Re: [PATCH v14 4/4] Documentation: trace/objtrace: Add documentation for objtrace

[Joel Fernandes]

On Thu, Aug 18, 2022 at 12:38 PM Jeff Xie <xiehuan09@xxxxxxxxx> wrote:
>
> Hi Joel,
>
> Thank you for your review.
>
> On Thu, Aug 18, 2022 at 11:53 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jun 28, 2022 at 10:55:52PM +0800, Jeff Xie wrote:
> > > Add documentation explaining how to use objtrace trigger to get the value
> > > of the object.
> > >
> > > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > > Cc: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
> > > Cc: linux-doc@xxxxxxxxxxxxxxx
> > > Signed-off-by: Jeff Xie <xiehuan09@xxxxxxxxx>
> > > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > > ---
> > > Changelog:
> > > v14:
> > > - make documentation more readable and fix literal code block by Bagas Sanjaya
> > >
> > >  Documentation/trace/events.rst | 87 ++++++++++++++++++++++++++++++++++
> > >  1 file changed, 87 insertions(+)
> > >
> > > diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
> > > index c47f381d0c00..c15f1d25d4a0 100644
> > > --- a/Documentation/trace/events.rst
> > > +++ b/Documentation/trace/events.rst
> > > @@ -546,6 +546,93 @@ The following commands are supported:
> > >
> > >    See Documentation/trace/histogram.rst for details and examples.
> > >
> > > +- objtrace
> > > +
> > > +  This command provides a way to get the value of any object, The object
> > > +  can be obtained from the dynamic event (kprobe_event/uprobe_event) or the
> > > +  static event (tracepoint).
> > > +
> > > +  Usage:
> > > +  When using the kprobe event, by only need to set the objtrace (a new
> > > +  trigger), we can get the value of object that is set by kprobe event.
> > > +
> > > +  For example, for the function bio_add_page():
> > > +
> > > +  .. code-block:: c
> > > +
> > > +     int bio_add_page(struct bio *bio, struct page *page,
> > > +                   unsigned int len, unsigned int offset)
> > > +
> > > +  Firstly, we can set the base of the object as first parameter (arg1) to
> > > +  to the function:
> > > +
> > > +  .. code-block::
> > > +
> > > +     # echo 'p bio_add_page arg1=$arg1' > ./kprobe_events
> > > +
> > > +  Secondly, we can get the value dynamically based on the object:
> > > +
> > > +  .. code-block::
> > > +
> > > +     find the offset of the bi_size in struct bio:
> > > +     $ gdb vmlinux
> > > +     (gdb) p &(((struct bio *)0)->bi_iter.bi_size)
> > > +     $1 = (unsigned int *) 0x28
> > > +
> > > +     # echo 'objtrace:add:arg1,0x28:u32:1 if comm == "cat"' > ./events/kprobes/ \
> > > +       p_bio_add_page_0/trigger
> > > +
> > > +     # cd /sys/kernel/debug/tracing/
> > > +     # echo 'p bio_add_page arg1=$arg1' > ./kprobe_events
> > > +     # echo 'objtrace:add:arg1,0x28:u32:1 if comm == "cat"' > ./events/kprobes/p_bio_add_page_0/trigger
> >
> > No offense but this documentation is not well written and hard to read.
> >
> > Admittedly though I am just casually browsing through, so apologies.
> >
> > So basically, 0x28 is the offset of the u32 within the bio, that you want to
> > track down, as it passes through functions?
>
> Yes, Not only track the bio, but also get a value with an offset of
> 0x28 relative to the bio.

Right.

> >
> > The example is good, but I suggest breakdown each of the commands separated
> > by ':' and document those as well.
>
> I don't know how to explain it in more detail, maybe need to be
> familiar with kprobe event and trigger in advance ;-)

That's not a strong argument IMO.

Shouldn't it be super easy to add the following to the documentation
since you already mentioned it in the commit log? Or am I missing
something?

Syntax:
        objtrace:add:obj[,offset][:type][:count][if <filter>]

Thanks,

- Joel