[LTP] [PATCH] doc: Add testers guide

Tue Jun 10 12:45:38 CEST 2025

While we have added a documentation on supported kernels, compilers, how
to compile and run LTP etc. Some kind of a comprehensive guide for
people new to LTP and kernel testing was missing. This patch adds a
short guide that tries to explain some of the pitfalls of kernel
testing. Feel free to point out what is missing and suggest additional
chapters.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Andrea Cervesato <andrea.cervesato@suse.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: : Wei Gao <wegao@suse.com>
---

Changes againts the RFC:

- fixed many typos
- added a paragraph about filesystems
- added links and some formatting

 doc/index.rst               |   1 +
 doc/users/testers_guide.rst | 151 ++++++++++++++++++++++++++++++++++++
 2 files changed, 152 insertions(+)
 create mode 100644 doc/users/testers_guide.rst

diff --git a/doc/index.rst b/doc/index.rst
index c00a59d31..acd16cdbf 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -9,6 +9,7 @@
 
    users/quick_start
    users/setup_tests
+   users/testers_guide
    users/supported_systems
    users/stats
    users/test_catalog
diff --git a/doc/users/testers_guide.rst b/doc/users/testers_guide.rst
new file mode 100644
index 000000000..86f8006f9
--- /dev/null
+++ b/doc/users/testers_guide.rst
@@ -0,0 +1,151 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Testers guide to the Linux test project
+=======================================
+
+While we try to make LTP work out of the box as much as possible there are
+still many things that testers need to consider before the actual testing
+starts. It's advisable to make a test plans in order to asses and formalize the
+expected test coverage or even just sit down for a while and consider different
+aspects of the problem at hand.
+
+
+Is testing even required?
+-------------------------
+
+Sometimes people tend to make an argument that there is no need to test the
+Linux kernel because it's heavily tested in upstream and hence very stable.
+That may be true, or at least one can say that upstream releases are not
+terribly unstable.
+
+However that argument holds only as long as you take the upstream sources
+verbatim, i.e. without any modifications and that includes taking the default
+`.config` options too. Once you stray away from what you consider the stable
+upstream source you introduce differences in behavior, possible bugs and
+problems into your codebase. That applies to patch backports too, it's quite
+easy to miss pre-requisites when backporting patches which may result in all
+kinds of breakages.
+
+
+Multi dimensionality
+--------------------
+
+First of all kernel testing is a multi dimensional problem, just compiling and
+running LTP will give you some coverage but very likely not enough. There are
+several big gaps that may be easily missed.
+
+For example 64bit Linux kernel provides compatibility layer for 32bit
+applications which code quality is usually a bit worse than the 64bit ABI.
+Hence recompiling LTP with `-m32` in compiler flags and running both 64bit
+and 32bit test binaries is a good start. If you try to make an argument that
+your application does not need 32bit support it's better to disable the compat
+layer completely since it's possible source of security bugs.
+
+Another dimension is the number of architectures you need to test, for a
+general distribution testing you may end up with a couple of them. Different
+architectures have different platform code as well as differences in memory
+orderings, etc. that all means that running tests on one architecture out of
+several will give you incomplete coverage.
+
+Since most of the POSIX API deals with files the choise of filesystem for the
+testing changes the focus and coverage too. LTP defaults to using `/tmp/` as
+a temporary directory for the tests. If `/tmp/` is mounted as tmpfs subset of
+tests will be skipped, if that is the case it's advisable to point environment
+variable `TMPDIR` to a path with a different filesystem instead. Then there
+are tests that format a device with a filesystem. LTP defaults to `ext2` and
+loop devices for these testcases, that can be changed with environment
+variables as well. Lastly but not least a few testcases repeat the test for all
+supported filesystem, if you are interested in testing on a single filesystem
+only you can limit these tests to a single filesystem too. See the tests setup
+for a comprehensive list of the `evironment variables
+<setup_tests.html#library-environment-variables>`_.
+
+They you also have to decide if you are going to run tests in virtual machine
+e.g. `qemu-kvm`, on bare metal or both. Testing in virtual machine will give you
+about 90% of the coverage for bare metal and vice versa.
+
+There are other options worth of consideration too, Linux kernel has many
+debugging options that are usually disabled on runtime since they incur
+significant performance penalty. Having a few more LTP testruns with different
+debug options enabled e.g. `KASAN
+<https://www.kernel.org/doc/html/latest/dev-tools/kasan.html>`_ or `KMEMLEAK
+<https://www.kernel.org/doc/html/latest/dev-tools/kmemleak.html>`_ may help
+catch bugs before they materialize in production.
+
+In practice your test matrix may easily explode and you may end up with dozens
+of differently configured testruns based on different considerations. The hard
+task at hand is not to have too many since computing power is not an infinite
+resource and does not scale that easily. If you managed to read up to this
+point *"Don't Panic!"* things are almost never as bad as they may seem at first
+glance.
+
+It's a good idea to start small with an environment that models your
+production.  Once that works well you can try different configurations. Select
+a few interesting ones and run them for some time in order to get an idea of
+their usefulness. If you are feeling adventurous you may try to measure and
+compare actual test coverage with one of the tools such as `gcov
+<https://www.kernel.org/doc/html/latest/dev-tools/gcov.html>`_ and `lcov
+<https://github.com/linux-test-project/lcov>`. If you do so do not fall into a
+trap of attempting to have 100% line coverage. Having 100% of lines executed
+during the test does not mean that your test coverage is 100%.  Good tests
+validate much more than just how much code from the tested binary was executed.
+
+You may need to sacrifice some coverage in order to match the tests runtime to
+the available computing power. When doing so `Pareto principle
+<https://en.wikipedia.org/wiki/Pareto_principle>`_ is your friend.
+
+
+Test scope
+----------
+
+So far we were talking about a code coverage from a point of maximizing test
+coverage while keeping our test matrix as small as possible. While that is a
+noble goal it's not the universal holy grail of testing. Different use cases
+have different considerations and scope. For a testing before a final release
+such testing is very desirable, however for a continuous integration or smoke
+testing the main requirement is that feedback loops are as short as possible.
+
+When a developer changes the kernel and submits the changes to be merged it's
+desirable to run some tests. Again the hard question is which tests. If we run
+all possible tests in all possible combinations it may take a day or two and
+the developer will move to a different tasks before the tests have a chance to
+finish. If you multiply that by a number of developers in the team you may end
+up in a situation where a developer will retire before tests for his patch may
+have had a chance to finish.
+
+In this case careful selection of tests is even more important. Having less is
+more in this context. One of the first ideas for CI is to skip tests that run
+for more than a second or so, happily this can be easily done with `kirk
+<https://github.com/linux-test-project/kirk/>`_. In the future we may want to
+explore some heuristics that would map the code changes in kernel into a subset
+of tests, which would allow for a very quick feedback.
+
+
+Debugging test failures
+-----------------------
+
+You may think that you will enjoy some rest once you have your test matrix
+ready and your tests are running. Unfortunately that's where the actual work
+starts. Debugging test failures is probably the hardest part of the testing
+process. In some cases failures are easily reproducible and it's not that hard
+to locate the bug, either in the test or in the kernel itself. There are
+however, quite common, cases where the test failure reproduces only in 10% or
+even 1% of the test runs. Sometimes tests are not failing in isolation, that is
+because operating system has a huge internal state and a test failure manifests
+only after running right sequence of tests. All of that does not mean that
+there is no bug, that usually means that the bug depends on more prerequisites
+that have to manifest at the right time in order to trigger the failure. Sadly
+for modern systems that are asynchronous in nature such bugs are more and more
+common.
+
+The debugging process itself is not complicated by its nature. You have to
+attempt to understand the failure by checking the logs, reading and
+understanding the source code, debugging with strace, gdb, etc. Then form a
+hypothesis and either prove or disprove it. Rinse and repeat until you end up
+with a clear description of what went wrong. Hopefully you will manage to find
+the root cause, but you should not be discouraged, if you do not. Debugging
+kernel bugs takes a lot of experience and skill one can say as much as is
+needed to write the kernel code.
+
+
+Happy testing!
-- 
2.49.0

