bionic (8) funcgraph-perf.8.gz
NAME
funcgraph - trace kernel function graph, showing child function calls and times. Uses Linux ftrace.
SYNOPSIS
funcgraph [-aCDhHPtT] [-m maxdepth] [-p PID] [-L TID] [-d secs] funcstring
DESCRIPTION
This is an exploratory tool that shows the graph of child function calls for a given kernel function.
This can cost moderate overhead to execute, and should only be used to understand kernel behavior before
using other, lower overhead tools. This is a proof of concept using Linux ftrace capabilities on older
kernels.
The output format is the same as the ftrace function graph trace format, described in the kernel source
under Documentation/trace/ftrace.txt. Note that the output may be shuffled when different CPU buffers
are read; check the CPU column for changes, or include timestamps (-t) and post sort.
The "-d duration" mode leaves the trace data in the kernel buffer, and only reads it at the end. If the
trace data is large, beware of exhausting buffer space (/sys/kernel/debug/tracing/buffer_size_kb) and
losing data.
Also beware of feedback loops: tracing tcp* functions over an ssh session, or writing ext4* functions to
an ext4 file system. For the former, tcp trace data could be redirected to a file (as in the usage
message). For the latter, trace to the screen or a different file system.
WARNING: This uses dynamic tracing of kernel functions, and could cause kernel panics or freezes. Test,
and know what you are doing, before use. Also see the OVERHEAD section.
Since this uses ftrace, only the root user can use this tool.
REQUIREMENTS
FTRACE CONFIG, which you may already have enabled and available on recent kernels.
OPTIONS
-a All info. Same as -HPt. (But no -T, which isn't available in older kernels.)
-C Function durations measure on-CPU time only (exclude sleep time).
-d seconds
Set the duration of tracing, in seconds. Trace output will be buffered and printed at the end.
This also reduces overheads by buffering in-kernel, instead of printing events as they occur.
The ftrace buffer has a fixed size per-CPU (see /sys/kernel/debug/tracing/buffer_size_kb). If you
think events are missing, try increasing that size.
-D Do not show function duration times.
-h Print usage message.
-H Print column headers.
-m Max depth to trace functions. By default, unlimited (0). This feature is only available for newer
Linux kernel versions.
-p PID Only trace kernel functions when this process ID is on-CPU.
-L TID Only trace kernel functions when this thread ID is on-CPU.
-P Show process names and process IDs with every line of output.
-t Show timestamps on every line of output.
-T Tail mode: decorate function return lines with the name of the function. This option may not be
available for older kernels.
funcstring
A function name to trace, which may include file glob style wildcards ("*") at the beginning or
ending of a string only. Eg, "vfs*" means match "vfs" followed by anything. Since the output is
verbose, you probably only want to trace single functions, and not use wildcards.
EXAMPLES
Trace calls to do_nanosleep(), showing child functions and durations:
# funcgraph do_nanosleep
Same as above, but include column headers:
# funcgraph -H do_nanosleep
Same as above, but include timestamps and process names as well:
# funcgraph -HtP do_nanosleep
Trace all vfs_read() kernel function calls, and child functions, for PID 198 only:
# funcgraph -p 198 vfs_read
Trace all vfs_read() kernel function calls, and child functions, for 1 second then write to a file.
# funcgraph -d 1 vfs_read > out
FIELDS
The output format depends on the kernel version, and headings can be printed using -H. The format is the
same as the ftrace function trace format, described in the kernel source under
Documentation/trace/ftrace.txt.
Typical fields are:
TIME (Shown with -t.) Time of event, in seconds.
CPU The CPU this event occurred on.
TASK/PID
(Shown with -P.) The process name (which could include dashes), a dash, and the process ID.
DURATION
Elapsed time during the function call, inclusive of children. This is also inclusive of sleep
time, unless -C is used. The time is either displayed on the return of a function ("}"), or for a
leaf function (no children), on the same line.
If the trace output begins with some returns that lack entries, their durations may not be
trusted. This is usually only the case for the first dozen or so lines.
FUNCTION CALLS
Entries and returns from kernel functions.
OVERHEAD
This tool causes moderate to high overheads. Use with caution for exploratory purposes, then switch to
lower overhead techniques based on findings. It's expected that the kernel will run at least 50% slower
while this tool is running -- even while no output is being generated. This is because ALL kernel
functions are traced, and filtered based on the function of interest. When output is generated, it can
generate many lines quickly depending on the traced event. Such data will cause performance overheads.
This also works without buffering by default, printing function events as they happen (uses trace_pipe),
context switching and consuming CPU to do so. If needed, you can try the "-d secs" option, which buffers
events instead, reducing overhead. If you think the buffer option is losing events, try increasing the
buffer size (buffer_size_kb).
It's a good idea to use funccount(8) first, which is lower overhead, to help you select which functions
you may want to trace using funcgraph(8).
SOURCE
This is from the perf-tools collection:
https://github.com/brendangregg/perf-tools
Also look under the examples directory for a text file containing example usage, output, and commentary
for this tool.
OS
Linux
STABILITY
Unstable - in development.
AUTHOR
Brendan Gregg
SEE ALSO
funccount(8), functrace(8), kprobe(8)