[PATCH v3 0/3] perf sched latency: Refine outputs, unit scaling, and histogram support

[Aaron Tomlin]

This patch series improves 'perf sched latency' by fixing misleading empty
table output, introducing dynamic unit auto-scaling for latency and runtime
statistics, and adding latency histogram visualisation alongside time-span
filtering.

Patch 1 fixes a bug where 'perf sched latency' would fall through and
return success (0) when perf_session__has_traces() failed due to missing
tracepoint events in perf.data. This resulted in empty header tables and
zeroed summary statistics being printed. The fix ensures an early return of
a suitable error code.

Patch 2 introduces dynamic auto-scaling for display columns (Runtime, Avg
delay, Max delay). Previously, all values were unconditionally formatted in
milliseconds (ms), making microsecond or second-scale latencies difficult
to read. Columns are now scaled dynamically to the most appropriate unit
(ns, us, ms, s) and headers are updated accordingly.

Patch 3 adds three new command-line options to 'perf sched latency':
    --histogram (-H):
            Displays an ASCII bar chart of CPU wait latencies between
            snapshots

    --hist-mode:
            Configures the bucketing scheme to either logarithmic (log) or
            100 us equal-width linear (linear) mode

    --time:
            Filters event processing to a specified [start,stop] time span

Changes since v2:

 - Ensured vertical pipe separators align across all latency table columns
  
 - Excluded "swapper" (idle thread) latency samples from global_hist
  
 - Preserved task state machine transitions across --time boundaries
  
 - Suppressed empty table headers, total lines, and histogram graphs when
   no matching trace samples exist (e.g., when --time, --CPU, or --pids
   exclude all samples), outputting "No matching trace samples found."
   instead

 - Linked to v2: https://lore.kernel.org/lkml/20260725173341.679782-1-atomlin@xxxxxxxxxxx/

Changes since v1:

 - Fixed integer overflow in latency_bucket() (Ian Rogers)

 - Linked to v1: https://lore.kernel.org/lkml/20260724142901.634761-1-atomlin@xxxxxxxxxxx/

Aaron Tomlin (3):
  perf sched: Suppress latency table output when trace samples are
    missing
  perf sched latency: Auto-scale latency and runtime display units
  perf sched latency: Add histogram and time interval options

 tools/perf/Documentation/perf-sched.txt |   6 +
 tools/perf/builtin-sched.c              | 246 ++++++++++++++++++++++--
 2 files changed, 231 insertions(+), 21 deletions(-)

-- 
2.54.0