Important note: both wult and ndl are research, debugging and tracing tools which require superuser privileges and must not be used in production environment.
Wult stands for "Wake Up Latency Tracer", and this is a project providing tools for measuring C-state latency in Linux. The project provides 2 tools:
These web pages are the primary source of wult and ndl. In addition to this, man pages include many details:
Here is an old wult presentation video recording by Artem Bityutskiy at Linux Plumbers Conference in 2019. The presentation begins at "2:17:50" and it gives a high level wult introduction.
The presentation focuses on wult kernel drivers, but some of the details are out of date.
And finally, if you do not find the information you are looking for, feel free to ask questions by filing GitHub issues or sending an e-mail to Artem Bityutskiy <email@example.com>.