[PATCH 0/5] Support metric group constraint

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Some metric groups, e.g. Page_Walks_Utilization, will never count when
NMI watchdog is enabled.

 $echo 1 > /proc/sys/kernel/nmi_watchdog
 $perf stat -M Page_Walks_Utilization

 Performance counter stats for 'system wide':

 <not counted>      itlb_misses.walk_pending       (0.00%)
 <not counted>      dtlb_load_misses.walk_pending  (0.00%)
 <not counted>      dtlb_store_misses.walk_pending (0.00%)
 <not counted>      ept.walk_pending               (0.00%)
 <not counted>      cycles                         (0.00%)

       2.343460588 seconds time elapsed

 Some events weren't counted. Try disabling the NMI watchdog:
        echo 0 > /proc/sys/kernel/nmi_watchdog
        perf stat ...
        echo 1 > /proc/sys/kernel/nmi_watchdog
 The events in group usually have to be from the same PMU. Try
 reorganizing the group.

A metric group is a weak group, which relies on group validation
code in the kernel to determine whether to be opened as a group or
a non-group. However, group validation code may return false-positives,
especially when NMI watchdog is enabled. (The metric group is allowed
as a group but will never be scheduled.)

The attempt to fix the group validation code has been rejected.
https://lore.kernel.org/lkml/20200117091341.GX2827@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Because we cannot accurately predict whether the group can be scheduled
as a group, only by checking current status.

This patch set provides another solution to mitigate the issue.
Add "MetricConstraint" in event list, which provides a hint for perf tool,
e.g. "MetricConstraint": "NO_NMI_WATCHDOG". Perf tool can change the
metric group to non-group (standalone metrics) if NMI watchdog is enabled.

After applying the patch,

 $echo 1 > /proc/sys/kernel/nmi_watchdog
 $perf stat -M Page_Walks_Utilization
  Splitting metric group Page_Walks_Utilization into standalone metrics.
  Try disabling the NMI watchdog to comply NO_NMI_WATCHDOG metric constraint:
        echo 0 > /proc/sys/kernel/nmi_watchdog
        perf stat ...
        echo 1 > /proc/sys/kernel/nmi_watchdog

 Performance counter stats for 'system wide':

        18,253,454      itlb_misses.walk_pending  #      0.0
                              Page_Walks_Utilization   (50.55%)
        78,051,525      dtlb_load_misses.walk_pending  (50.55%)
        29,213,063      dtlb_store_misses.walk_pending (50.55%)
                 0      ept.walk_pending               (50.55%)
     2,542,132,364      cycles                         (49.92%)

       1.037095993 seconds time elapsed

Kan Liang (5):
  perf jevents: Support metric constraint
  perf metricgroup: Factor out metricgroup__add_metric_weak_group()
  perf util: Factor out sysctl__nmi_watchdog_enabled()
  perf metricgroup: Support metric constraint
  perf vendor events: Add NO_NMI_WATCHDOG metric constraint

 .../arch/x86/cascadelakex/clx-metrics.json         |  3 +-
 .../pmu-events/arch/x86/skylake/skl-metrics.json   |  3 +-
 .../pmu-events/arch/x86/skylakex/skx-metrics.json  |  3 +-
 tools/perf/pmu-events/jevents.c                    | 19 +++--
 tools/perf/pmu-events/jevents.h                    |  2 +-
 tools/perf/pmu-events/pmu-events.h                 |  1 +
 tools/perf/util/metricgroup.c                      | 97 ++++++++++++++++------
 tools/perf/util/stat-display.c                     |  6 +-
 tools/perf/util/util.c                             | 18 ++++
 tools/perf/util/util.h                             |  2 +
 10 files changed, 116 insertions(+), 38 deletions(-)

-- 
2.7.4