Quoting Tvrtko Ursulin (2021-02-16 15:59:33)

On 16/02/2021 12:49, Chris Wilson wrote:

quoted

Quoting Tvrtko Ursulin (2021-02-16 10:50:50)

quoted

From: Tvrtko Ursulin <redacted>

Test that periodic reads of engine busyness against a constant 100% load
are within the 5000ppm tolerance when comparing perf timestamp versus
counter values.

Signed-off-by: Tvrtko Ursulin <redacted>
---
  tests/i915/perf_pmu.c | 46 ++++++++++++++++++++++++++++++++++++++-----
  1 file changed, 41 insertions(+), 5 deletions(-)

diff --git a/tests/i915/perf_pmu.c b/tests/i915/perf_pmu.c
index 50b5c82bc472..728312be5293 100644
--- a/tests/i915/perf_pmu.c
+++ b/tests/i915/perf_pmu.c

@@ -26,6 +26,7 @@
  #include <stdio.h>
  #include <string.h>
  #include <fcntl.h>
+#include <float.h>
  #include <inttypes.h>
  #include <errno.h>
  #include <signal.h>

@@ -46,6 +47,7 @@
  #include "igt_perf.h"
  #include "igt_sysfs.h"
  #include "igt_pm.h"
+#include "igt_stats.h"
  #include "sw_sync.h"
  
  IGT_TEST_DESCRIPTION("Test the i915 pmu perf interface");

@@ -278,8 +280,11 @@ static void end_spin(int fd, igt_spin_t *spin, unsigned int flags)
  static void
  single(int gem_fd, const struct intel_execution_engine2 *e, unsigned int flags)
  {
+       unsigned int loops = flags & FLAG_LONG ? 20 : 1;
+       double err_min = DBL_MAX, err_max = -DBL_MAX;
         unsigned long slept;
         igt_spin_t *spin;
+       igt_stats_t s;
         uint64_t val;
         int fd;
  

@@ -290,11 +295,40 @@ single(int gem_fd, const struct intel_execution_engine2 *e, unsigned int flags)
         else
                 spin = NULL;
  
-       val = pmu_read_single(fd);
-       slept = measured_usleep(batch_duration_ns / 1000);
-       if (flags & TEST_TRAILING_IDLE)
-               end_spin(gem_fd, spin, flags);
-       val = pmu_read_single(fd) - val;
+       igt_stats_init_with_size(&s, loops);
+
+       while (--loops) {

while (loops--)

/o\

Yeah.. At least I know the oddity is related to sampling. Since even on 
Haswell:

(perf_pmu:1591) DEBUG: time=500207720 busy=500037022 error=-341.25ppm
(perf_pmu:1591) DEBUG: time=500252520 busy=500033517 error=-437.78ppm
(perf_pmu:1591) DEBUG: time=500187490 busy=499999817 error=-375.21ppm
(perf_pmu:1591) DEBUG: time=500244871 busy=499999837 error=-489.83ppm
(perf_pmu:1591) DEBUG: time=500268670 busy=499999477 error=-538.10ppm
(perf_pmu:1591) DEBUG: time=500245246 busy=500000432 error=-489.39ppm
(perf_pmu:1591) DEBUG: time=500245735 busy=499999306 error=-492.62ppm
(perf_pmu:1591) DEBUG: time=500270045 busy=500001747 error=-536.31ppm
(perf_pmu:1591) DEBUG: time=500254286 busy=499998162 error=-511.99ppm
(perf_pmu:1591) DEBUG: time=500247790 busy=500000347 error=-494.64ppm
(perf_pmu:1591) DEBUG: time=500250261 busy=500000257 error=-499.76ppm
(perf_pmu:1591) DEBUG: time=500250005 busy=500008177 error=-483.41ppm
(perf_pmu:1591) DEBUG: time=500249065 busy=499991867 error=-514.14ppm
(perf_pmu:1591) DEBUG: time=500249725 busy=500000371 error=-498.46ppm
(perf_pmu:1591) DEBUG: time=500250335 busy=499999772 error=-500.88ppm
(perf_pmu:1591) DEBUG: time=500258691 busy=499999937 error=-517.24ppm
(perf_pmu:1591) DEBUG: time=500239980 busy=500001037 error=-477.66ppm
(perf_pmu:1591) DEBUG: time=500240791 busy=504999361 error=9512.56ppm

And this last one is way more than one sampling period. I'll be thinking 
about this in the background.

One thing to add would be the cumulative error. It does feel like a
corrective factor is applied to the sampling period.
-Chris
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