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    Subject[PATCH v13 4/4] Documentation: trace/objtrace: Add documentation for objtrace
    Date
    Added documentation explaining how to use objtrace trigger to get the value
    of the object.

    Signed-off-by: Jeff Xie <xiehuan09@gmail.com>
    Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
    ---
    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
    ============================

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