All About Ftrace

Ftrace is a Kernel function tracer.  It uses the file system debugfs.

To use Ftrace, these options needs to be enabled if it's not.

CONFIG_FTRACE,
CONFIG_HAVE_DYNAMIC_FTRACE,
CONFIG_HAVE_FUNCTION_TRACER,
CONFIG_HAVE_FUNCTION_GRAPH_TRACER,
CONFIG_STACKTRACE

To mount debugfs

mount -t debugfs nodev /sys/kernel/debug

If /sys/kernel/debug is present re-run the above command again.

ftrace directory will be created under /sys/kernel/debug/tracing

The directory contains all the control knobs in regard to kernel tracing.

Some of the important files can be referred when using ftrace
1) available_filter_funtions:  All the functions ftrace is able to trace.
2) available_tracer: List all the tracer compiled into the kernel.
3) current_tracer: shows the currently selected tracer.
4) trace : It holds the output of what being traced in readable format.
5) trace_options: To control the level of output in trace output
                 To enable block tracing echo block > trace_options
                 To disable block tracing echo noblock > trace_options
  

6) tracing_enabled : To start or stop tracing activity


To enable ftrace

echo function > /sys/kernel/debug/tracing/current_tracer

To enable/disable tracing

echo 1 > tracing_on  : To enable tracing
echo 0 > tracing_on   : To disable tracing
echo > trace   : To clear trace log file

echo nop > current_tracer

To Trace or Monitor Block IO
echo 1 > events/block/enable  (enable block I/O subsystem)

cat set_event  (To display all the subsystem event enabled)

echo 1 > tracing_on
run your program
echo 0 > tracing_on

cat trace to output the ftrace output.

Example of tracing a specific process

traceme.sh

 #!/bin/sh
DEBUGFS=`grep debugfs /proc/mounts | awk '{ print $2; }'`
echo nop > $DEBUGFS/tracing/current_tracer
echo > $DEBUGFS/tracing/trace
echo $1
echo $$ > $DEBUGFS/tracing/set_ftrace_pid
echo function > $DEBUGFS/tracing/current_tracer
#echo function_graph > $DEBUGFS/tracing/current_tracer
echo 1 > $DEBUGFS/tracing/tracing_on
exec $*
#echo nop > $DEBUGFS/tracing/current_tracer
echo 0 > $DEBUGFS/tracing/tracing_on

echo sys_* > set_ftrace_filter
echo vfs_* >> set_ftrace_filter

traceme.sh ls -al