Re: [PATCH -mm] kprobes: kretprobe user entry-handler (updated)

[Abhishek Sagar]

On 1/28/08, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> Neither the changelog nor the newly-added documentation explain why Linux
> needs this feature.  What will it be used for??

There's a detailed discussion along with an example on this thread:
http://lkml.org/lkml/2007/11/13/58

and a bit on:
http://sourceware.org/ml/systemtap/2005-q3/msg00593.html

The real advantage of this patch is in scenarios where some data (like
function paramters, system time etc) needs to be shared between
function entry and exit. E.g: Viewing the change in system time across
a call to profile it. Here, we have a need to share data between the
entry and exit events of a function call. Also, the correct
association needs to be maintained between the corresponding function
entry-exit pairs. This is already done using a 'return-instance' in
kretprobes. This patch allows these instances to pouch some data as
well. The patch also has a module example which does trivial function
time-duration profiling using entry-handlers. It makes writing
function profilers simpler using kretprobes.

Currently, doing such a thing would require an extra kprobe to be
planted at function entry-point and whose pre-handler must have all
the complexity to do the function entry-exit data association. Also,
using an entry-handler is optional, and is completely backward
compatible.

> > +1.3.2 Kretprobe entry-handler
> > +
> > +Kretprobes also provides an optional user-specified handler which runs
>
> I think "caller-specified" would be a better term here.  Generally "user"
> refers to Aunt Tillie sitting at the keyboard.

Just followed suit from exising kprobes.txt which had quite a few
references to such a 'user'.

Cheerfully Acked by -> Aunt Tillie :-)

--
Thanks,
Abhishek
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