Add demo readme

Author: isbm

demo/perfetto/perfetto-to-logjet/README.md

@@ -0,0 +1,47 @@
+# Perfetto-to-logjet Demo
+
+End-to-end pipeline: record a Linux ftrace, import it via the perfetto-ingest
+plugin into a `.logjet` spool, and view the result in `ljx view`.
+
+## Build First
+
+```bash
+# From workspace root
+make dev
+./scripts/build-perfetto.sh
+```
+
+## Run
+
+```bash
+cd demo/perfetto/perfetto-to-logjet
+./run-demo.sh
+```
+
+Requires sudo for ftrace access.
+
+## What Happens
+
+1. `traced` + `traced_probes` start in the background.
+2. `tracebox` records 5s of scheduler events (CPU switches) via ftrace.
+3. `ljd` loads the perfetto-ingest plugin, which spawns `trace_processor`,
+   exports the trace as SQLite, maps `sched_slice` rows to OTel log records
+   with CPU/state/duration, and streams them into a `.logjet` spool.
+4. `ljx view` opens the spool — each CPU scheduling event appears as one line.
+
+## What You Should See
+
+- Thousands of log lines, each showing a CPU scheduling event:
+  ```
+  May  7 10:43:15 I  cpu=7 dur=7.2us state=R utid=19 ts=...
+  May  7 10:43:15 I  cpu=7 dur=2.0us state=R utid=21 ts=...
+  ```
+- Press `Enter` to see full OTel attributes (perfetto.sched.id, cpu, end_state).
+- Press `F` for field filter, `/` to search, `q` to quit.
+
+## Troubleshooting
+
+- **0 records**: The trace needs ftrace events — they require root. The script
+  uses `sudo tracebox`. If passwordless sudo isn't configured, run `sudo ./run-demo.sh`.
+- **Fewer records than expected in ljx view**: Delete stale index cache:
+  `rm -rf ~/.cache/ljx && ./run-demo.sh`