Hi Joel, Thank you for your review. On Thu, Aug 18, 2022 at 11:53 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > On Tue, Jun 28, 2022 at 10:55:52PM +0800, Jeff Xie wrote: > > Add documentation explaining how to use objtrace trigger to get the value > > of the object. > > > > Cc: Jonathan Corbet <corbet@xxxxxxx> > > Cc: Bagas Sanjaya <bagasdotme@xxxxxxxxx> > > Cc: linux-doc@xxxxxxxxxxxxxxx > > Signed-off-by: Jeff Xie <xiehuan09@xxxxxxxxx> > > Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> > > --- > > Changelog: > > v14: > > - make documentation more readable and fix literal code block by Bagas Sanjaya > > > > Documentation/trace/events.rst | 87 ++++++++++++++++++++++++++++++++++ > > 1 file changed, 87 insertions(+) > > > > diff --git a/Documentation/trace/events.rst b/Documentation/trace/events.rst > > index c47f381d0c00..c15f1d25d4a0 100644 > > --- a/Documentation/trace/events.rst > > +++ b/Documentation/trace/events.rst > > @@ -546,6 +546,93 @@ 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, 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 > > No offense but this documentation is not well written and hard to read. > > Admittedly though I am just casually browsing through, so apologies. > > So basically, 0x28 is the offset of the u32 within the bio, that you want to > track down, as it passes through functions? Yes, Not only track the bio, but also get a value with an offset of 0x28 relative to the bio. > > The example is good, but I suggest breakdown each of the commands separated > by ':' and document those as well. I don't know how to explain it in more detail, maybe need to be familiar with kprobe event and trigger in advance ;-) > Cool feature though, I can see myself using it for something (dunno what yet) :) Thank you, hope you have a good experience ;-) > > thanks, > > - Joel > > > > > + > > + # 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 > > -- Thanks, JeffXie
