// 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_warn("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_warn("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);