Re: proposed instruction trace support in SystemTap

[Dave Nomura <dcnltc at us dot ibm dot com>]

Would the performance of utrace be acceptable for doing instruction 
tracing?  It sounds like a lot of the signal overhead of ptrace is 
eliminated.
Would utrace only be useful for user-mode stepping, or could it somehow 
handle the in-kernel case?
Roland McGrath wrote:
> For user-mode stepping (all you can do via ptrace), this is what the utrace
> in-kernel APIs give you.  The in-kernel case has enough different issues
> that I think it's appropriate to consider it an entirely separate case.
> For that, kprobes already has its fingers in this area of machine-specific
> code.  It might make most sense for in-kernel stepping to be an extension
> of the kprobes code.  OTOH, with the hw_breakpoint (nee kwatch) work by
> Alan Stern <stern@rowland.harvard.edu> we have a second in-kernel case that
> (on some machines) wants to get involved with single-stepping.  Perhaps it
> makes sense to consolidate the efforts on some shared low-level part that
> deals with the stepping part. 
>  Or there may not be enough to be done there
> that anything beyond current machine-specific calls and trap notifiers are
> really required.  (Off hand I think at least some kind of coordination will
> be required to avoid these three things stepping on each other's toes.)
>
>
> Thanks,
> Roland
>
>   


--
Dave Nomura
LTC Linux Power Toolchain