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trace_benchmark.c
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229 lines (193 loc) · 5.19 KB
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// 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 _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();
}
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);