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1// SPDX-License-Identifier: GPL-2.0
2#include <linux/delay.h>
3#include <linux/module.h>
4#include <linux/kthread.h>
5#include <linux/trace_clock.h>
6
7#define CREATE_TRACE_POINTS
8#include "trace_benchmark.h"
9
10static struct task_struct *bm_event_thread;
11
12static char bm_str[BENCHMARK_EVENT_STRLEN] = "START";
13
14static u64 bm_total;
15static u64 bm_totalsq;
16static u64 bm_last;
17static u64 bm_max;
18static u64 bm_min;
19static u64 bm_first;
20static u64 bm_cnt;
21static u64 bm_stddev;
22static unsigned int bm_avg;
23static unsigned int bm_std;
24
25static bool ok_to_run;
26
27/*
28 * This gets called in a loop recording the time it took to write
29 * the tracepoint. What it writes is the time statistics of the last
30 * tracepoint write. As there is nothing to write the first time
31 * it simply writes "START". As the first write is cold cache and
32 * the rest is hot, we save off that time in bm_first and it is
33 * reported as "first", which is shown in the second write to the
34 * tracepoint. The "first" field is written within the statics from
35 * then on but never changes.
36 */
37static void trace_do_benchmark(void)
38{
39 u64 start;
40 u64 stop;
41 u64 delta;
42 u64 stddev;
43 u64 seed;
44 u64 last_seed;
45 unsigned int avg;
46 unsigned int std = 0;
47
48 /* Only run if the tracepoint is actually active */
49 if (!trace_benchmark_event_enabled() || !tracing_is_on())
50 return;
51
52 local_irq_disable();
53 start = trace_clock_local();
54 trace_benchmark_event(bm_str, bm_last);
55 stop = trace_clock_local();
56 local_irq_enable();
57
58 bm_cnt++;
59
60 delta = stop - start;
61
62 /*
63 * The first read is cold cached, keep it separate from the
64 * other calculations.
65 */
66 if (bm_cnt == 1) {
67 bm_first = delta;
68 scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
69 "first=%llu [COLD CACHED]", bm_first);
70 return;
71 }
72
73 bm_last = delta;
74
75 if (delta > bm_max)
76 bm_max = delta;
77 if (!bm_min || delta < bm_min)
78 bm_min = delta;
79
80 /*
81 * When bm_cnt is greater than UINT_MAX, it breaks the statistics
82 * accounting. Freeze the statistics when that happens.
83 * We should have enough data for the avg and stddev anyway.
84 */
85 if (bm_cnt > UINT_MAX) {
86 scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
87 "last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
88 bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
89 return;
90 }
91
92 bm_total += delta;
93 bm_totalsq += delta * delta;
94
95 if (bm_cnt > 1) {
96 /*
97 * Apply Welford's method to calculate standard deviation:
98 * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
99 */
100 stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
101 do_div(stddev, (u32)bm_cnt);
102 do_div(stddev, (u32)bm_cnt - 1);
103 } else
104 stddev = 0;
105
106 delta = bm_total;
107 do_div(delta, (u32)bm_cnt);
108 avg = delta;
109
110 if (stddev > 0) {
111 int i = 0;
112 /*
113 * stddev is the square of standard deviation but
114 * we want the actually number. Use the average
115 * as our seed to find the std.
116 *
117 * The next try is:
118 * x = (x + N/x) / 2
119 *
120 * Where N is the squared number to find the square
121 * root of.
122 */
123 seed = avg;
124 do {
125 last_seed = seed;
126 seed = stddev;
127 if (!last_seed)
128 break;
129 seed = div64_u64(seed, last_seed);
130 seed += last_seed;
131 do_div(seed, 2);
132 } while (i++ < 10 && last_seed != seed);
133
134 std = seed;
135 }
136
137 scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
138 "last=%llu first=%llu max=%llu min=%llu avg=%u std=%d std^2=%lld",
139 bm_last, bm_first, bm_max, bm_min, avg, std, stddev);
140
141 bm_std = std;
142 bm_avg = avg;
143 bm_stddev = stddev;
144}
145
146static int benchmark_event_kthread(void *arg)
147{
148 /* sleep a bit to make sure the tracepoint gets activated */
149 msleep(100);
150
151 while (!kthread_should_stop()) {
152
153 trace_do_benchmark();
154
155 /*
156 * We don't go to sleep, but let others run as well.
157 * This is basically a "yield()" to let any task that
158 * wants to run, schedule in, but if the CPU is idle,
159 * we'll keep burning cycles.
160 *
161 * Note the tasks_rcu_qs() version of cond_resched() will
162 * notify synchronize_rcu_tasks() that this thread has
163 * passed a quiescent state for rcu_tasks. Otherwise
164 * this thread will never voluntarily schedule which would
165 * block synchronize_rcu_tasks() indefinitely.
166 */
167 cond_resched_tasks_rcu_qs();
168 }
169
170 return 0;
171}
172
173/*
174 * When the benchmark tracepoint is enabled, it calls this
175 * function and the thread that calls the tracepoint is created.
176 */
177int trace_benchmark_reg(void)
178{
179 if (!ok_to_run) {
180 pr_warn("trace benchmark cannot be started via kernel command line\n");
181 return -EBUSY;
182 }
183
184 bm_event_thread = kthread_run(benchmark_event_kthread,
185 NULL, "event_benchmark");
186 if (IS_ERR(bm_event_thread)) {
187 pr_warn("trace benchmark failed to create kernel thread\n");
188 return PTR_ERR(bm_event_thread);
189 }
190
191 return 0;
192}
193
194/*
195 * When the benchmark tracepoint is disabled, it calls this
196 * function and the thread that calls the tracepoint is deleted
197 * and all the numbers are reset.
198 */
199void trace_benchmark_unreg(void)
200{
201 if (!bm_event_thread)
202 return;
203
204 kthread_stop(bm_event_thread);
205 bm_event_thread = NULL;
206
207 strcpy(bm_str, "START");
208 bm_total = 0;
209 bm_totalsq = 0;
210 bm_last = 0;
211 bm_max = 0;
212 bm_min = 0;
213 bm_cnt = 0;
214 /* These don't need to be reset but reset them anyway */
215 bm_first = 0;
216 bm_std = 0;
217 bm_avg = 0;
218 bm_stddev = 0;
219}
220
221static __init int ok_to_run_trace_benchmark(void)
222{
223 ok_to_run = true;
224
225 return 0;
226}
227
228early_initcall(ok_to_run_trace_benchmark);