Search notes:

/sys/kernel/tracing

scripts/tracing/draw_functrace.py has the following example: 
# mount -t tracefs nodev /sys/kernel/tracing
# echo function > /sys/kernel/tracing/current_tracer
$ cat /sys/kernel/tracing/trace_pipe > ~/raw_trace_func
Wait some times but not too much, the script is a bit slow.
Break the pipe (Ctrl + Z) 
$ scripts/tracing/draw_functrace.py < ~/raw_trace_func > draw_functrace
Apparently, CONFIG_FUNCTION_TRACER needs to be set when compiling the kernel.

See also

/sys/kernel, /sys/kernel/debug
/sys/kernel/tracing/README contains the tracing mini-HOWTO.
Because the tracefs filesystem is mounted also on /sys/kernel/debug/tracing, these directories have the same structure.

Index

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php:78 Stack trace: #0 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(78): PDOStatement->execute(Array) #1 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(30): insert_webrequest_('/notes/Linux/fh...', 1759399478, '216.73.216.42', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/Linux/fhs/sys/kernel/tracing/index(55): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78