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Message-ID: <CAEr6+ED7UovW1BbrK4s5tCRrTrfkESpa4m3VO4a4PBAY9nK_JA@mail.gmail.com>
Date: Fri, 19 Aug 2022 00:37:59 +0800
From: Jeff Xie <xiehuan09@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: rostedt@...dmis.org, mingo@...hat.com, mhiramat@...nel.org,
zanussi@...nel.org, linux-kernel@...r.kernel.org,
chensong_2000@....cn, Jonathan Corbet <corbet@....net>,
Bagas Sanjaya <bagasdotme@...il.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v14 4/4] Documentation: trace/objtrace: Add documentation
for objtrace
Hi Joel,
Thank you for your review.
On Thu, Aug 18, 2022 at 11:53 PM Joel Fernandes <joel@...lfernandes.org> 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@....net>
> > Cc: Bagas Sanjaya <bagasdotme@...il.com>
> > Cc: linux-doc@...r.kernel.org
> > Signed-off-by: Jeff Xie <xiehuan09@...il.com>
> > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> > 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.
>
> 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 ;-)
> Cool feature though, I can see myself using it for something (dunno what yet) :)
Thank you, hope you have a good experience ;-)
>
> thanks,
>
> - Joel
>
>
>
> > +
> > + # du -sh /test.txt
> > + 12.0K /test.txt
> > +
> > + # cat /test.txt > /dev/null
> > + # cat ./trace
> > + # tracer: nop
> > + #
> > + # entries-in-buffer/entries-written: 128/128 #P:4
> > + #
> > + # _-----=> irqs-off/BH-disabled
> > + # / _----=> need-resched
> > + # | / _---=> hardirq/softirq
> > + # || / _--=> preempt-depth
> > + # ||| / _-=> migrate-disable
> > + # |||| / delay
> > + # TASK-PID CPU# ||||| TIMESTAMP FUNCTION
> > + # | | | ||||| | |
> > + cat-117 [002] ...1. 1.602243: __bio_try_merge_page <-bio_add_page object:0xffff88811bee4000 value:0x0
> > + cat-117 [002] ...1. 1.602244: __bio_add_page <-bio_add_page object:0xffff88811bee4000 value:0x0
> > + cat-117 [002] ...2. 1.602244: bio_add_page <-ext4_mpage_readpages object:0xffff88811bee4000 value:0x1000
> > + cat-117 [002] ...1. 1.602245: __bio_try_merge_page <-bio_add_page object:0xffff88811bee4000 value:0x1000
> > + cat-117 [002] ...1. 1.602245: __bio_add_page <-bio_add_page object:0xffff88811bee4000 value:0x1000
> > + cat-117 [002] ...2. 1.602245: bio_add_page <-ext4_mpage_readpages object:0xffff88811bee4000 value:0x2000
> > + cat-117 [002] ...1. 1.602245: __bio_try_merge_page <-bio_add_page object:0xffff88811bee4000 value:0x2000
> > + cat-117 [002] ...1. 1.602245: __bio_add_page <-bio_add_page object:0xffff88811bee4000 value:0x2000
> > + cat-117 [002] ...1. 1.602245: submit_bio <-ext4_mpage_readpages object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602245: submit_bio_noacct <-ext4_mpage_readpages object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: __submit_bio <-submit_bio_noacct object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: submit_bio_checks <-__submit_bio object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: __cond_resched <-submit_bio_checks object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: should_fail_bio <-submit_bio_checks object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: blk_mq_submit_bio <-submit_bio_noacct object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: blk_attempt_plug_merge <-blk_mq_submit_bio object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602246: blk_mq_sched_bio_merge <-blk_mq_submit_bio object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602247: __rcu_read_lock <-blk_mq_submit_bio object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602247: __rcu_read_unlock <-blk_mq_submit_bio object:0xffff88811bee4000 value:0x3000
> > + cat-117 [002] ...1. 1.602247: __blk_mq_alloc_requests <-blk_mq_submit_bio object:0xffff88811bee4000 value:0x3000
> > + <idle>-0 [002] d..3. 1.602298: bio_endio <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602298: mpage_end_io <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602298: __read_end_io <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602300: bio_put <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602300: bio_free <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602300: mempool_free <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602300: mempool_free_slab <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + <idle>-0 [002] d..3. 1.602300: kmem_cache_free <-blk_update_request object:0xffff88811bee4000 value:0x0
> > + ...
> > +
> > 7. In-kernel trace event API
> > ============================
> >
> > --
> > 2.25.1
> >
--
Thanks,
JeffXie
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