Re: tracepoint maintainance models

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

mingo wrote:

> [...]
>   static int x;
>   void func(int a)
>        MARK(event, a);
>
> if a dynamic tracer installs a probe into that MARK() spot, it will
> have access to 'a', but it can also have access to 'x'. While a
> static in-source markup for _static tracers_, if it also wanted to
> have the 'x' information, would also have to add 'x' as a parameter:
> [...]

Without heroic measures taken by by a static tracer type of tool, this
is correct.

> For dynamic tracers no such 'parameter preparation' instructions
> would need to be generated by gcc.  (thus for example the runtime
> overhead would be lower for inactive tracepoints)

Any such additional code would be small, plus if properly marked up
with unlikely() and compiled with -freorder-blocks, it would all be
out-of-line.  This small cost could be worth the added benefit of
systemtap being able to probe that point without debugging information
present, and avoiding its slow & deliberate way of accessing
target-side variables like $x.  (The slow & deliberate part comes in
from the need to check any pointer dereferences involved.)

- FChE