Re: [PATCH v13 4/4] Documentation: trace/objtrace: Add documentation for objtrace

[Bagas Sanjaya]

On Sun, Jun 26, 2022 at 10:56:04AM +0800, Jeff Xie wrote:
> Added documentation explaining how to use objtrace trigger to get the value
> of the object.
> 

Write "Add documentation..." instead.

> +- 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.
> +

Did you mean "the object value can be obtained by only needing to set the
objtrace?"

> +  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
> +

Did you mean "the first parameter (arg1) is the parameter value of function
that is passed to kprobe_event, in this case the object base"?

> +  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
> +             ...
> +

The output is messy, because for code example and output, literal code
blocks aren't used, hence trigger new warnings:

Documentation/trace/events.rst:564: WARNING: Inline emphasis start-string without end-string.
Documentation/trace/events.rst:564: WARNING: Inline emphasis start-string without end-string.
Documentation/trace/events.rst:573: WARNING: Inline emphasis start-string without end-string.
Documentation/trace/events.rst:573: WARNING: Inline emphasis start-string without end-string.
Documentation/trace/events.rst:602: WARNING: Unexpected indentation.
Documentation/trace/events.rst:622: WARNING: Block quote ends without a blank line; unexpected unindent.
Documentation/trace/events.rst:630: WARNING: Unexpected indentation.

I had to apply the following fixup (with rewording):

---- >8 ----

diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
index 0dc47516013331..c15f1d25d4a071 100644
--- a/Documentation/trace/events.rst
+++ b/Documentation/trace/events.rst
@@ -549,85 +549,89 @@ The following commands are supported:
 - 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).
+  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.
+  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():
+  For example, for the function bio_add_page():
 
-  int bio_add_page(struct bio *bio, struct page *page,
-	unsigned int len, unsigned int offset)
+  .. code-block:: c
 
-  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(),
+     int bio_add_page(struct bio *bio, struct page *page,
+	              unsigned int len, unsigned int offset)
 
-  # echo 'p bio_add_page arg1=$arg1' > ./kprobe_events
+  Firstly, we can set the base of the object as first parameter (arg1) to
+  to the function:
 
-  Secondly, we can get the value dynamically based on above object.
+  .. code-block::
 
-  find the offset of the bi_size in struct bio:
-  $ gdb vmlinux
-  (gdb) p &(((struct bio *)0)->bi_iter.bi_size)
-  $1 = (unsigned int *) 0x28
+     # echo 'p bio_add_page arg1=$arg1' > ./kprobe_events
 
-  # echo 'objtrace:add:arg1,0x28:u32:1 if comm == "cat"' > ./events/kprobes/ \
-	p_bio_add_page_0/trigger
+  Secondly, we can get the value dynamically based on the object:
 
-  # 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
+  .. code-block::
 
-  # du -sh /test.txt
-  12.0K   /test.txt
+     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
 
-  # 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
-             ...
+     # 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
 ============================


Thanks.

-- 
An old man doll... just what I always wanted! - Clara