Documentation/admin-guide/bug-bisect.rst - linux - Git at Google

 .. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
 .. [see the bottom of this file for redistribution information]

 ======================
 Bisecting a regression
 ======================

 This document describes how to use a ``git bisect`` to find the source code
 change that broke something -- for example when some functionality stopped
 working after upgrading from Linux 6.0 to 6.1.

 The text focuses on the gist of the process. If you are new to bisecting the
 kernel, better follow Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst
 instead: it depicts everything from start to finish while covering multiple
 aspects even kernel developers occasionally forget. This includes detecting
 situations early where a bisection would be a waste of time, as nobody would
 care about the result -- for example, because the problem happens after the
 kernel marked itself as 'tainted', occurs in an abandoned version, was already
 fixed, or is caused by a .config change you or your Linux distributor performed.

 Finding the change causing a kernel issue using a bisection
 ===========================================================

 *Note: the following process assumes you prepared everything for a bisection.
 This includes having a Git clone with the appropriate sources, installing the
 software required to build and install kernels, as well as a .config file stored
 in a safe place (the following example assumes '~/prepared_kernel_.config') to
 use as pristine base at each bisection step; ideally, you have also worked out
 a fully reliable and straight-forward way to reproduce the regression, too.*

 * Preparation: start the bisection and tell Git about the points in the history
   you consider to be working and broken, which Git calls 'good' and 'bad'::

      git bisect start
      git bisect good v6.0
      git bisect bad v6.1

   Instead of Git tags like 'v6.0' and 'v6.1' you can specify commit-ids, too.

 1. Copy your prepared .config into the build directory and adjust it to the
    needs of the codebase Git checked out for testing::

      cp ~/prepared_kernel_.config .config
      make olddefconfig

 2. Now build, install, and boot a kernel. This might fail for unrelated reasons,
    for example, when a compile error happens at the current stage of the
    bisection a later change resolves. In such cases run ``git bisect skip`` and
    go back to step 1.

 3. Check if the functionality that regressed works in the kernel you just built.

    If it works, execute::

      git bisect good

    If it is broken, run::

      git bisect bad

    Note, getting this wrong just once will send the rest of the bisection
    totally off course. To prevent having to start anew later you thus want to
    ensure what you tell Git is correct; it is thus often wise to spend a few
    minutes more on testing in case your reproducer is unreliable.

    After issuing one of these two commands, Git will usually check out another
    bisection point and print something like 'Bisecting: 675 revisions left to
    test after this (roughly 10 steps)'. In that case go back to step 1.

    If Git instead prints something like 'cafecaca0c0dacafecaca0c0dacafecaca0c0da
    is the first bad commit', then you have finished the bisection. In that case
    move to the next point below. Note, right after displaying that line Git will
    show some details about the culprit including its patch description; this can
    easily fill your terminal, so you might need to scroll up to see the message
    mentioning the culprit's commit-id.

    In case you missed Git's output, you can always run ``git bisect log`` to
    print the status: it will show how many steps remain or mention the result of
    the bisection.

 * Recommended complementary task: put the bisection log and the current .config
   file aside for the bug report; furthermore tell Git to reset the sources to
   the state before the bisection::

      git bisect log > ~/bisection-log
      cp .config ~/bisection-config-culprit
      git bisect reset

 * Recommended optional task: try reverting the culprit on top of the latest
   codebase and check if that fixes your bug; if that is the case, it validates
   the bisection and enables developers to resolve the regression through a
   revert.

   To try this, update your clone and check out latest mainline. Then tell Git
   to revert the change by specifying its commit-id::

      git revert --no-edit cafec0cacaca0

   Git might reject this, for example when the bisection landed on a merge
   commit. In that case, abandon the attempt. Do the same, if Git fails to revert
   the culprit on its own because later changes depend on it -- at least unless
   you bisected a stable or longterm kernel series, in which case you want to
   check out its latest codebase and try a revert there.

   If a revert succeeds, build and test another kernel to check if reverting
   resolved your regression.

 With that the process is complete. Now report the regression as described by
 Documentation/admin-guide/reporting-issues.rst.


 Additional reading material
 ---------------------------

 * The `man page for 'git bisect' <https://git-scm.com/docs/git-bisect>`_ and
   `fighting regressions with 'git bisect' <https://git-scm.com/docs/git-bisect-lk2009.html>`_
   in the Git documentation.
 * `Working with git bisect <https://nathanchance.dev/posts/working-with-git-bisect/>`_
   from kernel developer Nathan Chancellor.
 * `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_.
 * `Fully automated bisecting with 'git bisect run' <https://lwn.net/Articles/317154>`_.

 ..
    end-of-content
 ..
    This document is maintained by Thorsten Leemhuis <linux@leemhuis.info>. If
    you spot a typo or small mistake, feel free to let him know directly and
    he'll fix it. You are free to do the same in a mostly informal way if you
    want to contribute changes to the text -- but for copyright reasons please CC
    linux-doc@vger.kernel.org and 'sign-off' your contribution as
    Documentation/process/submitting-patches.rst explains in the section 'Sign
    your work - the Developer's Certificate of Origin'.
 ..
    This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
    of the file. If you want to distribute this text under CC-BY-4.0 only,
    please use 'The Linux kernel development community' for author attribution
    and link this as source:
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/bug-bisect.rst

 ..
    Note: Only the content of this RST file as found in the Linux kernel sources
    is available under CC-BY-4.0, as versions of this text that were processed
    (for example by the kernel's build system) might contain content taken from
    files which use a more restrictive license.
	.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
	.. [see the bottom of this file for redistribution information]

	======================
	Bisecting a regression
	======================

	This document describes how to use a ``git bisect`` to find the source code
	change that broke something -- for example when some functionality stopped
	working after upgrading from Linux 6.0 to 6.1.

	The text focuses on the gist of the process. If you are new to bisecting the
	kernel, better follow Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst
	instead: it depicts everything from start to finish while covering multiple
	aspects even kernel developers occasionally forget. This includes detecting
	situations early where a bisection would be a waste of time, as nobody would
	care about the result -- for example, because the problem happens after the
	kernel marked itself as 'tainted', occurs in an abandoned version, was already
	fixed, or is caused by a .config change you or your Linux distributor performed.

	Finding the change causing a kernel issue using a bisection
	===========================================================

	*Note: the following process assumes you prepared everything for a bisection.
	This includes having a Git clone with the appropriate sources, installing the
	software required to build and install kernels, as well as a .config file stored
	in a safe place (the following example assumes '~/prepared_kernel_.config') to
	use as pristine base at each bisection step; ideally, you have also worked out
	a fully reliable and straight-forward way to reproduce the regression, too.*

	* Preparation: start the bisection and tell Git about the points in the history
	you consider to be working and broken, which Git calls 'good' and 'bad'::

	git bisect start
	git bisect good v6.0
	git bisect bad v6.1

	Instead of Git tags like 'v6.0' and 'v6.1' you can specify commit-ids, too.

	1. Copy your prepared .config into the build directory and adjust it to the
	needs of the codebase Git checked out for testing::

	cp ~/prepared_kernel_.config .config
	make olddefconfig

	2. Now build, install, and boot a kernel. This might fail for unrelated reasons,
	for example, when a compile error happens at the current stage of the
	bisection a later change resolves. In such cases run ``git bisect skip`` and
	go back to step 1.

	3. Check if the functionality that regressed works in the kernel you just built.

	If it works, execute::

	git bisect good

	If it is broken, run::

	git bisect bad

	Note, getting this wrong just once will send the rest of the bisection
	totally off course. To prevent having to start anew later you thus want to
	ensure what you tell Git is correct; it is thus often wise to spend a few
	minutes more on testing in case your reproducer is unreliable.

	After issuing one of these two commands, Git will usually check out another
	bisection point and print something like 'Bisecting: 675 revisions left to
	test after this (roughly 10 steps)'. In that case go back to step 1.

	If Git instead prints something like 'cafecaca0c0dacafecaca0c0dacafecaca0c0da
	is the first bad commit', then you have finished the bisection. In that case
	move to the next point below. Note, right after displaying that line Git will
	show some details about the culprit including its patch description; this can
	easily fill your terminal, so you might need to scroll up to see the message
	mentioning the culprit's commit-id.

	In case you missed Git's output, you can always run ``git bisect log`` to
	print the status: it will show how many steps remain or mention the result of
	the bisection.

	* Recommended complementary task: put the bisection log and the current .config
	file aside for the bug report; furthermore tell Git to reset the sources to
	the state before the bisection::

	git bisect log > ~/bisection-log
	cp .config ~/bisection-config-culprit
	git bisect reset

	* Recommended optional task: try reverting the culprit on top of the latest
	codebase and check if that fixes your bug; if that is the case, it validates
	the bisection and enables developers to resolve the regression through a
	revert.

	To try this, update your clone and check out latest mainline. Then tell Git
	to revert the change by specifying its commit-id::

	git revert --no-edit cafec0cacaca0

	Git might reject this, for example when the bisection landed on a merge
	commit. In that case, abandon the attempt. Do the same, if Git fails to revert
	the culprit on its own because later changes depend on it -- at least unless
	you bisected a stable or longterm kernel series, in which case you want to
	check out its latest codebase and try a revert there.

	If a revert succeeds, build and test another kernel to check if reverting
	resolved your regression.

	With that the process is complete. Now report the regression as described by
	Documentation/admin-guide/reporting-issues.rst.


	Additional reading material
	---------------------------

	* The `man page for 'git bisect' <https://git-scm.com/docs/git-bisect>`_ and
	`fighting regressions with 'git bisect' <https://git-scm.com/docs/git-bisect-lk2009.html>`_
	in the Git documentation.
	* `Working with git bisect <https://nathanchance.dev/posts/working-with-git-bisect/>`_
	from kernel developer Nathan Chancellor.
	* `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_.
	* `Fully automated bisecting with 'git bisect run' <https://lwn.net/Articles/317154>`_.

	..
	end-of-content
	..
	This document is maintained by Thorsten Leemhuis <linux@leemhuis.info>. If
	you spot a typo or small mistake, feel free to let him know directly and
	he'll fix it. You are free to do the same in a mostly informal way if you
	want to contribute changes to the text -- but for copyright reasons please CC
	linux-doc@vger.kernel.org and 'sign-off' your contribution as
	Documentation/process/submitting-patches.rst explains in the section 'Sign
	your work - the Developer's Certificate of Origin'.
	..
	This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
	of the file. If you want to distribute this text under CC-BY-4.0 only,
	please use 'The Linux kernel development community' for author attribution
	and link this as source:
	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/bug-bisect.rst

	..
	Note: Only the content of this RST file as found in the Linux kernel sources
	is available under CC-BY-4.0, as versions of this text that were processed
	(for example by the kernel's build system) might contain content taken from
	files which use a more restrictive license.