| =========== | |
| 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. | |
| The state and the reference count can be retrieved with 'static_key_enabled()' | |
| and 'static_key_count()'. In general, if you use these functions, they | |
| should be protected with the same mutex used around the enable/disable | |
| or increment/decrement function. | |
| Note that switching branches results in some locks being taken, | |
| particularly the CPU hotplug lock (in order to avoid races against | |
| CPUs being brought in the kernel while the kernel is getting | |
| patched). Calling the static key API from within a hotplug notifier is | |
| thus a sure deadlock recipe. In order to still allow use of the | |
| functionality, the following functions are provided: | |
| static_key_enable_cpuslocked() | |
| static_key_disable_cpuslocked() | |
| static_branch_enable_cpuslocked() | |
| static_branch_disable_cpuslocked() | |
| These functions are *not* general purpose, and must only be used when | |
| you really know that you're in the above context, and no other. | |
| 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. |