Tracefs

In-kernel tracing is rapidly becoming a feature that developers and users count on. In current kernels, though, the virtual files used to control tracing and access data are all found in the debugfs filesystem, in the tracing directory. That is not seen as a long-term solution; debugfs is meant for volatile, debugging information, but tracing users want to see a stable ABI in a non-debugging location.

Following up on some conference discussions, Greg Kroah-Hartman decided to regularize the tracing file hierarchy through the creation of a new tracefs virtual filesystem. Tracefs looks a lot like .../debug/tracing in that the files have simply been moved from one location to the other. Tracefs has a simpler internal API, though, since it does not require all of the features supported by debugfs.

The idea of tracefs is universally supported, but this particular patch looks like it will not be going in anytime soon. The concern is that anything moved out of debugfs and into something more stable will instantly become part of the kernel ABI. Much of the current tracing interface has been thrown together to meet immediate needs; the sort of longer-term thinking which is needed to define an interface which can remain stable for years is just beginning to happen.

Ingo Molnar thinks that the virtual files which describe the available events could be exported now, but not much else. That leaves most of the interface in an unstable state, still. So Greg has withdrawn the patch for now; expect it to come back with the tracing developers are more ready to commit to their ABI. At that point, we can expect the debate to begin on the truly important question: /tracing or /sys/kernel/tracing?

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