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(I'm so sorry!) Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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==========================Userspace debugging advice==========================
This document provides a brief overview of common tools to debug the LinuxKernel from userspace.For debugging advice aimed at driver developers go :doc:`here</process/debugging/driver_development_debugging_guide>`.For general debugging advice, see :doc:`general advice document</process/debugging/index>`.
.. contents:: :depth: 3
The following sections show you the available tools.
Dynamic debug-------------
Mechanism to filter what ends up in the kernel log by dis-/en-abling logmessages.
Prerequisite: ``CONFIG_DYNAMIC_DEBUG``
Dynamic debug is only able to target:
- pr_debug()- dev_dbg()- print_hex_dump_debug()- print_hex_dump_bytes()
Therefore the usability of this tool is, as of now, quite limited as there isno uniform rule for adding debug prints to the codebase, resulting in a varietyof ways these prints are implemented.
Also, note that most debug statements are implemented as a variation ofdprintk(), which have to be activated via a parameter in respective module,dynamic debug is unable to do that step for you.
Here is one example, that enables all available pr_debug()'s within the file::
$ alias ddcmd='echo $* > /proc/dynamic_debug/control' $ ddcmd '-p; file v4l2-h264.c +p' $ grep =p /proc/dynamic_debug/control drivers/media/v4l2-core/v4l2-h264.c:372 [v4l2_h264]print_ref_list_b =p "ref_pic_list_b%u (cur_poc %u%c) %s" drivers/media/v4l2-core/v4l2-h264.c:333 [v4l2_h264]print_ref_list_p =p "ref_pic_list_p (cur_poc %u%c) %s\n"
**When should you use this over Ftrace ?**
- When the code contains one of the valid print statements (see above) or when you have added multiple pr_debug() statements during development- When timing is not an issue, meaning if multiple pr_debug() statements in the code won't cause delays- When you care more about receiving specific log messages than tracing the pattern of how a function is called
For the full documentation see :doc:`/admin-guide/dynamic-debug-howto`
Ftrace------
Prerequisite: ``CONFIG_DYNAMIC_FTRACE``
This tool uses the tracefs file system for the control files and output files.That file system will be mounted as a ``tracing`` directory, which can be foundin either ``/sys/kernel/`` or ``/sys/debug/kernel/``.
Some of the most important operations for debugging are:
- You can perform a function trace by adding a function name to the ``set_ftrace_filter`` file (which accepts any function name found within the ``available_filter_functions`` file) or you can specifically disable certain functions by adding their names to the ``set_ftrace_notrace`` file (more info at: :ref:`trace/ftrace:dynamic ftrace`).- In order to find out where calls originate from you can activate the ``func_stack_trace`` option under ``options/func_stack_trace``.- Tracing the children of a function call and showing the return values are possible by adding the desired function in the ``set_graph_function`` file (requires config ``FUNCTION_GRAPH_RETVAL``); more info at :ref:`trace/ftrace:dynamic ftrace with the function graph tracer`.
For the full Ftrace documentation see :doc:`/trace/ftrace`
Or you could also trace for specific events by :ref:`using event tracing<trace/events:2. using event tracing>`, which can be defined as described here::ref:`Creating a custom Ftrace tracepoint<process/debugging/driver_development_debugging_guide:ftrace>`.
For the full Ftrace event tracing documentation see :doc:`/trace/events`
.. _read_ftrace_log:
Reading the ftrace log~~~~~~~~~~~~~~~~~~~~~~
The ``trace`` file can be read just like any other file (``cat``, ``tail``,``head``, ``vim``, etc.), the size of the file is limited by the``buffer_size_kb`` (``echo 1000 > buffer_size_kb``). The:ref:`trace/ftrace:trace_pipe` will behave similarly to the ``trace`` file, butwhenever you read from the file the content is consumed.
Kernelshark~~~~~~~~~~~
A GUI interface to visualize the traces as a graph and list view from theoutput of the `trace-cmd<https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/>`__ application.
For the full documentation see `<https://kernelshark.org/Documentation.html>`__
Perf & alternatives-------------------
The tools mentioned above provide ways to inspect kernel code, results,variable values, etc. Sometimes you have to find out first where to look andfor those cases, a box of performance tracking tools can help you to frame theissue.
Why should you do a performance analysis?~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A performance analysis is a good first step when among other reasons:
- you cannot define the issue- you do not know where it occurs- the running system should not be interrupted or it is a remote system, where you cannot install a new module/kernel
How to do a simple analysis with linux tools?~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For the start of a performance analysis, you can start with the usual toolslike:
- ``top`` / ``htop`` / ``atop`` (*get an overview of the system load, see spikes on specific processes*)- ``mpstat -P ALL`` (*look at the load distribution among CPUs*)- ``iostat -x`` (*observe input and output devices utilization and performance*)- ``vmstat`` (*overview of memory usage on the system*)- ``pidstat`` (*similar to* ``vmstat`` *but per process, to dial it down to the target*)- ``strace -tp $PID`` (*once you know the process, you can figure out how it communicates with the Kernel*)
These should help to narrow down the areas to look at sufficiently.
Diving deeper with perf~~~~~~~~~~~~~~~~~~~~~~~
The **perf** tool provides a series of metrics and events to further dial downon issues.
Prerequisite: build or install perf on your system
Gather statistics data for finding all files starting with ``gcc`` in ``/usr``::
# perf stat -d find /usr -name 'gcc*' | wc -l
Performance counter stats for 'find /usr -name gcc*':
1277.81 msec task-clock # 0.997 CPUs utilized 9 context-switches # 7.043 /sec 1 cpu-migrations # 0.783 /sec 704 page-faults # 550.943 /sec 766548897 cycles # 0.600 GHz (97.15%) 798285467 instructions # 1.04 insn per cycle (97.15%) 57582731 branches # 45.064 M/sec (2.85%) 3842573 branch-misses # 6.67% of all branches (97.15%) 281616097 L1-dcache-loads # 220.390 M/sec (97.15%) 4220975 L1-dcache-load-misses # 1.50% of all L1-dcache accesses (97.15%) <not supported> LLC-loads <not supported> LLC-load-misses
1.281746009 seconds time elapsed
0.508796000 seconds user 0.773209000 seconds sys
52
The availability of events and metrics depends on the system you are running.
For the full documentation see`<https://perf.wiki.kernel.org/index.php/Main_Page>`__
Perfetto~~~~~~~~
A set of tools to measure and analyze how well applications and systems perform.You can use it to:
* identify bottlenecks* optimize code* make software run faster and more efficiently.
**What is the difference between perfetto and perf?**
* perf is tool as part of and specialized for the Linux Kernel and has CLI user interface.* perfetto cross-platform performance analysis stack, has extended functionality into userspace and provides a WEB user interface.
For the full documentation see `<https://perfetto.dev/docs/>`__
Kernel panic analysis tools---------------------------
To capture the crash dump please use ``Kdump`` & ``Kexec``. Below you can find some advice for analysing the data.
For the full documentation see the :doc:`/admin-guide/kdump/kdump`
In order to find the corresponding line in the code you can use `faddr2line <https://elixir.bootlin.com/linux/v6.11.6/source/scripts/faddr2line>`__; note that you need to enable ``CONFIG_DEBUG_INFO`` for that to work.
An alternative to using ``faddr2line`` is the use of ``objdump`` (and its derivatives for the different platforms like ``aarch64-linux-gnu-objdump``). Take this line as an example:
``[ +0.000240] rkvdec_device_run+0x50/0x138 [rockchip_vdec]``.
We can find the corresponding line of code by executing::
aarch64-linux-gnu-objdump -dS drivers/staging/media/rkvdec/rockchip-vdec.ko | grep rkvdec_device_run\>: -A 40 0000000000000ac8 <rkvdec_device_run>: ac8: d503201f nop acc: d503201f nop { ad0: d503233f paciasp ad4: a9bd7bfd stp x29, x30, [sp, #-48]! ad8: 910003fd mov x29, sp adc: a90153f3 stp x19, x20, [sp, #16] ae0: a9025bf5 stp x21, x22, [sp, #32] const struct rkvdec_coded_fmt_desc *desc = ctx->coded_fmt_desc; ae4: f9411814 ldr x20, [x0, #560] struct rkvdec_dev *rkvdec = ctx->dev; ae8: f9418015 ldr x21, [x0, #768] if (WARN_ON(!desc)) aec: b4000654 cbz x20, bb4 <rkvdec_device_run+0xec> ret = pm_runtime_resume_and_get(rkvdec->dev); af0: f943d2b6 ldr x22, [x21, #1952] ret = __pm_runtime_resume(dev, RPM_GET_PUT); af4: aa0003f3 mov x19, x0 af8: 52800081 mov w1, #0x4 // #4 afc: aa1603e0 mov x0, x22 b00: 94000000 bl 0 <__pm_runtime_resume> if (ret < 0) { b04: 37f80340 tbnz w0, #31, b6c <rkvdec_device_run+0xa4> dev_warn(rkvdec->dev, "Not good\n"); b08: f943d2a0 ldr x0, [x21, #1952] b0c: 90000001 adrp x1, 0 <rkvdec_try_ctrl-0x8> b10: 91000021 add x1, x1, #0x0 b14: 94000000 bl 0 <_dev_warn> *bad = 1; b18: d2800001 mov x1, #0x0 // #0 ...
Meaning, in this line from the crash dump::
[ +0.000240] rkvdec_device_run+0x50/0x138 [rockchip_vdec]
I can take the ``0x50`` as offset, which I have to add to the base address of the corresponding function, which I find in this line::
0000000000000ac8 <rkvdec_device_run>:
The result of ``0xac8 + 0x50 = 0xb18`` And when I search for that address within the function I get the following line::
*bad = 1; b18: d2800001 mov x1, #0x0
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