Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

[Karim Yaghmour <karim@opersys.com>]

Jes Sorensen wrote:
> There very few tracepoints in this category,

Wow, that's progress.

> the only things you can
> claim are more or less generic are syscalls, and tracing syscall
> handling is tricky.

If there are implementation issue, I trust an adequate solution can be
found by using the tested-and-proven method of posting stuff on the
lkml for review.

> This is grossly over simplifying things and why the whole things doesn't
> hold water. There is no such thing as 'the place' to put a specific
> tracepoint.
> 
> Especially when we start talking about things like tracepoints in the
> scheduler.

I do not underestimate the difficulty of selecting such tracepoints.
This is why I chose not to maintain other people's specific tracepoints.
I realize this is a tough problem, but I also trust subsystem maintainers
are smart enough to make the appropriate decision. Obviously for such
things like the scheduler, any fine-grained instrumentation will draw
a barrage of criticism from anyone since a lot of stuff depends on it.
Either the lkml process works or it doesn't, but it isn't for me to
decide.

> Note that I haven't been referring to debug tracepoints at any point in
> this debate.

You're right, but others have happily intermingled the whole lot, and
I just wanted to document my personal categorization on lkml for all
to see.

> You seem to think that it's fine to add instrumentation in the syscall
> path as an example as long as it's compiled out. Well on some
> architectures, the syscall path is very sensitive to alignment and there
> may be restrictions on how large the stub of code is allowed to be, like
> a few hundred bytes. Just because things work one way on x86, doesn't
> mean they work like that everywhere.

If ltt failed to implement such things appropriately, then we apologize.
That fact doesn't preclude proper implementation in the future, however.

Karim