[PATCH v10 4/4] trace/objtrace: Add documentation for objtrace

[Jeff Xie <xiehuan09@gmail.com>]

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