auto-configure spin loop in rtspin

Don't just blindly overrun the budget.
Instead, try to measure a spin loop and hope that we get good results.

1 files changed, 110 insertions, 13 deletions

diff --git a/bin/rtspin.c b/bin/rtspin.c
index 435e496..56f5549 100644
--- a/bin/rtspin.c
+++ b/bin/rtspin.c
@@ -9,11 +9,6 @@
 #include "litmus.h"
 #include "common.h"
 
-void usage(char *error) {
-        fprintf(stderr, "Error: %s\n", error);
-        fprintf(stderr, "Usage: rt_spin [-w] [-p PARTITION] [-c CLASS] WCET PERIOD DURATION\n");
-        exit(1);
-}
 
 static double wctime()
 {
@@ -22,11 +17,99 @@ static double wctime()
         return (tv.tv_sec + 1E-6 * tv.tv_usec);
 }
 
-#define OPTSTR "p:c:w"
+void usage(char *error) {
+        fprintf(stderr, "Error: %s\n", error);
+        fprintf(stderr,
+                "Usage: rt_spin [-w] [-p PARTITION] [-c CLASS] WCET PERIOD DURATION\n"
+                "       rt_spin -l\n");
+        exit(1);
+}
+
+#define NUMS 4096
+static int num[NUMS];
+static double loop_length = 1.0;
+
+static int loop_once(void)
+{
+        int i, j = 0;
+        for (i = 0; i < NUMS; i++)
+                j += num[i]++;
+        return j;
+}
+
+static int loop_for(double exec_time)
+{
+        double t = 0;
+        int tmp = 0;
+        while (t + loop_length < exec_time) {
+                tmp += loop_once();
+                t += loop_length;
+        }
+        return tmp;
+}
+
+static void fine_tune(double interval)
+{
+        double start, end, delta;
+
+        start = wctime();
+        loop_for(interval);
+        end = wctime();
+        delta = (end - start - interval) / interval;
+        if (delta != 0)
+                loop_length = loop_length / (1 - delta);
+}
+
+static void configure_loop(void)
+{
+        double start;
+
+        /* prime cache */
+        loop_once();
+        loop_once();
+        loop_once();
+
+        /* measure */
+        start = wctime();
+        loop_once(); /* hope we didn't get preempted  */
+        loop_length = wctime();
+        loop_length -= start;
+
+        /* fine tune */
+        fine_tune(0.1);
+        fine_tune(0.1);
+        fine_tune(0.1);
+}
+
+static void debug_delay_loop(void)
+{
+        double start, end, delay;
+        printf("loop_length=%f\n", loop_length);
+        while (1) {
+                for (delay = 0.5; delay > 0.01; delay -= 0.01) {
+                        start = wctime();
+                        loop_for(delay);
+                        end = wctime();
+                        printf("%6.4fs: looped for %10.8fs, delta=%11.8fs, error=%7.4f%%\n",
+                               delay,
+                               end - start,
+                               end - start - delay,
+                               100 * (end - start - delay) / delay);
+                }
+        }
+}
+
+static int job(double exec_time)
+{
+        loop_for(exec_time);
+        sleep_next_period();
+        return 0;
+}
+
+#define OPTSTR "p:c:wl"
 
 int main(int argc, char** argv) 
 {
-        int i;
         int ret;
         lt_t wcet;
         lt_t period;
@@ -34,6 +117,7 @@ int main(int argc, char** argv)
         int cpu = 0;
         int opt;
         int wait = 0;
+        int test_loop = 0;
         double duration, start;
         task_class_t class = RT_CLASS_HARD;
 
@@ -51,6 +135,9 @@ int main(int argc, char** argv)
                         if (class == -1)
                                 usage("Unknown task class.");
                         break;
+                case 'l':
+                        test_loop = 1;
+                        break;
                 case ':':
                         usage("Argument missing.");
                         break;
@@ -61,6 +148,14 @@ int main(int argc, char** argv)
                 }
         }
 
+
+        configure_loop();
+
+        if (test_loop) {
+                debug_delay_loop();
+                return 0;
+        }
+
         if (argc - optind < 3)
                 usage("Arguments missing.");       
         wcet     = atoi(argv[optind + 0]);
@@ -75,21 +170,21 @@ int main(int argc, char** argv)
                 usage("The worst-case execution time must not "
                       "exceed the period.");
         }
+
         if (migrate) {
                 ret = be_migrate_to(cpu);
                 if (ret < 0)
                         bail_out("could not migrate to target partition");
         }
-        
-//      printf("wcet: %llu\nperiod: %llu\n", wcet, period);
+
         ret = sporadic_task(wcet, period, 0, cpu, class, migrate);
         
         if (ret < 0)
                 bail_out("could not become rt tasks.");
 
-//      ret = init_litmus();
-//      if (ret < 0)
-//              perror("init_litmus()");
+        ret = init_litmus();
+        if (ret < 0)
+                perror("init_litmus()");
 
         ret = task_mode(LITMUS_RT_TASK);
         if (ret != 0)
@@ -100,10 +195,12 @@ int main(int argc, char** argv)
                 if (ret != 0)
                         bail_out("wait_for_ts_release()");
         }
+
+
         start = wctime();
 
         while (start + duration > wctime()) {
-                for (i = 0; i < 500000; i++);
+                job(wcet * 0.0009); /* 90% wcet, in seconds */
         }
 
         return 0;