[PATCH v4 0/9] perf c2c: add a function view

[Jiebin Sun <jiebin.sun@intel.com>]

This series adds a new "function view" to perf c2c report, on top of the
existing cacheline view. It does not change the cacheline view; it adds a
second, complementary way to look at the same cache-to-cache (C2C) data.

v1: https://lore.kernel.org/linux-perf-users/20260626070355.1556721-1-jiebin.sun@intel.com/
v2: https://lore.kernel.org/linux-perf-users/20260710084247.3576706-1-jiebin.sun@intel.com/
v3: https://lore.kernel.org/linux-perf-users/20260717020530.1645123-1-jiebin.sun@intel.com/

Changes since v3
================

Reworked the function view into a true function-centric hierarchy, as
discussed with Namhyung:

  - Level 1 is now the read-side function itself, aggregating all of its
    code addresses into a single entry (v3 keyed level 1 on the code
    address, which read like a "code-address view"). Suggested by
    Namhyung.

  - Level 2 is the contending function that writes the shared lines
    (causing the level-1 function's HITMs), aggregated across all the
    cachelines it contends over and sorted by store count. Level 3 is
    the specific cachelines. This lets a contending pair show its total
    on one row, instead of repeating a writer under every cacheline and
    leaving the reader to sum by hand.

  - Every level shows only functions and cachelines; code addresses and
    offsets are no longer shown in the main view. They remain available
    in the per-cacheline detail view ('d').

  - Lower-level entries are indented per hierarchy level, like the normal
    perf report hierarchy view. Suggested by Namhyung.

  - Cachelines with no writing function, and functions left with no
    contending writer, are pruned so the view shows only real contention.

  - Reorganized the series from 14 mechanical patches into 9 logically
    self-contained ones (skeleton, column rendering, HPP parsing, stats
    merge, entry creation, build/finalize, browser UI, doc).

  - Updated the perf-c2c man page example to the new layout.

  - Rebased onto the latest perf-tools-next.

Changes since v2
================

  - perf c2c: add a c2c_ prefix to the functions exported from
    builtin-c2c.c (fmt_free() -> c2c_fmt_free(), fmt_equal() ->
    c2c_fmt_equal()), so the shared symbols do not pollute the global
    namespace. Suggested by Namhyung.

Changes since v1
================

  - Add the worked man-page example, as suggested by Namhyung.

What it does
============

In the perf c2c TUI, press TAB in the cacheline view to switch to the
function view. It presents a 3-level hierarchy of the same data,
organized around functions rather than cachelines:

  Level 1: the read-side function, sorted by Cycles % (estimated load
           cycles: HITM, peer-snoop and other-load cycles -- on systems
           whose default display mode is peer, such as Arm64, the
           peer-snoop component dominates)
  Level 2: the functions that write the shared lines, causing the
           level-1 function's HITMs, sorted by store count
  Level 3: the specific cachelines where the two functions contend

Every level shows functions and cachelines only; code addresses are not
shown in the main view -- they remain available in the per-cacheline
detail view ('d'). Each function aggregates all of its code addresses
into a single entry, and a level-2 writer aggregates all of the
cachelines it contends over, so a contending pair is a single row with
its total shown.

Keys in the function view:
  TAB/ESC/q   return to the cacheline view
  d           show cacheline details for the selected entry
  e / +       expand / collapse the selected entry
  ?           help

The cacheline view and the --stdio output are unchanged.

Example
=======

A level-1 function is expanded (press 'e') to reveal the functions that
write the lines it reads, and one of those is expanded again to reveal
the specific shared cachelines:

  Shared Data Functions Table     (18 entries, sorted on Cycles %)
     Cycles    Store
          %    count  Function / Contending function / Cacheline
  ----------------------------------------------------------------------
  -  16.54%      419  - [k] dequeue_pushable_task
                 145    - [k] pull_rt_task
                 145        0xff2d0082809da080
                 139    - [k] enqueue_pushable_task
                  70        0xff2d00a2071f9640
                  69        0xff2d0082809da000

Here dequeue_pushable_task pays the read-side HITM penalty (16.54% of the
estimated load cycles). It is contended by pull_rt_task and
enqueue_pushable_task, which write the shared lines; enqueue_pushable_task
contends over two cachelines but stays a single row with its total (139)
shown, and the individual cachelines are underneath. This is the
false-sharing chain that the cacheline view otherwise leaves to be
reconstructed by hand.

Implementation
==============

The function view is a separate hist_browser in
tools/perf/ui/browsers/c2c-function.c. Shared types and helpers used by
both views are factored out of builtin-c2c.c into a new c2c.h. The
hierarchy is constructed from the existing cacheline histograms into a
dedicated set of hists and rendered with custom column formatters.

Testing
=======

  - Each of the 9 commits builds individually and as a full series.
  - perf c2c report --stdio (cacheline view) output is unchanged versus
    the baseline.
  - The function view was exercised on c2c recordings; the level-1
    ordering and the level-2/3 sharing breakdown match the underlying
    cacheline data.

Jiebin Sun (9):
  perf c2c: extract shared data structures into c2c.h
  perf c2c: add function view browser skeleton
  perf c2c: add column rendering for function view
  perf c2c: add HPP list parsing for function view columns
  perf c2c: add function view stats merge and memory management
  perf c2c: add function view hierarchy entry creation
  perf c2c: build and finalize the function view hierarchy
  perf c2c: add function view browser UI and cacheline detail
  perf c2c: document function view in perf-c2c man page

 tools/perf/Documentation/perf-c2c.txt |   46 +
 tools/perf/builtin-c2c.c              |  132 +-
 tools/perf/c2c.h                      |  154 +++
 tools/perf/ui/browsers/Build          |    1 +
 tools/perf/ui/browsers/c2c-function.c | 1789 +++++++++++++++++++++++++
 5 files changed, 2002 insertions(+), 120 deletions(-)
 create mode 100644 tools/perf/c2c.h
 create mode 100644 tools/perf/ui/browsers/c2c-function.c


base-commit: 87ec3437f37b9fe44c524ba967cb12e78de06f15
-- 
2.52.0