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TOMOYO Linux Cross Reference
Linux/kernel/trace/trace_benchmark.c

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  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 
  9 static struct task_struct *bm_event_thread;
 10 
 11 static char bm_str[BENCHMARK_EVENT_STRLEN] = "START";
 12 
 13 static u64 bm_total;
 14 static u64 bm_totalsq;
 15 static u64 bm_last;
 16 static u64 bm_max;
 17 static u64 bm_min;
 18 static u64 bm_first;
 19 static u64 bm_cnt;
 20 static u64 bm_stddev;
 21 static unsigned int bm_avg;
 22 static unsigned int bm_std;
 23 
 24 static bool ok_to_run;
 25 
 26 /*
 27  * This gets called in a loop recording the time it took to write
 28  * the tracepoint. What it writes is the time statistics of the last
 29  * tracepoint write. As there is nothing to write the first time
 30  * it simply writes "START". As the first write is cold cache and
 31  * the rest is hot, we save off that time in bm_first and it is
 32  * reported as "first", which is shown in the second write to the
 33  * tracepoint. The "first" field is writen within the statics from
 34  * then on but never changes.
 35  */
 36 static void trace_do_benchmark(void)
 37 {
 38         u64 start;
 39         u64 stop;
 40         u64 delta;
 41         u64 stddev;
 42         u64 seed;
 43         u64 last_seed;
 44         unsigned int avg;
 45         unsigned int std = 0;
 46 
 47         /* Only run if the tracepoint is actually active */
 48         if (!trace_benchmark_event_enabled() || !tracing_is_on())
 49                 return;
 50 
 51         local_irq_disable();
 52         start = trace_clock_local();
 53         trace_benchmark_event(bm_str);
 54         stop = trace_clock_local();
 55         local_irq_enable();
 56 
 57         bm_cnt++;
 58 
 59         delta = stop - start;
 60 
 61         /*
 62          * The first read is cold cached, keep it separate from the
 63          * other calculations.
 64          */
 65         if (bm_cnt == 1) {
 66                 bm_first = delta;
 67                 scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
 68                           "first=%llu [COLD CACHED]", bm_first);
 69                 return;
 70         }
 71 
 72         bm_last = delta;
 73 
 74         if (delta > bm_max)
 75                 bm_max = delta;
 76         if (!bm_min || delta < bm_min)
 77                 bm_min = delta;
 78 
 79         /*
 80          * When bm_cnt is greater than UINT_MAX, it breaks the statistics
 81          * accounting. Freeze the statistics when that happens.
 82          * We should have enough data for the avg and stddev anyway.
 83          */
 84         if (bm_cnt > UINT_MAX) {
 85                 scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
 86                     "last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
 87                           bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
 88                 return;
 89         }
 90 
 91         bm_total += delta;
 92         bm_totalsq += delta * delta;
 93 
 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, 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 actualy 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                         do_div(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 
146 static 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
157                  * run as well.
158                  */
159                 cond_resched();
160         }
161 
162         return 0;
163 }
164 
165 /*
166  * When the benchmark tracepoint is enabled, it calls this
167  * function and the thread that calls the tracepoint is created.
168  */
169 int trace_benchmark_reg(void)
170 {
171         if (!ok_to_run) {
172                 pr_warning("trace benchmark cannot be started via kernel command line\n");
173                 return -EBUSY;
174         }
175 
176         bm_event_thread = kthread_run(benchmark_event_kthread,
177                                       NULL, "event_benchmark");
178         if (IS_ERR(bm_event_thread)) {
179                 pr_warning("trace benchmark failed to create kernel thread\n");
180                 return PTR_ERR(bm_event_thread);
181         }
182 
183         return 0;
184 }
185 
186 /*
187  * When the benchmark tracepoint is disabled, it calls this
188  * function and the thread that calls the tracepoint is deleted
189  * and all the numbers are reset.
190  */
191 void trace_benchmark_unreg(void)
192 {
193         if (!bm_event_thread)
194                 return;
195 
196         kthread_stop(bm_event_thread);
197         bm_event_thread = NULL;
198 
199         strcpy(bm_str, "START");
200         bm_total = 0;
201         bm_totalsq = 0;
202         bm_last = 0;
203         bm_max = 0;
204         bm_min = 0;
205         bm_cnt = 0;
206         /* These don't need to be reset but reset them anyway */
207         bm_first = 0;
208         bm_std = 0;
209         bm_avg = 0;
210         bm_stddev = 0;
211 }
212 
213 static __init int ok_to_run_trace_benchmark(void)
214 {
215         ok_to_run = true;
216 
217         return 0;
218 }
219 
220 early_initcall(ok_to_run_trace_benchmark);
221 

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