Re: [PATCH v12 4/4] trace/objtrace: Add documentation for objtrace

[Jeff Xie]

Hi Masami,

On Sun, Jun 26, 2022 at 12:10 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> Hi Jeff,
>
> OK, this looks good to me.
>
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>
> And just a note that please add 'Documentation:' tag to the title
> and Cc to linux-doc@xxxxxxxxxxxxxxx and Jonathan Corbet too since
> this is a patch for the Documentation/*.

Thanks for the reminder, I will do it like this in my next version.

> Thank you,
>
> On Tue,  7 Jun 2022 00:09:43 +0800
> Jeff Xie <xiehuan09@xxxxxxxxx> wrote:
>
> > Added documentation explaining how to use objtrace trigger to get the value
> > of the object.
> >
> > Signed-off-by: Jeff Xie <xiehuan09@xxxxxxxxx>
> > ---
> >  Documentation/trace/events.rst | 83 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 83 insertions(+)
> >
> > diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
> > index c47f381d0c00..0dc475160133 100644
> > --- a/Documentation/trace/events.rst
> > +++ b/Documentation/trace/events.rst
> > @@ -546,6 +546,89 @@ 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, only need to set the objtrace(a new trigger),
> > +  we can get the value of the object. The object is from the setting of the
> > +  kprobe event.
> > +
> > +  For example:
> > +  For the function bio_add_page():
> > +
> > +  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, thus the first string "arg1"
> > +  stands for the value of the first parameter of this function bio_add_gage(),
> > +
> > +  # echo 'p bio_add_page arg1=$arg1' > ./kprobe_events
> > +
> > +  Secondly, we can get the value dynamically based on above object.
> > +
> > +  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
> > +
> > +  # 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
> >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Thanks,
JeffXie