perf stat: Change noise calculation to use stddev

The current noise computation does:

 \Sum abs(n_i - avg(n)) * N^-1.5

Which is (afaik) not a regular noise function, and needs the
complete sample set available to post-process.

Change this to use a regular stddev computation which can be
done by keeping a two sums:

 stddev = sqrt( 1/N (\Sum n_i^2) - avg(n)^2 )

For which we only need to keep \Sum n_i and \Sum n_i^2.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 69 insertions, 101 deletions

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 1a2626230660..31ffc4d3ba60 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -83,19 +83,32 @@ static u64			runtime_cycles[MAX_RUN];
 static u64                      event_res[MAX_RUN][MAX_COUNTERS][3];
 static u64                      event_scaled[MAX_RUN][MAX_COUNTERS];
 
-static u64                      event_res_avg[MAX_COUNTERS][3];
-static u64                      event_res_noise[MAX_COUNTERS][3];
+struct stats
+{
+        double sum;
+        double sum_sq;
+};
 
-static u64                      event_scaled_avg[MAX_COUNTERS];
+static double avg_stats(struct stats *stats)
+{
+        return stats->sum / run_count;
+}
 
-static u64                      runtime_nsecs_avg;
-static u64                      runtime_nsecs_noise;
+/*
+ * stddev = sqrt(1/N (\Sum n_i^2) - avg(n)^2)
+ */
+static double stddev_stats(struct stats *stats)
+{
+        double avg = stats->sum / run_count;
 
-static u64                      walltime_nsecs_avg;
-static u64                      walltime_nsecs_noise;
+        return sqrt(stats->sum_sq/run_count - avg*avg);
+}
 
-static u64                      runtime_cycles_avg;
-static u64                      runtime_cycles_noise;
+struct stats                    event_res_stats[MAX_COUNTERS][3];
+struct stats                    event_scaled_stats[MAX_COUNTERS];
+struct stats                    runtime_nsecs_stats;
+struct stats                    walltime_nsecs_stats;
+struct stats                    runtime_cycles_stats;
 
 #define MATCH_EVENT(t, c, counter)                      \
         (attrs[counter].type == PERF_TYPE_##t &&        \
@@ -279,42 +292,37 @@ static int run_perf_stat(int argc __used, const char **argv)
         return WEXITSTATUS(status);
 }
 
-static void print_noise(u64 *count, u64 *noise)
+static void print_noise(double avg, double stddev)
 {
         if (run_count > 1)
-                fprintf(stderr, "   ( +- %7.3f%% )",
-                        (double)noise[0]/(count[0]+1)*100.0);
+                fprintf(stderr, "   ( +- %7.3f%% )", 100*stddev / avg);
 }
 
-static void nsec_printout(int counter, u64 *count, u64 *noise)
+static void nsec_printout(int counter, double avg, double stddev)
 {
-        double msecs = (double)count[0] / 1000000;
+        double msecs = avg / 1e6;
 
         fprintf(stderr, " %14.6f  %-24s", msecs, event_name(counter));
 
         if (MATCH_EVENT(SOFTWARE, SW_TASK_CLOCK, counter)) {
-                if (walltime_nsecs_avg)
-                        fprintf(stderr, " # %10.3f CPUs ",
-                                (double)count[0] / (double)walltime_nsecs_avg);
+                fprintf(stderr, " # %10.3f CPUs ",
+                                avg / avg_stats(&walltime_nsecs_stats));
         }
-        print_noise(count, noise);
+        print_noise(avg, stddev);
 }
 
-static void abs_printout(int counter, u64 *count, u64 *noise)
+static void abs_printout(int counter, double avg, double stddev)
 {
-        fprintf(stderr, " %14Ld  %-24s", count[0], event_name(counter));
+        fprintf(stderr, " %14.0f  %-24s", avg, event_name(counter));
 
-        if (runtime_cycles_avg &&
-            MATCH_EVENT(HARDWARE, HW_INSTRUCTIONS, counter)) {
+        if (MATCH_EVENT(HARDWARE, HW_INSTRUCTIONS, counter)) {
                 fprintf(stderr, " # %10.3f IPC  ",
-                        (double)count[0] / (double)runtime_cycles_avg);
+                                avg / avg_stats(&runtime_cycles_stats));
         } else {
-                if (runtime_nsecs_avg) {
-                        fprintf(stderr, " # %10.3f M/sec",
-                                (double)count[0]/runtime_nsecs_avg*1000.0);
-                }
+                fprintf(stderr, " # %10.3f M/sec",
+                                1000.0 * avg / avg_stats(&runtime_nsecs_stats));
         }
-        print_noise(count, noise);
+        print_noise(avg, stddev);
 }
 
 /*
@@ -322,12 +330,12 @@ static void abs_printout(int counter, u64 *count, u64 *noise)
  */
 static void print_counter(int counter)
 {
-        u64 *count, *noise;
+        double avg, stddev;
         int scaled;
 
-        count = event_res_avg[counter];
-        noise = event_res_noise[counter];
-        scaled = event_scaled_avg[counter];
+        avg    = avg_stats(&event_res_stats[counter][0]);
+        stddev = stddev_stats(&event_res_stats[counter][0]);
+        scaled = avg_stats(&event_scaled_stats[counter]);
 
         if (scaled == -1) {
                 fprintf(stderr, " %14s  %-24s\n",
@@ -336,36 +344,34 @@ static void print_counter(int counter)
         }
 
         if (nsec_counter(counter))
-                nsec_printout(counter, count, noise);
+                nsec_printout(counter, avg, stddev);
         else
-                abs_printout(counter, count, noise);
+                abs_printout(counter, avg, stddev);
+
+        if (scaled) {
+                double avg_enabled, avg_running;
+
+                avg_enabled = avg_stats(&event_res_stats[counter][1]);
+                avg_running = avg_stats(&event_res_stats[counter][2]);
 
-        if (scaled)
                 fprintf(stderr, "  (scaled from %.2f%%)",
-                        (double) count[2] / count[1] * 100);
+                                100 * avg_running / avg_enabled);
+        }
 
         fprintf(stderr, "\n");
 }
 
-/*
- * normalize_noise noise values down to stddev:
- */
-static void normalize_noise(u64 *val)
+static void update_stats(const char *name, int idx, struct stats *stats, u64 *val)
 {
-        double res;
+        double sq = *val;
 
-        res = (double)*val / (run_count * sqrt((double)run_count));
-
-        *val = (u64)res;
-}
-
-static void update_avg(const char *name, int idx, u64 *avg, u64 *val)
-{
-        *avg += *val;
+        stats->sum += *val;
+        stats->sum_sq += sq * sq;
 
         if (verbose > 1)
                 fprintf(stderr, "debug: %20s[%d]: %Ld\n", name, idx, *val);
 }
+
 /*
  * Calculate the averages and noises:
  */
@@ -377,61 +383,22 @@ static void calc_avg(void)
                 fprintf(stderr, "\n");
 
         for (i = 0; i < run_count; i++) {
-                update_avg("runtime", 0, &runtime_nsecs_avg, runtime_nsecs + i);
-                update_avg("walltime", 0, &walltime_nsecs_avg, walltime_nsecs + i);
-                update_avg("runtime_cycles", 0, &runtime_cycles_avg, runtime_cycles + i);
+                update_stats("runtime", 0, &runtime_nsecs_stats, runtime_nsecs + i);
+                update_stats("walltime", 0, &walltime_nsecs_stats, walltime_nsecs + i);
+                update_stats("runtime_cycles", 0, &runtime_cycles_stats, runtime_cycles + i);
 
                 for (j = 0; j < nr_counters; j++) {
-                        update_avg("counter/0", j,
-                                event_res_avg[j]+0, event_res[i][j]+0);
-                        update_avg("counter/1", j,
-                                event_res_avg[j]+1, event_res[i][j]+1);
-                        update_avg("counter/2", j,
-                                event_res_avg[j]+2, event_res[i][j]+2);
+                        update_stats("counter/0", j,
+                                event_res_stats[j]+0, event_res[i][j]+0);
+                        update_stats("counter/1", j,
+                                event_res_stats[j]+1, event_res[i][j]+1);
+                        update_stats("counter/2", j,
+                                event_res_stats[j]+2, event_res[i][j]+2);
                         if (event_scaled[i][j] != (u64)-1)
-                                update_avg("scaled", j,
-                                        event_scaled_avg + j, event_scaled[i]+j);
-                        else
-                                event_scaled_avg[j] = -1;
+                                update_stats("scaled", j,
+                                        event_scaled_stats + j, event_scaled[i]+j);
                 }
         }
-        runtime_nsecs_avg /= run_count;
-        walltime_nsecs_avg /= run_count;
-        runtime_cycles_avg /= run_count;
-
-        for (j = 0; j < nr_counters; j++) {
-                event_res_avg[j][0] /= run_count;
-                event_res_avg[j][1] /= run_count;
-                event_res_avg[j][2] /= run_count;
-        }
-
-        for (i = 0; i < run_count; i++) {
-                runtime_nsecs_noise +=
-                        abs((s64)(runtime_nsecs[i] - runtime_nsecs_avg));
-                walltime_nsecs_noise +=
-                        abs((s64)(walltime_nsecs[i] - walltime_nsecs_avg));
-                runtime_cycles_noise +=
-                        abs((s64)(runtime_cycles[i] - runtime_cycles_avg));
-
-                for (j = 0; j < nr_counters; j++) {
-                        event_res_noise[j][0] +=
-                                abs((s64)(event_res[i][j][0] - event_res_avg[j][0]));
-                        event_res_noise[j][1] +=
-                                abs((s64)(event_res[i][j][1] - event_res_avg[j][1]));
-                        event_res_noise[j][2] +=
-                                abs((s64)(event_res[i][j][2] - event_res_avg[j][2]));
-                }
-        }
-
-        normalize_noise(&runtime_nsecs_noise);
-        normalize_noise(&walltime_nsecs_noise);
-        normalize_noise(&runtime_cycles_noise);
-
-        for (j = 0; j < nr_counters; j++) {
-                normalize_noise(&event_res_noise[j][0]);
-                normalize_noise(&event_res_noise[j][1]);
-                normalize_noise(&event_res_noise[j][2]);
-        }
 }
 
 static void print_stat(int argc, const char **argv)
@@ -458,10 +425,11 @@ static void print_stat(int argc, const char **argv)
 
         fprintf(stderr, "\n");
         fprintf(stderr, " %14.9f  seconds time elapsed",
-                        (double)walltime_nsecs_avg/1e9);
+                        avg_stats(&walltime_nsecs_stats)/1e9);
         if (run_count > 1) {
                 fprintf(stderr, "   ( +- %7.3f%% )",
-                        100.0*(double)walltime_nsecs_noise/(double)walltime_nsecs_avg);
+                                100*stddev_stats(&walltime_nsecs_stats) /
+                                avg_stats(&walltime_nsecs_stats));
         }
         fprintf(stderr, "\n\n");
 }