reset-trace_example.txt

Demonstrations of reset-trace, for Linux bcc/BPF.


You will probably never need this tool. If you kill -9 a bcc tool (plus other
signals, like SIGTERM), or if a bcc tool crashes, then kernel tracing can be
left in a semi-enabled state. It's not as bad as it sounds: there may just be
overhead for writing to ring buffers that are never read. This tool can be
used to clean up the tracing state, and reset and disable active tracing.

WARNING: Make sure no other tracing sessions are active, as it will likely
stop them from functioning (perhaps ungracefully).

This specifically clears the state in at least the following files in
/sys/kernel/debug/tracing: kprobe_events, uprobe_events, trace_pipe.
Other tracing facilities (ftrace) are checked, and if not in an expected state,
a note is printed. All tracing files can be reset with -F for force, but this
will interfere with any other running tracing sessions (eg, ftrace).

Here's an example:

# ./reset-trace.sh
#

That's it.


You can use -v to see what it does:

# ./reset-trace.sh -v
Resetting tracing state...

Checking /sys/kernel/debug/tracing/kprobe_events
Checking /sys/kernel/debug/tracing/uprobe_events
Checking /sys/kernel/debug/tracing/trace
Checking /sys/kernel/debug/tracing/current_tracer
Checking /sys/kernel/debug/tracing/set_ftrace_filter
Checking /sys/kernel/debug/tracing/set_graph_function
Checking /sys/kernel/debug/tracing/set_ftrace_pid
Checking /sys/kernel/debug/tracing/events/enable
Checking /sys/kernel/debug/tracing/tracing_thresh
Checking /sys/kernel/debug/tracing/tracing_on

Done.

In this example, no resetting was necessary.


Here's an example of actually needing it:

# ./funccount 'bash:r*'
Tracing 317 functions for "bash:r*"... Hit Ctrl-C to end.
^C
FUNC                                    COUNT
rl_free_undo_list                           1
rl_deprep_terminal                          1
readline_internal_teardown                  1
rl_on_new_line                              1
rl_crlf                                     1
rl_clear_signals                            1
rl_prep_terminal                            1
rl_reset_line_state                         1
rl_initialize                               1
rl_newline                                  1
readline_internal_setup                     1
rl_set_screen_size                          1
readline                                    1
rl_set_signals                              1
rl_expand_prompt                            1
replace_history_data                        1
rl_set_prompt                               1
rl_add_undo                                 1
rl_insert_text                              2
rl_insert                                   2
rl_redisplay                                3
rl_read_key                                 3
rl_getc                                     3
readline_internal_char                      3
restore_parser_state                        6
reap_dead_jobs                              6
reset_parser                                6
restore_input_line_state                    6
realloc                                     7
read_octal                                 10
read_tty_modified                          13
run_exit_trap                              13
redirection_expand                         13
restore_pipestatus_array                   18
reader_loop                                20
run_return_trap                            21
remember_args                              25
reset_signal_handlers                      30
remove_quoted_escapes                      60
run_unwind_frame                          102
reset_terminating_signals                 125
restore_original_signals                  139
reset_internal_getopt                     405
run_debug_trap                            719
read_command                              940
remove_quoted_nulls                      1830
run_pending_traps                        3207
^C
^C
^C

I've traced 317 functions using funccount, and when I hit Ctrl-C, funccount is
not exiting (it can normally take many seconds, but this really looks stuck):

# pidstat 1
Linux 4.9.0-rc1-virtual (bgregg-xenial-bpf-i-xxx) 	10/18/2016 	_x86_64_	(8 CPU)

10:00:33 PM   UID       PID    %usr %system  %guest    %CPU   CPU  Command
10:00:34 PM 60004      3277    0.00    0.98    0.00    0.98     0  redis-server
10:00:34 PM     0     27980   87.25   10.78    0.00   98.04     3  funccount.py
10:00:34 PM     0     29965    0.00    0.98    0.00    0.98     6  pidstat

10:00:34 PM   UID       PID    %usr %system  %guest    %CPU   CPU  Command
10:00:35 PM 65534      3276    0.00    1.00    0.00    1.00     2  multilog
10:00:35 PM     0     27980   77.00   23.00    0.00  100.00     3  funccount.py
10:00:35 PM     0     29965    0.00    1.00    0.00    1.00     6  pidstat
10:00:35 PM 60004     29990    0.00    1.00    0.00    1.00     6  catalina.sh

funccount looks a lot like it's in an infinite loop (I can use a stack-sampling
profiler to confirm). This is a known bug (#665) and may be fixed by the time
you read this. But right now it's a good example of needing reset-trace.

I'll send a SIGTERM, before resorting to a SIGKILL:

# kill 27980
Terminated

Ok, so the process is now gone, but it did leave tracing in a semi-enabled
state. Using reset-trace:

# ./reset-trace.sh -v
Resetting tracing state...

Checking /sys/kernel/debug/tracing/kprobe_events
Checking /sys/kernel/debug/tracing/uprobe_events
Needed to reset /sys/kernel/debug/tracing/uprobe_events
uprobe_events, before (line enumerated):
     1	p:uprobes/p__bin_bash_0xa2540 /bin/bash:0x00000000000a2540
     2	p:uprobes/p__bin_bash_0x21220 /bin/bash:0x0000000000021220
     3	p:uprobes/p__bin_bash_0x78530 /bin/bash:0x0000000000078530
     4	p:uprobes/p__bin_bash_0xa3840 /bin/bash:0x00000000000a3840
     5	p:uprobes/p__bin_bash_0x9c550 /bin/bash:0x000000000009c550
     6	p:uprobes/p__bin_bash_0x5e360 /bin/bash:0x000000000005e360
     7	p:uprobes/p__bin_bash_0xb2630 /bin/bash:0x00000000000b2630
     8	p:uprobes/p__bin_bash_0xb1e70 /bin/bash:0x00000000000b1e70
     9	p:uprobes/p__bin_bash_0xb2540 /bin/bash:0x00000000000b2540
    10	p:uprobes/p__bin_bash_0xb16e0 /bin/bash:0x00000000000b16e0
[...]
   312	p:uprobes/p__bin_bash_0xa80b0 /bin/bash:0x00000000000a80b0
   313	p:uprobes/p__bin_bash_0x9e280 /bin/bash:0x000000000009e280
   314	p:uprobes/p__bin_bash_0x9e100 /bin/bash:0x000000000009e100
   315	p:uprobes/p__bin_bash_0xb2bd0 /bin/bash:0x00000000000b2bd0
   316	p:uprobes/p__bin_bash_0x9d9c0 /bin/bash:0x000000000009d9c0
   317	p:uprobes/p__bin_bash_0x4a930 /bin/bash:0x000000000004a930
uprobe_events, after (line enumerated):

Checking /sys/kernel/debug/tracing/trace
Checking /sys/kernel/debug/tracing/current_tracer
Checking /sys/kernel/debug/tracing/set_ftrace_filter
Checking /sys/kernel/debug/tracing/set_graph_function
Checking /sys/kernel/debug/tracing/set_ftrace_pid
Checking /sys/kernel/debug/tracing/events/enable
Checking /sys/kernel/debug/tracing/tracing_thresh
Checking /sys/kernel/debug/tracing/tracing_on

Done.

Now looks clean. I did truncate the output here: there were a few hundred lines
from uprobe_events.

Here's the same situation, but without the verbose option:

# ./reset-trace.sh
Needed to reset /sys/kernel/debug/tracing/uprobe_events
#

And again with quiet:

# ./reset-trace.sh -q
#


Here is an example of reset-trace detecting an unrelated tracing session:

# ./reset-trace.sh 
Noticed unrelated tracing file /sys/kernel/debug/tracing/set_ftrace_filter isn't set as expected. Not resetting (-F to force, -v for verbose).

And verbose:

# ./reset-trace.sh -v
Resetting tracing state...

Checking /sys/kernel/debug/tracing/kprobe_events
Checking /sys/kernel/debug/tracing/uprobe_events
Checking /sys/kernel/debug/tracing/trace
Checking /sys/kernel/debug/tracing/current_tracer
Checking /sys/kernel/debug/tracing/set_ftrace_filter
Noticed unrelated tracing file /sys/kernel/debug/tracing/set_ftrace_filter isn't set as expected. Not resetting (-F to force, -v for verbose).
Contents of set_ftrace_filter is (line enumerated):
     1	tcp_send_mss
     2	tcp_sendpage
     3	tcp_sendmsg
     4	tcp_send_dupack
     5	tcp_send_challenge_ack.isra.53
     6	tcp_send_rcvq
     7	tcp_send_ack
     8	tcp_send_loss_probe
     9	tcp_send_fin
    10	tcp_send_active_reset
    11	tcp_send_synack
    12	tcp_send_delayed_ack
    13	tcp_send_window_probe
    14	tcp_send_probe0
Expected "".
Checking /sys/kernel/debug/tracing/set_graph_function
Checking /sys/kernel/debug/tracing/set_ftrace_pid
Checking /sys/kernel/debug/tracing/events/enable
Checking /sys/kernel/debug/tracing/tracing_thresh
Checking /sys/kernel/debug/tracing/tracing_on

Done.

So this file is not currently used by bcc, but it may be useful to know that
it's not in the default state -- something is either using it or has left it
enabled. These files can be reset with -F, but that may break other tools that
are currently using them.


Use -h to print the USAGE message:

# ./reset-trace.sh -h
USAGE: reset-trace [-Fhqv]
                 -F             # force: reset all tracing files
                 -v             # verbose: print details while working
                 -h             # this usage message
                 -q             # quiet: no output
  eg,
       reset-trace              # disable semi-enabled tracing