On Thu, Aug 18, 2022 at 1:34 PM Jeff Xie <xiehuan09@xxxxxxxxx> wrote: > > Hi Joel, > > On Fri, Aug 19, 2022 at 1:05 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > 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>] > > I'am so sorry, I misunderstood you, I thought it needed to be > explained every word like this, e.g. > objtrace: > ... > add: > ... > ... > > Thanks, I will add the above syntax in the next version. > Yeah no problem, I am also OK with that being a follow-up patch instead of the series if it matches with Steven's review timing better. Thanks, - Joel
