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Date:   Wed, 19 Jan 2022 10:32:49 +0800
From:   Jeff Xie <xiehuan09@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, mingo@...hat.com,
        Tom Zanussi <zanussi@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/4] trace: Introduce objtrace trigger to trace the
 kernel object

Hi Masami,

On Tue, Jan 18, 2022 at 10:24 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Thu, 13 Jan 2022 09:38:31 +0800
> Jeff Xie <xiehuan09@...il.com> wrote:
>
> > Introduce a method based on function tracer to trace any object and get
> > the value of the object dynamically. 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
> >
> > Almost all changelogs were suggested by Masami(mhiramat@...nel.org)
> > and steve(rostedt@...dmis.org), thank you all so much.
> >
> > v7:
> > - use fixed-size array for object pool instead of list structure
> > - use ftrace_test_recursion_trylock for function trace hook function
> > - fix trace_object_ref reference count in the init_trace_object
> > - invoke exit_trace_object no matter whether data->ops->free is null
> >   in the unregister_object_trigger
> > - release private_data of event_trigger_data in the trace_object_trigger_free
> > - remove [RFC] tag
> >
> > Note: when change to use the ftrace_test_recursion_trylock, all the functions
> > will call the copy_from_kernel_nofault, I don't know where this is the problem now,
> > maybe should fall back to the usage in v6.
> >
> > for example:
> >
> > cat-118     [000] ...1.     1.458998: __bio_add_page <-bio_add_page object:0xffff88811a12e9e8 value:0x0
> > cat-118     [000] ...2.     1.458998: copy_from_kernel_nofault <-trace_object_events_call object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...2.     1.458998: copy_from_kernel_nofault_allowed <-copy_from_kernel_nofault object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...1.     1.458998: __rcu_read_lock <-xa_load object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...2.     1.458998: copy_from_kernel_nofault <-trace_object_events_call object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...2.     1.458998: copy_from_kernel_nofault_allowed <-copy_from_kernel_nofault object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...1.     1.458998: __rcu_read_unlock <-xa_load object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...3.     1.458998: copy_from_kernel_nofault <-trace_object_events_call object:0xffff88811a12e9e8 value:0x1000
> > cat-118     [000] ...3.     1.458998: copy_from_kernel_nofault_allowed <-copy_from_kernel_nofault object:0xffff88811a12e9e8 value:0x1000
> > ....
>
> Hmm, this is strange, but I got it is the expected behavior, since the
> ftrace_test_recursion_trylock() accepts one stage recursion for the
> first event in the interrupt as transition event.
> Steve, any good way to limit probing this transition events?
>
>
> BTW, I tried your series on the
> git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git ftrace/core
> and got below build errors.
>
> linux/kernel/trace/trace_object.c:266:3: error: ‘struct event_trigger_ops’ has no member named ‘func’
>   266 |  .func   = trace_object_trigger,
>       |   ^~~~
> linux/kernel/trace/trace_object.c:273:3: error: ‘struct event_trigger_ops’ has no member named ‘func’
>   273 |  .func   = trace_object_count_trigger,
>       |   ^~~~
> linux/kernel/trace/trace_object.c:535:3: error: ‘struct event_command’ has no member named ‘func’
>   535 |  .func   = event_object_trigger_callback,
>       |   ^~~~
>   CC      net/ipv6/sysctl_net_ipv6.o
>
> This is because commit 7d28e1e7d4fa ("tracing: Change event_trigger_ops func() to trigger()")
> and commit 4dfe5dff80a4 ("tracing: Change event_command func() to parse()") changed the field
> names. Please update it.
>
Thanks, I will update it.
>
> Thank you,
>
> --
> Masami Hiramatsu <mhiramat@...nel.org>
---
JeffXie

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