reset-ftrace_example.txt

Demonstrations of reset-ftrace, the Linux ftrace tool.


You will probably never need this tool. If you kill -9 an ftrace-based tool,
leaving the kernel in a tracing enabled state, you could try using this tool
to reset ftrace and disable tracing. Make sure no other ftrace sessions are
in use on your system, or it will kill those.

Here's an example:

# ./opensnoop
Tracing open()s. Ctrl-C to end.
ERROR: ftrace may be in use by PID 2197 /var/tmp/.ftrace-lock

I tried to run opensnoop, but there's a lock file for PID 2197. Checking if it
exists:

# ps -fp 2197
UID        PID  PPID  C STIME TTY          TIME CMD
#

No.

I also know that no one is using ftrace on this system. So I'll use reset-ftrace
to clean up this lock file and ftrace state:

# ./reset-ftrace 
ERROR: ftrace lock (/var/tmp/.ftrace-lock) exists. It shows ftrace may be in use by PID 2197.
Double check to see if that PID is still active. If not, consider using -f to force a reset. Exiting.

... except it's complaining about the lock file too. I'm already sure that this
PID doesn't exist, so I'll add the -f option:

# ./reset-ftrace -f
Reseting ftrace state...

current_tracer, before:
     1	nop
current_tracer, after:
     1	nop

set_ftrace_filter, before:
     1	#### all functions enabled ####
set_ftrace_filter, after:
     1	#### all functions enabled ####

set_ftrace_pid, before:
     1	no pid
set_ftrace_pid, after:
     1	no pid

kprobe_events, before:
kprobe_events, after:

Done.

The output shows what has been reset, including the before and after state of
these files.

Now I can try iosnoop again:

# ./iosnoop
Tracing block I/O. Ctrl-C to end.
COMM             PID    TYPE DEV      BLOCK        BYTES     LATms
supervise        1689   W    202,1    17039664     4096       0.58
supervise        1689   W    202,1    17039672     4096       0.47
supervise        1694   W    202,1    17039744     4096       0.98
supervise        1694   W    202,1    17039752     4096       0.74
supervise        1684   W    202,1    17039760     4096       0.63
[...]

Fixed.

Note that reset-ftrace currently only resets a few methods of enabling
tracing, such as set_ftrace_filter and kprobe_events. Static tracepoints could 
be enabled individually, and this script currently doesn't find and disable
those.


Use -h to print the USAGE message:

# ./reset-ftrace -h
USAGE: reset-ftrace [-fhq]
                 -f              # force: delete ftrace lock file
                 -q              # quiet: reset, but say nothing
                 -h              # this usage message
  eg,
       reset-ftrace              # disable active ftrace session