From c477b8a808d39da9e908738ad5d40e3e93096f82 Mon Sep 17 00:00:00 2001
From: Mika <kontakt@donau2space.de>
Date: Wed, 3 Dec 2025 13:45:00 +0000
Subject: [PATCH] Add main.c

---
 main.c | 149 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 149 insertions(+)
 create mode 100644 main.c

diff --git a/main.c b/main.c
new file mode 100644
index 0000000..6d30355
--- /dev/null
+++ b/main.c
@@ -0,0 +1,149 @@
+/*
+ * micro_benchmark - simple CLI tool to emulate clocksource switch latency patterns.
+ * It runs configurable loops, estimates timing statistics, and logs results to a file.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <math.h>
+
+struct benchmark_result {
+    int run_count;
+    double average_offset;
+    double std_deviation;
+};
+
+static struct benchmark_result g_result = {0};
+static int g_last_c_state = 0;
+static int g_last_governor = 0;
+
+static double timespec_diff(const struct timespec *start, const struct timespec *end)
+{
+    time_t sec = end->tv_sec - start->tv_sec;
+    long nsec = end->tv_nsec - start->tv_nsec;
+    return (double)sec + (double)nsec / 1e9;
+}
+
+static void simulate_switch_latency(int c_state, int governor)
+{
+    volatile double sink = 0.0;
+    unsigned int base_loops = 25000;
+    unsigned int c_factor = (unsigned int)((c_state >= 0 ? c_state : 0) + 1) * 12000U;
+    unsigned int g_factor = (unsigned int)((governor >= 0 ? governor : 0) + 1) * 9000U;
+    unsigned int loops = base_loops + c_factor + g_factor;
+
+    for (unsigned int i = 0; i < loops; ++i) {
+        sink += (i & 0x7U) * 0.0001;
+    }
+}
+
+double run_benchmark(int runs, int c_state, int governor)
+{
+    if (runs <= 0) {
+        g_result.run_count = 0;
+        g_result.average_offset = 0.0;
+        g_result.std_deviation = 0.0;
+        return 0.0;
+    }
+
+    double *samples = malloc(sizeof(double) * (size_t)runs);
+    if (!samples) {
+        fprintf(stderr, "Allocation failed\n");
+        exit(EXIT_FAILURE);
+    }
+
+    struct timespec seed_ts;
+    clock_gettime(CLOCK_REALTIME, &seed_ts);
+    srand((unsigned int)seed_ts.tv_nsec);
+
+    for (int i = 0; i < runs; ++i) {
+        struct timespec start, end;
+        clock_gettime(CLOCK_MONOTONIC, &start);
+        simulate_switch_latency(c_state, governor);
+        clock_gettime(CLOCK_MONOTONIC, &end);
+
+        double delta = timespec_diff(&start, &end);
+        double jitter = ((double)rand() / (double)RAND_MAX - 0.5) * 5e-6 * (c_state + 1);
+        delta += jitter;
+        if (delta < 0.0) {
+            delta = 0.0;
+        }
+        samples[i] = delta;
+    }
+
+    double sum = 0.0;
+    for (int i = 0; i < runs; ++i) {
+        sum += samples[i];
+    }
+    double avg = sum / (double)runs;
+
+    double variance = 0.0;
+    for (int i = 0; i < runs; ++i) {
+        double diff = samples[i] - avg;
+        variance += diff * diff;
+    }
+    variance /= (double)runs;
+
+    g_result.run_count = runs;
+    g_result.average_offset = avg;
+    g_result.std_deviation = sqrt(variance);
+
+    free(samples);
+    return g_result.average_offset;
+}
+
+void log_results(const char *filename)
+{
+    if (!filename) {
+        return;
+    }
+
+    FILE *fp = fopen(filename, "w");
+    if (!fp) {
+        fprintf(stderr, "Could not open log file %s\n", filename);
+        return;
+    }
+
+    time_t now = time(NULL);
+    fprintf(fp, "# micro_benchmark log\n");
+    fprintf(fp, "timestamp=%ld\n", (long)now);
+    fprintf(fp, "runs=%d\n", g_result.run_count);
+    fprintf(fp, "c_state=%d\n", g_last_c_state);
+    fprintf(fp, "governor=%d\n", g_last_governor);
+    fprintf(fp, "average_offset=%.9f\n", g_result.average_offset);
+    fprintf(fp, "std_deviation=%.9f\n", g_result.std_deviation);
+    fclose(fp);
+}
+
+int main(int argc, char **argv)
+{
+    if (argc < 5) {
+        fprintf(stderr, "Usage: %s <runs> <c_state> <governor> <logfile>\n", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    int runs = atoi(argv[1]);
+    int c_state = atoi(argv[2]);
+    int governor = atoi(argv[3]);
+    const char *logfile = argv[4];
+
+    if (runs <= 0) {
+        fprintf(stderr, "Runs must be positive\n");
+        return EXIT_FAILURE;
+    }
+
+    g_last_c_state = c_state;
+    g_last_governor = governor;
+
+    run_benchmark(runs, c_state, governor);
+
+    printf("Runs: %d\n", g_result.run_count);
+    printf("C-State: %d\n", g_last_c_state);
+    printf("Governor: %d\n", g_last_governor);
+    printf("Average offset: %.9f s\n", g_result.average_offset);
+    printf("Std dev: %.9f s\n", g_result.std_deviation);
+
+    log_results(logfile);
+    return EXIT_SUCCESS;
+}