[RFC][PATCH v6 5/5] 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 | 114 +++++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)

diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst
index 8ddb9b09451c..a6d8313ba24e 100644
--- a/Documentation/trace/events.rst
+++ b/Documentation/trace/events.rst
@@ -527,6 +527,120 @@ 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 base the above object.
+
+  find the offset of the bi_size in struct bio:
+  $ gdb vmlinux
+  (gdb) p &(((struct bio *)0)->bi_iter.bi_size)
+  $1 = (unsigned int *) 0x28
+
+  # echo 'objtrace:add:arg1,0x28:u32:1 if comm == "cat"' > ./events/kprobes/ \
+	p_bio_add_page_0/trigger
+
+  The best way to use this is that we can set the entrance event and exit
+  event, for example, the following example is to set the read_papes as
+  the entrance event, and set the __blk_account_io_start as the exit event.
+
+  # cd /sys/kernel/debug/tracing/
+  # echo 0 > ./tracing_on
+  # echo 'p read_pages' >> ./kprobe_events
+  # echo 'p __blk_account_io_start' >> ./kprobe_events
+  # echo 'traceon if comm == "cat"' > ./events/kprobes/p_read_pages_0/trigger
+  # echo 'traceoff if comm == "cat"' > ./events/kprobes/p___blk_account_io_start_0/trigger
+  # echo 'p bio_add_page arg1=$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: 50/50   #P:1
+  #
+  #                                _-----=> irqs-off
+  #                               / _----=> need-resched
+  #                              | / _---=> hardirq/softirq
+  #                              || / _--=> preempt-depth
+  #                              ||| / _-=> migrate-disable
+  #                              |||| /     delay
+  #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
+  #              | |         |   |||||     |         |
+             cat-95      [000] .....     1.412065: _raw_spin_unlock_irqrestore <-event_triggers_call object:0xffff888108af6328 value:0x0
+             cat-95      [000] .....     1.412066: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x0
+             cat-95      [000] .....     1.412066: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x0
+             cat-95      [000] .....     1.412066: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412066: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412066: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412067: __bio_add_page <-bio_add_page object:0xffff888108af6328 value:0x1000
+             cat-95      [000] .....     1.412067: rcu_read_unlock_strict <-xa_load object:0xffff888108af6328 value:0x2000
+             cat-95      [000] .....     1.412067: bio_add_page <-ext4_mpage_readpages object:0xffff888108af6328 value:0x2000
+             cat-95      [000] .....     1.412067: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x2000
+             cat-95      [000] .....     1.412067: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x2000
+             cat-95      [000] .....     1.412067: __bio_try_merge_page <-bio_add_page object:0xffff888108af6328 value:0x2000
+             cat-95      [000] .....     1.412068: submit_bio <-ext4_mpage_readpages object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: submit_bio_noacct <-ext4_mpage_readpages object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: __submit_bio <-submit_bio_noacct object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: blk_try_enter_queue <-__submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: rcu_read_unlock_strict <-blk_try_enter_queue object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: rcu_read_unlock_strict <-blk_try_enter_queue object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: submit_bio_checks <-__submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: __cond_resched <-submit_bio_checks object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: rcu_all_qs <-__cond_resched object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412068: should_fail_bio <-submit_bio_checks object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: create_task_io_context <-submit_bio_checks object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: kmem_cache_alloc_node <-create_task_io_context object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: should_failslab <-kmem_cache_alloc_node object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: _raw_spin_lock <-create_task_io_context object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: blk_mq_submit_bio <-__submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: __blk_queue_split <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: bvec_split_segs <-__blk_queue_split object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412069: blk_attempt_plug_merge <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: __blk_mq_sched_bio_merge <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: dd_bio_merge <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: _raw_spin_lock <-dd_bio_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: blk_mq_sched_try_merge <-dd_bio_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: elv_merge <-blk_mq_sched_try_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: elv_rqhash_find <-elv_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: dd_request_merge <-blk_mq_sched_try_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: elv_rb_find <-dd_request_merge object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412070: __blk_mq_alloc_request <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: dd_limit_depth <-__blk_mq_alloc_request object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: blk_mq_get_tag <-__blk_mq_alloc_request object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: __blk_mq_get_tag <-blk_mq_get_tag object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: blk_mq_rq_ctx_init.isra.0 <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: ktime_get <-blk_mq_rq_ctx_init.isra.0 object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: dd_prepare_request <-blk_mq_rq_ctx_init.isra.0 object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: __blk_account_io_start <-blk_mq_submit_bio object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: kprobe_ftrace_handler <-ftrace_ops_list_func object:0xffff888108af6328 value:0x3000
+             cat-95      [000] .....     1.412071: get_kprobe <-kprobe_ftrace_handler object:0xffff888108af6328 value:0x3000
+
 7. In-kernel trace event API
 ============================
 
-- 
2.25.1