1=========== 2Static Keys 3=========== 4 5.. warning:: 6 7 DEPRECATED API: 8 9 The use of 'struct static_key' directly, is now DEPRECATED. In addition 10 static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:: 11 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() 16 17 The updated API replacements are:: 18 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 26Abstract 27======== 28 29Static keys allows the inclusion of seldom used features in 30performance-sensitive fast-path kernel code, via a GCC feature and a code 31patching technique. A quick example:: 32 33 DEFINE_STATIC_KEY_FALSE(key); 34 35 ... 36 37 if (static_branch_unlikely(&key)) 38 do unlikely code 39 else 40 do likely code 41 42 ... 43 static_branch_enable(&key); 44 ... 45 static_branch_disable(&key); 46 ... 47 48The static_branch_unlikely() branch will be generated into the code with as little 49impact to the likely code path as possible. 50 51 52Motivation 53========== 54 55 56Currently, tracepoints are implemented using a conditional branch. The 57conditional check requires checking a global variable for each tracepoint. 58Although the overhead of this check is small, it increases when the memory 59cache comes under pressure (memory cache lines for these global variables may 60be shared with other memory accesses). As we increase the number of tracepoints 61in the kernel this overhead may become more of an issue. In addition, 62tracepoints are often dormant (disabled) and provide no direct kernel 63functionality. Thus, it is highly desirable to reduce their impact as much as 64possible. Although tracepoints are the original motivation for this work, other 65kernel code paths should be able to make use of the static keys facility. 66 67 68Solution 69======== 70 71 72gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label: 73 74http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html 75 76Using the 'asm goto', we can create branches that are either taken or not taken 77by default, without the need to check memory. Then, at run-time, we can patch 78the branch site to change the branch direction. 79 80For example, if we have a simple branch that is disabled by default:: 81 82 if (static_branch_unlikely(&key)) 83 printk("I am the true branch\n"); 84 85Thus, by default the 'printk' will not be emitted. And the code generated will 86consist of a single atomic 'no-op' instruction (5 bytes on x86), in the 87straight-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 89out-of-line true branch. Thus, changing branch direction is expensive but 90branch selection is basically 'free'. That is the basic tradeoff of this 91optimization. 92 93This lowlevel patching mechanism is called 'jump label patching', and it gives 94the basis for the static keys facility. 95 96Static key label API, usage and examples 97======================================== 98 99 100In order to make use of this optimization you must first define a key:: 101 102 DEFINE_STATIC_KEY_TRUE(key); 103 104or:: 105 106 DEFINE_STATIC_KEY_FALSE(key); 107 108 109The key must be global, that is, it can't be allocated on the stack or dynamically 110allocated at run-time. 111 112The key is then used in code as:: 113 114 if (static_branch_unlikely(&key)) 115 do unlikely code 116 else 117 do likely code 118 119Or:: 120 121 if (static_branch_likely(&key)) 122 do likely code 123 else 124 do unlikely code 125 126Keys defined via DEFINE_STATIC_KEY_TRUE(), or DEFINE_STATIC_KEY_FALSE, may 127be used in either static_branch_likely() or static_branch_unlikely() 128statements. 129 130Branch(es) can be set true via:: 131 132 static_branch_enable(&key); 133 134or false via:: 135 136 static_branch_disable(&key); 137 138The branch(es) can then be switched via reference counts:: 139 140 static_branch_inc(&key); 141 ... 142 static_branch_dec(&key); 143 144Thus, 'static_branch_inc()' means 'make the branch true', and 145'static_branch_dec()' means 'make the branch false' with appropriate 146reference counting. For example, if the key is initialized true, a 147static_branch_dec(), will switch the branch to false. And a subsequent 148static_branch_inc(), will change the branch back to true. Likewise, if the 149key is initialized false, a 'static_branch_inc()', will change the branch to 150true. And then a 'static_branch_dec()', will again make the branch false. 151 152The state and the reference count can be retrieved with 'static_key_enabled()' 153and 'static_key_count()'. In general, if you use these functions, they 154should be protected with the same mutex used around the enable/disable 155or increment/decrement function. 156 157Note that switching branches results in some locks being taken, 158particularly the CPU hotplug lock (in order to avoid races against 159CPUs being brought in the kernel while the kernel is getting 160patched). Calling the static key API from within a hotplug notifier is 161thus a sure deadlock recipe. In order to still allow use of the 162functionality, the following functions are provided: 163 164 static_key_enable_cpuslocked() 165 static_key_disable_cpuslocked() 166 static_branch_enable_cpuslocked() 167 static_branch_disable_cpuslocked() 168 169These functions are *not* general purpose, and must only be used when 170you really know that you're in the above context, and no other. 171 172Where an array of keys is required, it can be defined as:: 173 174 DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count); 175 176or:: 177 178 DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count); 179 1804) Architecture level code patching interface, 'jump labels' 181 182 183There are a few functions and macros that architectures must implement in order 184to take advantage of this optimization. If there is no architecture support, we 185simply fall back to a traditional, load, test, and jump sequence. Also, the 186struct jump_entry table must be at least 4-byte aligned because the 187static_key->entry field makes use of the two least significant bits. 188 189* ``select HAVE_ARCH_JUMP_LABEL``, 190 see: arch/x86/Kconfig 191 192* ``#define JUMP_LABEL_NOP_SIZE``, 193 see: arch/x86/include/asm/jump_label.h 194 195* ``__always_inline bool arch_static_branch(struct static_key *key, bool branch)``, 196 see: arch/x86/include/asm/jump_label.h 197 198* ``__always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)``, 199 see: arch/x86/include/asm/jump_label.h 200 201* ``void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type)``, 202 see: arch/x86/kernel/jump_label.c 203 204* ``__init_or_module void arch_jump_label_transform_static(struct jump_entry *entry, enum jump_label_type type)``, 205 see: arch/x86/kernel/jump_label.c 206 207* ``struct jump_entry``, 208 see: arch/x86/include/asm/jump_label.h 209 210 2115) Static keys / jump label analysis, results (x86_64): 212 213 214As an example, let's add the following branch to 'getppid()', such that the 215system call now looks like:: 216 217 SYSCALL_DEFINE0(getppid) 218 { 219 int pid; 220 221 + if (static_branch_unlikely(&key)) 222 + printk("I am the true branch\n"); 223 224 rcu_read_lock(); 225 pid = task_tgid_vnr(rcu_dereference(current->real_parent)); 226 rcu_read_unlock(); 227 228 return pid; 229 } 230 231The resulting instructions with jump labels generated by GCC is:: 232 233 ffffffff81044290 <sys_getppid>: 234 ffffffff81044290: 55 push %rbp 235 ffffffff81044291: 48 89 e5 mov %rsp,%rbp 236 ffffffff81044294: e9 00 00 00 00 jmpq ffffffff81044299 <sys_getppid+0x9> 237 ffffffff81044299: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax 238 ffffffff810442a0: 00 00 239 ffffffff810442a2: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax 240 ffffffff810442a9: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax 241 ffffffff810442b0: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi 242 ffffffff810442b7: e8 f4 d9 00 00 callq ffffffff81051cb0 <pid_vnr> 243 ffffffff810442bc: 5d pop %rbp 244 ffffffff810442bd: 48 98 cltq 245 ffffffff810442bf: c3 retq 246 ffffffff810442c0: 48 c7 c7 e3 54 98 81 mov $0xffffffff819854e3,%rdi 247 ffffffff810442c7: 31 c0 xor %eax,%eax 248 ffffffff810442c9: e8 71 13 6d 00 callq ffffffff8171563f <printk> 249 ffffffff810442ce: eb c9 jmp ffffffff81044299 <sys_getppid+0x9> 250 251Without the jump label optimization it looks like:: 252 253 ffffffff810441f0 <sys_getppid>: 254 ffffffff810441f0: 8b 05 8a 52 d8 00 mov 0xd8528a(%rip),%eax # ffffffff81dc9480 <key> 255 ffffffff810441f6: 55 push %rbp 256 ffffffff810441f7: 48 89 e5 mov %rsp,%rbp 257 ffffffff810441fa: 85 c0 test %eax,%eax 258 ffffffff810441fc: 75 27 jne ffffffff81044225 <sys_getppid+0x35> 259 ffffffff810441fe: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax 260 ffffffff81044205: 00 00 261 ffffffff81044207: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax 262 ffffffff8104420e: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax 263 ffffffff81044215: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi 264 ffffffff8104421c: e8 2f da 00 00 callq ffffffff81051c50 <pid_vnr> 265 ffffffff81044221: 5d pop %rbp 266 ffffffff81044222: 48 98 cltq 267 ffffffff81044224: c3 retq 268 ffffffff81044225: 48 c7 c7 13 53 98 81 mov $0xffffffff81985313,%rdi 269 ffffffff8104422c: 31 c0 xor %eax,%eax 270 ffffffff8104422e: e8 60 0f 6d 00 callq ffffffff81715193 <printk> 271 ffffffff81044233: eb c9 jmp ffffffff810441fe <sys_getppid+0xe> 272 ffffffff81044235: 66 66 2e 0f 1f 84 00 data32 nopw %cs:0x0(%rax,%rax,1) 273 ffffffff8104423c: 00 00 00 00 274 275Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction 276vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched 277to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump 278label case adds:: 279 280 6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes. 281 282If we then include the padding bytes, the jump label code saves, 16 total bytes 283of instruction memory for this small function. In this case the non-jump label 284function is 80 bytes long. Thus, we have saved 20% of the instruction 285footprint. We can in fact improve this even further, since the 5-byte no-op 286really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp. 287However, we have not yet implemented optimal no-op sizes (they are currently 288hard-coded). 289 290Since there are a number of static key API uses in the scheduler paths, 291'pipe-test' (also known as 'perf bench sched pipe') can be used to show the 292performance improvement. Testing done on 3.3.0-rc2: 293 294jump label disabled:: 295 296 Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs): 297 298 855.700314 task-clock # 0.534 CPUs utilized ( +- 0.11% ) 299 200,003 context-switches # 0.234 M/sec ( +- 0.00% ) 300 0 CPU-migrations # 0.000 M/sec ( +- 39.58% ) 301 487 page-faults # 0.001 M/sec ( +- 0.02% ) 302 1,474,374,262 cycles # 1.723 GHz ( +- 0.17% ) 303 <not supported> stalled-cycles-frontend 304 <not supported> stalled-cycles-backend 305 1,178,049,567 instructions # 0.80 insns per cycle ( +- 0.06% ) 306 208,368,926 branches # 243.507 M/sec ( +- 0.06% ) 307 5,569,188 branch-misses # 2.67% of all branches ( +- 0.54% ) 308 309 1.601607384 seconds time elapsed ( +- 0.07% ) 310 311jump label enabled:: 312 313 Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs): 314 315 841.043185 task-clock # 0.533 CPUs utilized ( +- 0.12% ) 316 200,004 context-switches # 0.238 M/sec ( +- 0.00% ) 317 0 CPU-migrations # 0.000 M/sec ( +- 40.87% ) 318 487 page-faults # 0.001 M/sec ( +- 0.05% ) 319 1,432,559,428 cycles # 1.703 GHz ( +- 0.18% ) 320 <not supported> stalled-cycles-frontend 321 <not supported> stalled-cycles-backend 322 1,175,363,994 instructions # 0.82 insns per cycle ( +- 0.04% ) 323 206,859,359 branches # 245.956 M/sec ( +- 0.04% ) 324 4,884,119 branch-misses # 2.36% of all branches ( +- 0.85% ) 325 326 1.579384366 seconds time elapsed 327 328The percentage of saved branches is .7%, and we've saved 12% on 329'branch-misses'. This is where we would expect to get the most savings, since 330this optimization is about reducing the number of branches. In addition, we've 331saved .2% on instructions, and 2.8% on cycles and 1.4% on elapsed time. 332
