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SubjectRe: [PATCH v12 4/4] trace/objtrace: Add documentation for objtrace
Hi Masami,

On Sun, Jun 26, 2022 at 12:10 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Hi Jeff,
>
> OK, this looks good to me.
>
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>
> And just a note that please add 'Documentation:' tag to the title
> and Cc to linux-doc@vger.kernel.org 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@gmail.com> wrote:
>
> > Added documentation explaining how to use objtrace trigger to get the value
> > of the object.
> >
> > Signed-off-by: Jeff Xie <xiehuan09@gmail.com>
> > ---
> > 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@kernel.org>

Thanks,
JeffXie

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