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