Root/Documentation/static-keys.txt

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1            Static Keys
2            -----------
3
4By: Jason Baron <jbaron@redhat.com>
5
60) Abstract
7
8Static keys allows the inclusion of seldom used features in
9performance-sensitive fast-path kernel code, via a GCC feature and a code
10patching 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
27The static_key_false() branch will be generated into the code with as little
28impact to the likely code path as possible.
29
30
311) Motivation
32
33
34Currently, tracepoints are implemented using a conditional branch. The
35conditional check requires checking a global variable for each tracepoint.
36Although the overhead of this check is small, it increases when the memory
37cache comes under pressure (memory cache lines for these global variables may
38be shared with other memory accesses). As we increase the number of tracepoints
39in the kernel this overhead may become more of an issue. In addition,
40tracepoints are often dormant (disabled) and provide no direct kernel
41functionality. Thus, it is highly desirable to reduce their impact as much as
42possible. Although tracepoints are the original motivation for this work, other
43kernel code paths should be able to make use of the static keys facility.
44
45
462) Solution
47
48
49gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label:
50
51http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
52
53Using the 'asm goto', we can create branches that are either taken or not taken
54by default, without the need to check memory. Then, at run-time, we can patch
55the branch site to change the branch direction.
56
57For 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
62Thus, by default the 'printk' will not be emitted. And the code generated will
63consist of a single atomic 'no-op' instruction (5 bytes on x86), in the
64straight-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
66out-of-line true branch. Thus, changing branch direction is expensive but
67branch selection is basically 'free'. That is the basic tradeoff of this
68optimization.
69
70This lowlevel patching mechanism is called 'jump label patching', and it gives
71the basis for the static keys facility.
72
733) Static key label API, usage and examples:
74
75
76In order to make use of this optimization you must first define a key:
77
78    struct static_key key;
79
80Which is initialized as:
81
82    struct static_key key = STATIC_KEY_INIT_TRUE;
83
84or:
85
86    struct static_key key = STATIC_KEY_INIT_FALSE;
87
88If the key is not initialized, it is default false. The 'struct static_key',
89must be a 'global'. That is, it can't be allocated on the stack or dynamically
90allocated at run-time.
91
92The 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
99Or:
100
101        if (static_key_true(&key))
102                do likely code
103        else
104                do unlikely code
105
106A 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
109single key can be used in many branches, but all the branches must match the
110way that the key has been initialized.
111
112The branch(es) can then be switched via:
113
114    static_key_slow_inc(&key);
115    ...
116    static_key_slow_dec(&key);
117
118Thus, 'static_key_slow_inc()' means 'make the branch true', and
119'static_key_slow_dec()' means 'make the the branch false' with appropriate
120reference counting. For example, if the key is initialized true, a
121static_key_slow_dec(), will switch the branch to false. And a subsequent
122static_key_slow_inc(), will change the branch back to true. Likewise, if the
123key is initialized false, a 'static_key_slow_inc()', will change the branch to
124true. And then a 'static_key_slow_dec()', will again make the branch false.
125
126An 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
137Tracepoints are disabled by default, and can be placed in performance critical
138pieces of the kernel. Thus, by using a static key, the tracepoints can have
139absolutely minimal impact when not in use.
140
141
1424) Architecture level code patching interface, 'jump labels'
143
144
145There are a few functions and macros that architectures must implement in order
146to take advantage of this optimization. If there is no architecture support, we
147simply 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
1665) Static keys / jump label analysis, results (x86_64):
167
168
169As an example, let's add the following branch to 'getppid()', such that the
170system call now looks like:
171
172SYSCALL_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
186The resulting instructions with jump labels generated by GCC is:
187
188ffffffff81044290 <sys_getppid>:
189ffffffff81044290: 55 push %rbp
190ffffffff81044291: 48 89 e5 mov %rsp,%rbp
191ffffffff81044294: e9 00 00 00 00 jmpq ffffffff81044299 <sys_getppid+0x9>
192ffffffff81044299: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
193ffffffff810442a0: 00 00
194ffffffff810442a2: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
195ffffffff810442a9: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
196ffffffff810442b0: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
197ffffffff810442b7: e8 f4 d9 00 00 callq ffffffff81051cb0 <pid_vnr>
198ffffffff810442bc: 5d pop %rbp
199ffffffff810442bd: 48 98 cltq
200ffffffff810442bf: c3 retq
201ffffffff810442c0: 48 c7 c7 e3 54 98 81 mov $0xffffffff819854e3,%rdi
202ffffffff810442c7: 31 c0 xor %eax,%eax
203ffffffff810442c9: e8 71 13 6d 00 callq ffffffff8171563f <printk>
204ffffffff810442ce: eb c9 jmp ffffffff81044299 <sys_getppid+0x9>
205
206Without the jump label optimization it looks like:
207
208ffffffff810441f0 <sys_getppid>:
209ffffffff810441f0: 8b 05 8a 52 d8 00 mov 0xd8528a(%rip),%eax # ffffffff81dc9480 <key>
210ffffffff810441f6: 55 push %rbp
211ffffffff810441f7: 48 89 e5 mov %rsp,%rbp
212ffffffff810441fa: 85 c0 test %eax,%eax
213ffffffff810441fc: 75 27 jne ffffffff81044225 <sys_getppid+0x35>
214ffffffff810441fe: 65 48 8b 04 25 c0 b6 mov %gs:0xb6c0,%rax
215ffffffff81044205: 00 00
216ffffffff81044207: 48 8b 80 80 02 00 00 mov 0x280(%rax),%rax
217ffffffff8104420e: 48 8b 80 b0 02 00 00 mov 0x2b0(%rax),%rax
218ffffffff81044215: 48 8b b8 e8 02 00 00 mov 0x2e8(%rax),%rdi
219ffffffff8104421c: e8 2f da 00 00 callq ffffffff81051c50 <pid_vnr>
220ffffffff81044221: 5d pop %rbp
221ffffffff81044222: 48 98 cltq
222ffffffff81044224: c3 retq
223ffffffff81044225: 48 c7 c7 13 53 98 81 mov $0xffffffff81985313,%rdi
224ffffffff8104422c: 31 c0 xor %eax,%eax
225ffffffff8104422e: e8 60 0f 6d 00 callq ffffffff81715193 <printk>
226ffffffff81044233: eb c9 jmp ffffffff810441fe <sys_getppid+0xe>
227ffffffff81044235: 66 66 2e 0f 1f 84 00 data32 nopw %cs:0x0(%rax,%rax,1)
228ffffffff8104423c: 00 00 00 00
229
230Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction
231vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched
232to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump
233label case adds:
234
2356 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.
236
237If we then include the padding bytes, the jump label code saves, 16 total bytes
238of instruction memory for this small function. In this case the non-jump label
239function is 80 bytes long. Thus, we have have saved 20% of the instruction
240footprint. We can in fact improve this even further, since the 5-byte no-op
241really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp.
242However, we have not yet implemented optimal no-op sizes (they are currently
243hard-coded).
244
245Since 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
247performance improvement. Testing done on 3.3.0-rc2:
248
249jump 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
266jump 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
283The 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
285this optimization is about reducing the number of branches. In addition, we've
286saved .2% on instructions, and 2.8% on cycles and 1.4% on elapsed time.
287

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