Normalize the variance to stddev, and remove some redundant steps in
computing the time from itself.

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
---
 tests/perf_pmu.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/tests/perf_pmu.c b/tests/perf_pmu.c
index 5a26d5272..4e8da3d94 100644
--- a/tests/perf_pmu.c
+++ b/tests/perf_pmu.c
@@ -1577,8 +1577,8 @@ accuracy(int gem_fd, const struct intel_execution_engine2 
*e,
                /* 1st pass is calibration, second pass is the test. */
                for (int pass = 0; pass < ARRAY_SIZE(timeout); pass++) {
                        unsigned int target_idle_us = idle_us;
-                       uint64_t busy_ns = 0, idle_ns = 0;
                        struct timespec start = { };
+                       uint64_t busy_ns = 0;
                        unsigned long pass_ns = 0;
                        double avg = 0.0, var = 0.0;
                        unsigned int n = 0;
@@ -1589,6 +1589,7 @@ accuracy(int gem_fd, const struct intel_execution_engine2 
*e,
                                unsigned long loop_ns, loop_busy;
                                struct timespec _ts = { };
                                double err, tmp;
+                               uint64_t now;
 
                                /* PWM idle sleep. */
                                _ts.tv_nsec = target_idle_us * 1000;
@@ -1605,14 +1606,13 @@ accuracy(int gem_fd, const struct 
intel_execution_engine2 *e,
                                igt_spin_batch_end(spin);
 
                                /* Time accounting. */
-                               loop_ns = igt_nsec_elapsed(&start);
-                               loop_busy = loop_ns - loop_busy;
-                               loop_ns -= pass_ns;
+                               now = igt_nsec_elapsed(&start);
+                               loop_busy = now - loop_busy;
+                               loop_ns = now - pass_ns;
+                               pass_ns = now;
 
                                busy_ns += loop_busy;
                                total_busy_ns += loop_busy;
-                               idle_ns += loop_ns - loop_busy;
-                               pass_ns += loop_ns;
                                total_ns += loop_ns;
 
                                /* Re-calibrate. */
@@ -1628,10 +1628,14 @@ accuracy(int gem_fd, const struct 
intel_execution_engine2 *e,
                                var += (err - avg) * (err - tmp);
                        } while (pass_ns < timeout[pass]);
 
+                       pass_ns = igt_nsec_elapsed(&start);
                        expected = (double)busy_ns / pass_ns;
-                       igt_info("%u: busy %"PRIu64"us, idle %"PRIu64"us -> 
%.2f%% (target: %lu%%; average=%.2f, variance=%f)\n",
-                                pass, busy_ns / 1000, idle_ns / 1000,
-                                100 * expected, target_busy_pct, avg, var / n);
+
+                       igt_info("%u: busy %"PRIu64"us, idle %"PRIu64"us -> 
%.2f%% (target: %lu%%; average=%.2f±%.3f%%)\n",
+                                pass, busy_ns / 1000, (pass_ns - busy_ns) / 
1000,
+                                100 * expected, target_busy_pct,
+                                avg, sqrt(var / n));
+
                        write(link[1], &expected, sizeof(expected));
                }
 
-- 
2.18.0

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