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1 .. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
7 "*We don't cause regressions*" is the first rule of Linux kernel development;
11 This document describes what the rule means for users and how the Linux kernel's
13 for kernel developers are left to Documentation/process/handling-regressions.rst.
19 #. It's a regression if something running fine with one Linux kernel works worse
20 or not at all with a newer version. Note, the newer kernel has to be compiled
22 and other fine print in more detail.
24 #. Report your issue as outlined in Documentation/admin-guide/reporting-issues.rst,
28 <https://lore.kernel.org/regressions/>`_ (regressions@lists.linux.dev).
31 Linux kernel regression tracking bot "regzbot" track the issue by specifying
34 #regzbot introduced: v5.13..v5.14-rc1
37 All the details on Linux kernel regressions relevant for users
42 --------------------
49 one Linux kernel works worse or not at all with a newer version compiled using a
57 behavior with a newer kernel version; such issues can be caused by changes in
59 software. But keep in mind, as mentioned earlier: 5.14 in this example needs to
61 using ``make olddefconfig``, as explained in more detail below.
63 Note the "practical use case" in the first sentence of this section: developers
64 despite the "no regressions" rule are free to change any aspect of the kernel
68 Also be aware the "no regressions" rule covers only interfaces the kernel
69 provides to the userland. It thus does not apply to kernel-internal interfaces
71 the kernel.
76 Just report the issue as outlined in
77 Documentation/admin-guide/reporting-issues.rst, it already describes the
82 Linux regressions mailing list <https://lore.kernel.org/regressions/>`_ and
83 `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_.
87 * In your report, clearly mention the last kernel version that worked fine and
89 regression using a bisection, as explained below in more detail.
98 mailing list for the subsystem in question.
102 <https://lore.kernel.org/stable/>`_ (stable@vger.kernel.org).
104 In case you performed a successful bisection, add everyone to the CC the
105 culprit's commit message mentions in lines starting with "Signed-off-by:".
108 aforementioned Linux kernel regression tracking bot about your report. To do
109 that, include a paragraph like this in your mail::
111 #regzbot introduced: v5.13..v5.14-rc1
113 Regzbot will then consider your mail a report for a regression introduced in the
114 specified version range. In above case Linux v5.13 still worked fine and Linux
115 v5.14-rc1 was the first version where you encountered the issue. If you
117 culprit's commit-id instead::
121 Placing such a "regzbot command" is in your interest, as it will ensure the
123 kernel's regressions tracker will take care of telling regzbot about your
127 Relying on this person thus will result in an unnecessary delay before the
129 kernel regressions <https://linux-regtracking.leemhuis.info/regzbot/>`_ and the
130 weekly regression reports sent by regzbot. Such delays can result in Linus
146 a lot of issues only occur in a particular environment outside the developer's
147 reach -- for example, a specific hardware platform, firmware, Linux distro,
148 system's configuration, or application. That's why in the end it's often up to
157 Perform a bisection, as roughly outlined in
158 Documentation/admin-guide/reporting-issues.rst and described in more detail by
159 Documentation/admin-guide/bug-bisect.rst. It might sound like a lot of work, but
160 in many cases finds the culprit relatively quickly. If it's hard or
161 time-consuming to reliably reproduce the issue, consider teaming up with other
168 CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
169 issue might better be dealt with in private, feel free to omit the list.
173 ------------------------------------
179 Users should feel safe when updating kernel versions and not have to worry
180 something might break. This is in the interest of the kernel developers to make
183 in everybody's interest, as `those series might have known bugs, security
184 issues, or other problematic aspects already fixed in later versions
185 <http://www.kroah.com/log/blog/2018/08/24/what-stable-kernel-should-i-use/>`_.
186 Additionally, the kernel developers want to make it simple and appealing for
187 users to test the latest pre-release or regular release. That's also in
191 Is the "no regressions" rule really adhered in practice?
195 Linux creator and lead developer Linus Torvalds, some of which are quoted in
196 Documentation/process/handling-regressions.rst.
198 Exceptions to this rule are extremely rare; in the past developers almost always
206 supported by the tree maintainers -- e.g. Linus Torvalds for mainline and
207 Greg Kroah-Hartman et al. for various stable/longterm series.
211 the Linux kernel's "regressions tracker"; to facilitate this work he relies on
212 regzbot, the Linux kernel regression tracking bot. That's why you want to bring
214 the regressions mailing list, ideally with a "regzbot command" in your mail to
221 affected users with a solution in a timely manner and prevent more users from
226 age, or the Linux series in which it occurs. In the end though, most regressions
235 Is it a regression, if a newer kernel works slower or consumes more energy?
238 Yes, but the difference has to be significant. A five percent slow-down in a
239 micro-benchmark thus is unlikely to qualify as regression, unless it also
240 influences the results of a broad benchmark by more than one percent. If in
243 Is it a regression, if an external kernel module breaks when updating Linux?
247 kernel provides to the userland. It thus does not cover building or running
248 externally developed kernel modules, as they run in kernel-space and hook into
249 the kernel using internal interfaces occasionally changed.
254 In extremely rare situations security issues can't be fixed without causing
255 regressions; those fixes are given way, as they are the lesser evil in the end.
261 tried their best to avoid the regression. If not, report it; if in doubt, ask
272 advice if in doubt.
275 regularly give mainline pre-releases (say v5.15-rc1 or -rc3) from each
277 integrated between Linux v5.14 and v5.15-rc1 which causes a regression, but at
279 5.15-rc1. All these changes often can simply be reverted and the regression thus
282 started to rely on aspects introduced by one of the follow-up changes: reverting
290 in the previous section. It hence needs to be dealt with on a case-by-case
291 basis. This is another reason why it's in everybody's interest to regularly test
292 mainline pre-releases.
298 remove support for hardware only to be found in attics and museums anymore.
300 Note, sometimes regressions can't be avoided to make progress -- and the latter
302 to be affected by a regression, it for the greater good might be in their and
305 software or using a kernel parameter created just for this purpose.
307 Does the regression rule apply for code in the staging tree as well?
311 staging code <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/stagi…
316 will be changed in the near future.
326 Because the Linux kernel developers sometimes integrate changes known to cause
327 regressions, but make them optional and disable them in the kernel's default
331 Consider for example a new security feature blocking access to some kernel
337 How to create a configuration similar to the one of an older kernel?
340 Start your machine with a known-good kernel and configure the newer Linux
341 version with ``make olddefconfig``. This makes the kernel's build scripts pick
342 up the configuration file (the ".config" file) from the running kernel as base
347 Can I report a regression I found with pre-compiled vanilla kernels?
350 You need to ensure the newer kernel was compiled with a similar configuration
352 some known-to-be incompatible feature for the newer kernel. If in doubt, report
353 the matter to the kernel's provider and ask for advice.
357 ---------------------------------------------
364 true for Linux kernel development as well. That's why Thorsten Leemhuis, the
365 Linux Kernel's regression tracker, and some people try to ensure all regression
369 Why and how are Linux kernel regressions tracked using a bot?
373 distributed and loosely structured nature of Linux kernel development process.
374 That's why the Linux kernel's regression tracker developed regzbot to facilitate
387 Check out `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_.
393 regular issues. But it's okay for the Linux kernel's regression tracker if you
400 By using a 'regzbot command' in a direct or indirect reply to the mail with the
401 report. The easiest way to do that: find the report in your "Sent" folder or the
402 mailing list archive and reply to it using your mailer's "Reply-all" function.
403 In that mail, use one of the following commands in a stand-alone paragraph (IOW:
416 * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
419 #regzbot monitor: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
420 #regzbot monitor: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
423 or a ticket in a bug tracker that are slightly related, but about a different
426 #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
434 commands can be found in the `getting started guide
435 <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_ and
436 the `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
440 end-of-content
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443 of the file. If you want to distribute this text under CC-BY-4.0 only,
444 please use "The Linux kernel developers" for author attribution and link
446 …https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide…
448 Note: Only the content of this RST file as found in the Linux kernel sources
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450 (for example by the kernel's build system) might contain content taken from