kernel/trace/trace_benchmark.c

   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
  10 static struct task_struct *bm_event_thread;
  11
  12 static char bm_str[BENCHMARK_EVENT_STRLEN] = "START";
  13
  14 static u64 bm_total;
  15 static u64 bm_totalsq;
  16 static u64 bm_last;
  17 static u64 bm_max;
  18 static u64 bm_min;
  19 static u64 bm_first;
  20 static u64 bm_cnt;
  21 static u64 bm_stddev;
  22 static unsigned int bm_avg;
  23 static unsigned int bm_std;
  24
  25 static 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  */
  37 static 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
 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 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  */
 177 int 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  */
 199 void 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
 221 static __init int ok_to_run_trace_benchmark(void)
 222 {
 223         ok_to_run = true;
 224
 225         return 0;
 226 }
 227
 228 early_initcall(ok_to_run_trace_benchmark);