summaryrefslogtreecommitdiff
path: root/kernel/trace/trace_benchmark.c
blob: 80e0b2aca703a8ee5e554a1583ee9ac82fe21fcb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
// SPDX-License-Identifier: GPL-2.0
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/trace_clock.h>

#define CREATE_TRACE_POINTS
#include "trace_benchmark.h"

static struct task_struct *bm_event_thread;

static char bm_str[BENCHMARK_EVENT_STRLEN] = "START";

static u64 bm_total;
static u64 bm_totalsq;
static u64 bm_last;
static u64 bm_max;
static u64 bm_min;
static u64 bm_first;
static u64 bm_cnt;
static u64 bm_stddev;
static unsigned int bm_avg;
static unsigned int bm_std;

static bool ok_to_run;

/*
 * This gets called in a loop recording the time it took to write
 * the tracepoint. What it writes is the time statistics of the last
 * tracepoint write. As there is nothing to write the first time
 * it simply writes "START". As the first write is cold cache and
 * the rest is hot, we save off that time in bm_first and it is
 * reported as "first", which is shown in the second write to the
 * tracepoint. The "first" field is writen within the statics from
 * then on but never changes.
 */
static void trace_do_benchmark(void)
{
	u64 start;
	u64 stop;
	u64 delta;
	u64 stddev;
	u64 seed;
	u64 last_seed;
	unsigned int avg;
	unsigned int std = 0;

	/* Only run if the tracepoint is actually active */
	if (!trace_benchmark_event_enabled() || !tracing_is_on())
		return;

	local_irq_disable();
	start = trace_clock_local();
	trace_benchmark_event(bm_str);
	stop = trace_clock_local();
	local_irq_enable();

	bm_cnt++;

	delta = stop - start;

	/*
	 * The first read is cold cached, keep it separate from the
	 * other calculations.
	 */
	if (bm_cnt == 1) {
		bm_first = delta;
		scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
			  "first=%llu [COLD CACHED]", bm_first);
		return;
	}

	bm_last = delta;

	if (delta > bm_max)
		bm_max = delta;
	if (!bm_min || delta < bm_min)
		bm_min = delta;

	/*
	 * When bm_cnt is greater than UINT_MAX, it breaks the statistics
	 * accounting. Freeze the statistics when that happens.
	 * We should have enough data for the avg and stddev anyway.
	 */
	if (bm_cnt > UINT_MAX) {
		scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
		    "last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
			  bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
		return;
	}

	bm_total += delta;
	bm_totalsq += delta * delta;


	if (bm_cnt > 1) {
		/*
		 * Apply Welford's method to calculate standard deviation:
		 * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
		 */
		stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
		do_div(stddev, (u32)bm_cnt);
		do_div(stddev, (u32)bm_cnt - 1);
	} else
		stddev = 0;

	delta = bm_total;
	do_div(delta, bm_cnt);
	avg = delta;

	if (stddev > 0) {
		int i = 0;
		/*
		 * stddev is the square of standard deviation but
		 * we want the actualy number. Use the average
		 * as our seed to find the std.
		 *
		 * The next try is:
		 *  x = (x + N/x) / 2
		 *
		 * Where N is the squared number to find the square
		 * root of.
		 */
		seed = avg;
		do {
			last_seed = seed;
			seed = stddev;
			if (!last_seed)
				break;
			do_div(seed, last_seed);
			seed += last_seed;
			do_div(seed, 2);
		} while (i++ < 10 && last_seed != seed);

		std = seed;
	}

	scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
		  "last=%llu first=%llu max=%llu min=%llu avg=%u std=%d std^2=%lld",
		  bm_last, bm_first, bm_max, bm_min, avg, std, stddev);

	bm_std = std;
	bm_avg = avg;
	bm_stddev = stddev;
}

static int benchmark_event_kthread(void *arg)
{
	/* sleep a bit to make sure the tracepoint gets activated */
	msleep(100);

	while (!kthread_should_stop()) {

		trace_do_benchmark();

		/*
		 * We don't go to sleep, but let others run as well.
		 * This is bascially a "yield()" to let any task that
		 * wants to run, schedule in, but if the CPU is idle,
		 * we'll keep burning cycles.
		 *
		 * Note the tasks_rcu_qs() version of cond_resched() will
		 * notify synchronize_rcu_tasks() that this thread has
		 * passed a quiescent state for rcu_tasks. Otherwise
		 * this thread will never voluntarily schedule which would
		 * block synchronize_rcu_tasks() indefinitely.
		 */
		cond_resched_tasks_rcu_qs();
	}

	return 0;
}

/*
 * When the benchmark tracepoint is enabled, it calls this
 * function and the thread that calls the tracepoint is created.
 */
int trace_benchmark_reg(void)
{
	if (!ok_to_run) {
		pr_warning("trace benchmark cannot be started via kernel command line\n");
		return -EBUSY;
	}

	bm_event_thread = kthread_run(benchmark_event_kthread,
				      NULL, "event_benchmark");
	if (IS_ERR(bm_event_thread)) {
		pr_warning("trace benchmark failed to create kernel thread\n");
		return PTR_ERR(bm_event_thread);
	}

	return 0;
}

/*
 * When the benchmark tracepoint is disabled, it calls this
 * function and the thread that calls the tracepoint is deleted
 * and all the numbers are reset.
 */
void trace_benchmark_unreg(void)
{
	if (!bm_event_thread)
		return;

	kthread_stop(bm_event_thread);
	bm_event_thread = NULL;

	strcpy(bm_str, "START");
	bm_total = 0;
	bm_totalsq = 0;
	bm_last = 0;
	bm_max = 0;
	bm_min = 0;
	bm_cnt = 0;
	/* These don't need to be reset but reset them anyway */
	bm_first = 0;
	bm_std = 0;
	bm_avg = 0;
	bm_stddev = 0;
}

static __init int ok_to_run_trace_benchmark(void)
{
	ok_to_run = true;

	return 0;
}

early_initcall(ok_to_run_trace_benchmark);