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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
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