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