[PATCH v15 4/4] Documentation: trace/objtrace: Add documentation for objtrace

[Jeff Xie <xiehuan09@gmail.com>]

Add documentation explaining how to use objtrace trigger to get the value
of the object.

Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jeff Xie <xiehuan09@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
Changelog:
V15:
- Add syntax suggested by Joel for usage of objtrace

 Documentation/trace/events.rst | 90 ++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)

diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
index c47f381d0c00..1209f3cd8f1d 100644
--- a/Documentation/trace/events.rst
+++ b/Documentation/trace/events.rst
@@ -546,6 +546,96 @@ 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).
+
+  Syntax:
+        objtrace:add:obj[,offset][:type][:count][if <filter>]
+
+  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
+
+     # 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