diff options
Diffstat (limited to 'Documentation/static-keys.txt')
| -rw-r--r-- | Documentation/static-keys.txt | 207 |
1 files changed, 108 insertions, 99 deletions
diff --git a/Documentation/static-keys.txt b/Documentation/static-keys.txt index ef419fd0897f..b83dfa1c0602 100644 --- a/Documentation/static-keys.txt +++ b/Documentation/static-keys.txt @@ -1,30 +1,34 @@ - Static Keys - ----------- +=========== +Static Keys +=========== -DEPRECATED API: +.. warning:: -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: + DEPRECATED API: -struct static_key false = STATIC_KEY_INIT_FALSE; -struct static_key true = STATIC_KEY_INIT_TRUE; -static_key_true() -static_key_false() + 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:: -The updated API replacements are: + struct static_key false = STATIC_KEY_INIT_FALSE; + struct static_key true = STATIC_KEY_INIT_TRUE; + static_key_true() + static_key_false() -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() + The updated API replacements are:: -0) Abstract + 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: +patching technique. A quick example:: DEFINE_STATIC_KEY_FALSE(key); @@ -45,7 +49,8 @@ The static_branch_unlikely() branch will be generated into the code with as litt impact to the likely code path as possible. -1) Motivation +Motivation +========== Currently, tracepoints are implemented using a conditional branch. The @@ -60,7 +65,8 @@ 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. -2) Solution +Solution +======== gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label: @@ -71,7 +77,7 @@ 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: +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"); @@ -87,14 +93,15 @@ optimization. This lowlevel patching mechanism is called 'jump label patching', and it gives the basis for the static keys facility. -3) Static key label API, usage and examples: +Static key label API, usage and examples +======================================== -In order to make use of this optimization you must first define a key: +In order to make use of this optimization you must first define a key:: DEFINE_STATIC_KEY_TRUE(key); -or: +or:: DEFINE_STATIC_KEY_FALSE(key); @@ -102,14 +109,14 @@ or: 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: +The key is then used in code as:: if (static_branch_unlikely(&key)) do unlikely code else do likely code -Or: +Or:: if (static_branch_likely(&key)) do likely code @@ -120,15 +127,15 @@ 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: +Branch(es) can be set true via:: -static_branch_enable(&key); + static_branch_enable(&key); -or false via: +or false via:: -static_branch_disable(&key); + static_branch_disable(&key); -The branch(es) can then be switched via reference counts: +The branch(es) can then be switched via reference counts:: static_branch_inc(&key); ... @@ -142,11 +149,11 @@ 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: +Where an array of keys is required, it can be defined as:: DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count); -or: +or:: DEFINE_STATIC_KEY_ARRAY_FALSE(keys, count); @@ -159,96 +166,98 @@ 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 +* ``select HAVE_ARCH_JUMP_LABEL``, + see: arch/x86/Kconfig -* __always_inline bool arch_static_branch(struct static_key *key, bool branch), see: - arch/x86/include/asm/jump_label.h +* ``#define JUMP_LABEL_NOP_SIZE``, + 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 +* ``__always_inline bool arch_static_branch(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 +* ``__always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)``, + see: arch/x86/include/asm/jump_label.h -* __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 +* ``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 +* ``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: +system call now looks like:: -SYSCALL_DEFINE0(getppid) -{ + SYSCALL_DEFINE0(getppid) + { int pid; -+ if (static_branch_unlikely(&key)) -+ printk("I am the true branch\n"); + + 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 + } + +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: +label case adds:: -6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes. + 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 @@ -262,7 +271,7 @@ 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: +jump label disabled:: Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs): @@ -279,7 +288,7 @@ jump label disabled: 1.601607384 seconds time elapsed ( +- 0.07% ) -jump label enabled: +jump label enabled:: Performance counter stats for 'bash -c /tmp/pipe-test' (50 runs): |
