Re: Unsafe mode for probes

[fche at redhat dot com (Frank Ch. Eigler)]

arkady.miasnikov wrote:

> My typical probe looks like this
>
> probe syscall.accept4.return ?
> {
>     %{HIT_MAP_INC(HIT_MAP_SYSCALL_ACCEPT4_RETURN)%}
>     send_incident_result(%{INCDENT_TYPE_SYSCALL_ACCEPT4%}, $return);
> }
> [...]

> The actual assembler of the probe will contain quite a bit of code
> handling local variables, call to lock, etc
> For short probes I can shave about 20% of CPU cycles.

A blanket "unsafe" probe cannot make do with no checks at all, e.g.
a proper context structure allocation for temporary values, etc. 
It's a matter of detail - which checks particularly should one skip?

It is not unlikely that some of that initialization / lock business in
the probe prologue could be elided entirely with some more cleverness
during translation.  Like a new pragma for embedded-C functions that
skip the context-struct based api, which in turn could make it
unnecessary to have a context struct at all for that probe.  Stuff
like that - but it takes analysis to figure out which is needed
and which is not.  stap -p3 ftw.

- FChE