Re: [PATCH] Linux Kernel Markers

[Martin Bligh <mbligh@google.com>]

> It is an interesting idea but there appears to be following hard issues 
> (some of which you have already listed) i am not able to see how we can 
> overcome them
> 
> 1) We are going to have a duplicate of the whole function which means 
> any significant changes in the original function needs to be done on the 
> copy as well, you think maintainers would like this double work idea.

No, no ... the duplicate function isn't duplicated source code, only
object code. Either a config option via the markup macros that we've
been discussing, or something I hack up on the fly to debug a problem
dynamically. In terms of how the debugging-type source code is kept,
it's no different than something like systemtap or LTT (either would
work, and a normal diff could be used to keep out of tree stuff),
it's just how it hooks in is different to kprobes.

> 2) Inline functions is often the place where we need a fast path to 
> overcome the current kprobes overhead.

You can still instrument inline functions, you just need to hook all
the callers, not the inline itself.

> 3) As you said it is not trivial across all the platforms to do a switch 
> to the instrumented function from the original during the execution.  
> This problem is similar to the issue we are dealing with djprobes.

If we just freeze all kernel operations for a split second whilst we do
this, does it matter? Or even if we don't ... there's a brief race where
some calls are traced, and some are not ... does that even matter?
Doesn't seem like most usages would care.

M.