[RFC PATCH v2 0/5] Add perf.data tracepoint events to trace.dat conversion

[Tanushree Shah <tshah@linux.ibm.com>]

This RFC patch series introduces support for converting perf.data files
containing tracepoint events into trace.dat format, enabling seamless
visualization and analysis using KernelShark.

======================
Background and Motivation
======================

Currently, perf and trace-cmd operate as separate tracing ecosystems with
incompatible data formats. Users who collect tracepoint data with
'perf record' cannot easily visualize it in KernelShark's graphical
timeline view or leverage trace-cmd's analysis capabilities.

This creates workflow friction when users need to:

- Visualize perf tracepoint data in KernelShark's interactive graphical
  timeline
- Share trace data between perf and trace-cmd workflows and toolchains
- Perform architecture-independent conversion and analysis of traces

This conversion bridge eliminates these barriers by enabling seamless
data exchange between perf and trace-cmd ecosystems, allowing users to
choose the best tool for each analysis phase.

======================
Implementation Overview
======================

The series implements the trace.dat file format specification (version 7)
within perf's data conversion framework.

**Patch 1/5: Core trace.dat Export Infrastructure**
Introduces util/trace-dat.c and util/trace-dat.h implementing:
- Per-CPU raw event buffer management (init, collect, free)
- Ftrace ring buffer page construction
- trace.dat section writers (strings, options, flyrecord sections)

**Patch 2/5: Metadata Integration**
Extends util/trace-event-read.c to write trace.dat metadata during
perf.data
parsing:
- Initial format header (magic, version, endian, page size, compression)
- Section 16: HEADER INFO (header_page + header_event)
- Section 17: FTRACE EVENT FORMATS
- Section 18: EVENT FORMATS (per system/event format files)
- Section 19: KALLSYMS
- Section 21: CMDLINES
- Section 15: STRINGS (written last after all sections)

**Patch 3/5: Conversion Backend**
Implements util/data-convert-trace.c with trace_convert__perf2dat()
function:
- Processes PERF_TYPE_TRACEPOINT samples via process_sample_event()
- Collects raw event data per-CPU using trace_dat__collect_cpu_event()
- Writes OPTIONS sections (CPUCOUNT, TRACECLOCK, metadata offsets)
- Writes FLYRECORD section with per-CPU ring buffer pages

**Patch 4/5: User Interface**
Extends tools/perf/builtin-data.c with --to-trace-dat option:
- Adds command-line option for trace.dat output
- Mutually exclusive with --to-ctf and --to-json
- Calls trace_convert__perf2dat() to perform conversion

**Patch 5/5: Shell Test**
Adds a shell test (tools/perf/tests/shell/) covering normal tracepoint
recordings, pipe mode and mixed tracepoint/non-tracepoint recording
conversions and --force flag behaviour.

======================
Current Implementation Details
======================

**trace.dat Format Version:**
The implementation currently targets trace.dat format version 7, which
is the stable version supported by current trace-cmd releases (v3.x).
This version is hardcoded to ensure compatibility with existing
trace-cmd and KernelShark installations. Future enhancements could add
version negotiation or support for newer format versions as they become
standardized.

**Compression Strategy:**
Compression is explicitly disabled (set to NONE) in the generated
trace.dat files.
This design choice:
- Simplifies the initial implementation and testing
- Ensures maximum compatibility across trace-cmd versions
- Avoids external compression library dependencies

Future work could add support for various compression algorithms (zlib,
zstd, lz4) with runtime selection via command-line options, significantly
reducing file sizes for large traces.

======================
Usage Example
======================

```bash
*Record tracepoint events with perf*
perf record -e sched:sched_switch -e sched:sched_wakeup -a sleep 10

*Convert to trace.dat format*
perf data convert --to-trace-dat=output.dat

*Verify trace.dat structure*
trace-cmd dump --summary output.dat

*Analyze with trace-cmd*
trace-cmd report output.dat

*Visualize in KernelShark*
kernelshark output.dat
```

**Conversion Output:**
```
[ perf data convert: Converted 'perf.data' into trace.dat format
'output.dat' ]
[ perf data convert: Converted 2684 events ]
```
**trace-cmd dump --summary Output:**
```
 Tracing meta data in file output.dat:
	[Initial format]
		7	[Version]
		0	[Little endian]
		8	[Bytes in a long]
		65536	[Page size, bytes]
		none	[Compression algorithm]
			[Compression version]
	[buffer "", "local" clock, 65536 page size, 16 cpus, 1048576 bytes
    flyrecord data]
	[10 options]
	[Saved command lines, 0 bytes]
	[Kallsyms, 0 bytes]
	[Ftrace format, 0 events]
	[Header page, 206 bytes]
	[Header event, 205 bytes]
	[Events format, 1 systems]
	[9 sections]
```	
======================
Testing and Verification
======================

The series has been extensively tested with:
- Various tracepoint events (sched, irq, syscalls, block I/O)
- Mixed recordings containing both tracepoint and non-tracepoint events
  only tracepoints converted)
- Verification with trace-cmd report and KernelShark visualization
- Memory leak testing with Valgrind (0 bytes leaked).
- Cross-architecture testing: v1 tested x86_64 and ppc64le. v2 adds
  s390 (big-endian) perf.data converted on both ppc64le
  (little-endian) and x86_64 (little-endian) hosts, in addition to
  same-arch x86_64 (LE->LE) and ppc64le (BE->BE) conversion.
- Pipe mode support has been tested end-to-end. (in v2)

All generated trace.dat files successfully open in:
- trace-cmd report (v3.1+)
- KernelShark (v2.0+)


======================
Next Steps
======================

We would highly appreciate reviews, comments, and feedback on:
- The overall architectural approach and integration points
- Compatibility considerations with trace-cmd ecosystem
- Performance characteristics for large-scale traces
- Additional use cases or workflow scenarios
- Future enhancement priorities

---
Changes in v2

Addressing the Sashiko AI review findings on v1:
 
Cross-arch correctness:
  - Introduce to_file_u16/u32/u64 helpers (wrapping tep_read_number())
    to write all multi-byte fields in the recorded machine's byte order;
    apply throughout metadata sections and flyrecord page/record headers
    (ts, commit, TIME_EXTEND, large-event data_len).
  - Fix flyrecord record header bit layout for big-endian files.
    The record header word bit layout differs by file endianness,
    matching kbuffer-parse.c type_len4host()/ts4host():
      LE: type_len in bits [4:0],   time_delta in bits [31:5]
      BE: type_len in bits [31:27], time_delta in bits [26:0]
 
Pipe mode:
  - Add process_attr(), process_feature(), and process_tracing_data()
    callbacks required for pipe mode operation.
  - Defer CPU buffer initialisation until the first tracepoint sample,
    after process_feature()/process_tracing_data() have populated the
    session header. This ensures the recorded machine's CPU count is
    used rather than the host's - critical for cross-platform analysis.
 
Format compliance:
  - Implement TIME_EXTEND records for timestamp deltas >27 bits to
    prevent silent truncation and maintain chronological ordering.
  - Fix large event encoding (>=29 words): use type_len=0 with a
    separate 32-bit length word, avoiding collision with reserved types
    (PADDING=29, TIME_EXTEND=30, TIME_STAMP=31).
  - Add bounds check rejecting records larger than a page payload before
    batching, preventing heap overflow in trace_dat__write_page().
  - Fix flyrecord section_size to exclude the 16-byte section header,
    matching trace.dat specification and trace-cmd behaviour.
 
CLI behavior:
  - Fix --force flag: open with O_CREAT|O_EXCL when force is not set,
    failing with -EEXIST instead of silently overwriting existing files.
 
Memory safety:
  - Fix realloc overwrite of cpu_events->events and page_records on
    failure: use temporary pointers, only commit on success.
  - Fix use-after-free/double-free in sequential page_records realloc
    failure: replace with malloc+memcpy+free pattern.
  - Fix section_size computed from before section header position.
  - Add NULL checks for get_tracing_file(), calloc() padding, and
    trace_dat_options_offset assignment on write failure.
  - Use goto out_free on record allocation failure to avoid leaking
    accumulated page_records entries.
  - Replace direct read() with do_read() in read_proc_kallsyms() to
    handle short reads correctly.
  - On fwrite failure, set trace_dat_write_failed and continue parsing
    so that perf.data processing completes normally.
 
Testing (new in v2):
  - Add shell test covering conversion, trace-cmd dump validation,
    sched_switch event verification, and --force flag behaviour.
 
Documentation (new in v2):
  - Add documentation for 'perf data convert --to-trace-dat', covering
    usage and supported options.

v1: https://lore.kernel.org/linux-perf-users/20260608125951.90425-2-tshah@linux.ibm.com/

Tanushree Shah (5):
  perf/trace-dat: Add trace.dat export infrastructure
  perf/trace-event: Write trace.dat metadata sections during parsing
  perf data-convert: Add perf.data to trace.dat conversion backend
  perf data: Add --to-trace-dat option for converting perf.data
    tracepoint events into trace.dat format
  perf test: Add test validating trace.dat generated by 'perf data
    convert --to-trace-dat'


 tools/perf/Documentation/perf-data.txt        |   7 +
 tools/perf/builtin-data.c                     |  40 +-
 ...rf_data_converter_tracepoints_trace_dat.sh | 169 ++++
 tools/perf/util/Build                         |   2 +
 tools/perf/util/data-convert-trace.c          | 240 +++++
 tools/perf/util/data-convert.h                |   4 +
 tools/perf/util/trace-dat.c                   | 876 ++++++++++++++++++
 tools/perf/util/trace-dat.h                   | 113 +++
 tools/perf/util/trace-event-read.c            | 307 +++++-
 9 files changed, 1749 insertions(+), 9 deletions(-)
 create mode 100755 tools/perf/tests/shell/test_perf_data_converter_tracepoints_trace_dat.sh
 create mode 100644 tools/perf/util/data-convert-trace.c
 create mode 100644 tools/perf/util/trace-dat.c
 create mode 100644 tools/perf/util/trace-dat.h

-- 
2.47.3