branch(es) can then be switched via reference counts::
1cfa60dc	139
412758cb JB	140	static_branch_inc(&key);
	141	...
	142	static_branch_dec(&key);
1cfa60dc	143
412758cb JB	144	Thus, 'static_branch_inc()' means 'make the branch true', and
	145	'static_branch_dec()' means 'make the branch false' with appropriate
	146	reference counting. For example, if the key is initialized true, a
	147	static_branch_dec(), will switch the branch to false. And a subsequent
	148	static_branch_inc(), will change the branch back to true. Likewise, if the
	149	key is initialized false, a 'static_branch_inc()', will change the branch to
	150	true. And then a 'static_branch_dec()', will again make the branch false.
1cfa60dc	151
603699bb	152	Where an array of keys is required, it can be defined as::
ef0da55a CM	153
	154	DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
	155
603699bb	156	or::
ef0da55a CM	157
ef0da55a CM	158	DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count);
1cfa60dc JB	159
	160	4) Architecture level code patching interface, 'jump labels'
	161
	162
	163	There are a few functions and macros that architectures must implement in order
	164	to take advantage of this optimization. If there is no architecture support, we
3821fd35 JB	165	simply fall back to a traditional, load, test, and jump sequence. Also, the
	166	struct jump_entry table must be at least 4-byte aligned because the
	167	static_key->entry field makes use of the two least significant bits.
1cfa60dc	168
603699bb MCC	169	* ``select HAVE_ARCH_JUMP_LABEL``,
603699bb MCC	170	see: arch/x86/Kconfig
1cfa60dc	171
603699bb MCC	172	* ``#define JUMP_LABEL_NOP_SIZE``,
603699bb MCC	173	see: arch/x86/include/asm/jump_label.h
1cfa60dc	174
603699bb MCC	175	* ``__always_inline bool arch_static_branch(struct static_key *key, bool branch)``,
603699bb MCC	176	see: arch/x86/include/asm/jump_label.h
412758cb	177
603699bb MCC	178	* ``__always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)``,
603699bb MCC	179	see: arch/x86/include/asm/jump_label.h
1cfa60dc	180
603699bb MCC	181	* ``void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type)``,
603699bb MCC	182	see: arch/x86/kernel/jump_label.c
1cfa60dc	183
603699bb MCC	184	* ``__init_or_module void arch_jump_label_transform_static(struct jump_entry *entry, enum jump_label_type type)``,
603699bb MCC	185	see: arch/x86/kernel/jump_label.c
1cfa60dc	186
603699bb MCC	187	* ``struct jump_entry``,
603699bb MCC	188	see: arch/x86/include/asm/jump_label.h
1cfa60dc JB	189
	190
	191	5) Static keys / jump label analysis, results (x86_64):
	192
	193
	194	As an example, let's add the following branch to 'getppid()', such that the
603699bb	195	system call now looks like::
1cfa60dc	196
603699bb MCC	197	SYSCALL_DEFINE0(getppid)
603699bb MCC	198	{
1cfa60dc JB	199	int pid;
1cfa60dc JB	200
603699bb MCC	201	+ if (static_branch_unlikely(&key))
603699bb MCC	202	+ printk("I am the true branch\n");
1cfa60dc JB	203
	204	rcu_read_lock();
	205	pid = task_tgid_vnr(rcu_dereference(current->real_parent));
	206	rcu_read_unlock();
	207
	208	return pid;
603699bb MCC	209	}
	210
	211	The resulting instructions with jump labels generated by GCC is::
	212
	213	ffffffff81044290 <sys_getppid>:
	214	ffffffff81044290: 55 push %rbp
	215	ffffffff81044291: 48 89 e5 mov %rsp,%rbp
	216	ffffffff81044294: e9 00 00 00 00 jmpq ffffffff81044299 <sys_getppid+0x9>
	217	ffffffff81044299: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
	218	ffffffff810442a0: 00 00
	219	ffffffff810442a2: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
	220	ffffffff810442a9: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
	221	ffffffff810442b0: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
	222	ffffffff810442b7: e8 f4 d9 00 00 callq ffffffff81051cb0 <pid_vnr>
	223	ffffffff810442bc: 5d pop %rbp
	224	ffffffff810442bd: 48 98 cltq
	225	ffffffff810442bf: c3 retq
	226	ffffffff810442c0: 48 c7 c7 e3 54 98 81 mov $0xffffffff819854e3,%rdi
	227	ffffffff810442c7: 31 c0 xor %eax,%eax
	228	ffffffff810442c9: e8 71 13 6d 00 callq ffffffff8171563f <printk>
	229	ffffffff810442ce: eb c9 jmp ffffffff81044299 <sys_getppid+0x9>
	230
	231	Without the jump label optimization it looks like::
	232
	233	ffffffff810441f0 <sys_getppid>:
	234	ffffffff810441f0: 8b 05 8a 52 d8 00 mov 0xd8528a(%rip),%eax # ffffffff81dc9480 <key>
	235	ffffffff810441f6: 55 push %rbp
	236	ffffffff810441f7: 48 89 e5 mov %rsp,%rbp
	237	ffffffff810441fa: 85 c0 test %eax,%eax
	238	ffffffff810441fc: 75 27 jne ffffffff81044225 <sys_getppid+0x35>
	239	ffffffff810441fe: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
	240	ffffffff81044205: 00 00
	241	ffffffff81044207: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
	242	ffffffff8104420e: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
	243	ffffffff81044215: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
	244	ffffffff8104421c: e8 2f da 00 00 callq ffffffff81051c50 <pid_vnr>
	245	ffffffff81044221: 5d pop %rbp
	246	ffffffff81044222: 48 98 cltq
	247	ffffffff81044224: c3 retq
	248	ffffffff81044225: 48 c7 c7 13 53 98 81 mov $0xffffffff81985313,%rdi
	249	ffffffff8104422c: 31 c0 xor %eax,%eax
	250	ffffffff8104422e: e8 60 0f 6d 00 callq ffffffff81715193 <printk>
	251	ffffffff81044233: eb c9 jmp ffffffff810441fe <sys_getppid+0xe>
	252	ffffffff81044235: 66 66 2e 0f 1f 84 00 data32 nopw %cs:0x0(%rax,%rax,1)
	253	ffffffff8104423c: 00 00 00 00
1cfa60dc JB	254
	255	Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction
	256	vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched
	257	to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump
603699bb	258	label case adds::
1cfa60dc	259
603699bb	260	6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.
1cfa60dc JB	261
1cfa60dc JB	262	If we then include the padding bytes, the jump label code saves, 16 total bytes
c94bed8e	263	of instruction memory for this small function. In this case the non-jump label
c79a8d85	264	function is 80 bytes long. Thus, we have saved 20% of the instruction
1cfa60dc JB	265	footprint. We can in fact improve this even further, since the 5-byte no-op
	266	really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp.
	267	However, we have not yet implemented optimal no-op sizes (they are currently
	268	hard-coded).
	269
	270	Since there are a number of static key API uses in the scheduler paths,
	271	'pipe-test' (also known as 'perf bench sched pipe') can be used to show the
	272	performance improvement. Testing done on 3.3.0-rc2:
	273
603699bb	274	jump label disabled::
1cfa60dc JB	275
	276	Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
	277
	278	855.700314 task-clock # 0.534 CPUs utilized ( +- 0.11% )
	279	200,003 context-switches # 0.234 M/sec ( +- 0.00% )
	280	0 CPU-migrations # 0.000 M/sec ( +- 39.58% )
	281	487 page-faults # 0.001 M/sec ( +- 0.02% )
	282	1,474,374,262 cycles # 1.723 GHz ( +- 0.17% )
	283	<not supported> stalled-cycles-frontend
	284	<not supported> stalled-cycles-backend
	285	1,178,049,567 instructions # 0.80 insns per cycle ( +- 0.06% )
	286	208,368,926 branches # 243.507 M/sec ( +- 0.06% )
	287	5,569,188 branch-misses # 2.67% of all branches ( +- 0.54% )
	288
	289	1.601607384 seconds time elapsed ( +- 0.07% )
	290
603699bb	291	jump label enabled::
1cfa60dc JB	292
	293	Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs):
	294
	295	841.043185 task-clock # 0.533 CPUs utilized ( +- 0.12% )
	296	200,004 context-switches # 0.238 M/sec ( +- 0.00% )
	297	0 CPU-migrations # 0.000 M/sec ( +- 40.87% )
	298	487 page-faults # 0.001 M/sec ( +- 0.05% )
	299	1,432,559,428 cycles # 1.703 GHz ( +- 0.18% )
	300	<not supported> stalled-cycles-frontend
	301	<not supported> stalled-cycles-backend
	302	1,175,363,994 instructions # 0.82 insns per cycle ( +- 0.04% )
	303	206,859,359 branches # 245.956 M/sec ( +- 0.04% )
	304	4,884,119 branch-misses # 2.36% of all branches ( +- 0.85% )
	305
	306	1.579384366 seconds time elapsed
	307
	308	The percentage of saved branches is .7%, and we've saved 12% on
	309	'branch-misses'. This is where we would expect to get the most savings, since
	310	this optimization is about reducing the number of branches. In addition, we've
	311	saved .2% on instructions, and 2.8% on cycles and 1.4% on elapsed time.