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docs: add two documents about regression handling

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Create two documents explaining various aspects around regression
handling and tracking; one is aimed at users, the other targets
developers.

The texts among others describes the first rule of Linux kernel
development and what it means in practice. They also explain what a
regression actually is and how to report one properly.

Both texts additionally provide a brief introduction to the bot the
kernel's regression tracker uses to facilitate the work, but mention the
use is optional.

To sum things up, provide a few quotes from Linus in the document for
developers to show how serious we take regressions.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/34e56d3588f22d7e0b4d635ef9c9c3b33ca4ac04.1644994117.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Thorsten Leemhuis 2022-02-16 07:51:33 +01:00 committed by Jonathan Corbet
parent 18ab307823
commit 1ecf393fc5
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@ -35,6 +35,7 @@ problems and bugs in particular.
:maxdepth: 1
reporting-issues
reporting-regressions
security-bugs
bug-hunting
bug-bisect

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.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
.. [see the bottom of this file for redistribution information]
Reporting regressions
+++++++++++++++++++++
"*We don't cause regressions*" is the first rule of Linux kernel development;
Linux founder and lead developer Linus Torvalds established it himself and
ensures it's obeyed.
This document describes what the rule means for users and how the Linux kernel's
development model ensures to address all reported regressions; aspects relevant
for kernel developers are left to Documentation/process/handling-regressions.rst.
The important bits (aka "TL;DR")
================================
#. It's a regression if something running fine with one Linux kernel works worse
or not at all with a newer version. Note, the newer kernel has to be compiled
using a similar configuration; the detailed explanations below describes this
and other fine print in more detail.
#. Report your issue as outlined in Documentation/admin-guide/reporting-issues.rst,
it already covers all aspects important for regressions and repeated
below for convenience. Two of them are important: start your report's subject
with "[REGRESSION]" and CC or forward it to `the regression mailing list
<https://lore.kernel.org/regressions/>`_ (regressions@lists.linux.dev).
#. Optional, but recommended: when sending or forwarding your report, make the
Linux kernel regression tracking bot "regzbot" track the issue by specifying
when the regression started like this::
#regzbot introduced v5.13..v5.14-rc1
All the details on Linux kernel regressions relevant for users
==============================================================
The important basics
--------------------
What is a "regression" and what is the "no regressions rule"?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's a regression if some application or practical use case running fine with
one Linux kernel works worse or not at all with a newer version compiled using a
similar configuration. The "no regressions rule" forbids this to take place; if
it happens by accident, developers that caused it are expected to quickly fix
the issue.
It thus is a regression when a WiFi driver from Linux 5.13 works fine, but with
5.14 doesn't work at all, works significantly slower, or misbehaves somehow.
It's also a regression if a perfectly working application suddenly shows erratic
behavior with a newer kernel version; such issues can be caused by changes in
procfs, sysfs, or one of the many other interfaces Linux provides to userland
software. But keep in mind, as mentioned earlier: 5.14 in this example needs to
be built from a configuration similar to the one from 5.13. This can be achieved
using ``make olddefconfig``, as explained in more detail below.
Note the "practical use case" in the first sentence of this section: developers
despite the "no regressions" rule are free to change any aspect of the kernel
and even APIs or ABIs to userland, as long as no existing application or use
case breaks.
Also be aware the "no regressions" rule covers only interfaces the kernel
provides to the userland. It thus does not apply to kernel-internal interfaces
like the module API, which some externally developed drivers use to hook into
the kernel.
How do I report a regression?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Just report the issue as outlined in
Documentation/admin-guide/reporting-issues.rst, it already describes the
important points. The following aspects outlined there are especially relevant
for regressions:
* When checking for existing reports to join, also search the `archives of the
Linux regressions mailing list <https://lore.kernel.org/regressions/>`_ and
`regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_.
* Start your report's subject with "[REGRESSION]".
* In your report, clearly mention the last kernel version that worked fine and
the first broken one. Ideally try to find the exact change causing the
regression using a bisection, as explained below in more detail.
* Remember to let the Linux regressions mailing list
(regressions@lists.linux.dev) know about your report:
* If you report the regression by mail, CC the regressions list.
* If you report your regression to some bug tracker, forward the submitted
report by mail to the regressions list while CCing the maintainer and the
mailing list for the subsystem in question.
If it's a regression within a stable or longterm series (e.g.
v5.15.3..v5.15.5), remember to CC the `Linux stable mailing list
<https://lore.kernel.org/stable/>`_ (stable@vger.kernel.org).
In case you performed a successful bisection, add everyone to the CC the
culprit's commit message mentions in lines starting with "Signed-off-by:".
When CCing for forwarding your report to the list, consider directly telling the
aforementioned Linux kernel regression tracking bot about your report. To do
that, include a paragraph like this in your mail::
#regzbot introduced: v5.13..v5.14-rc1
Regzbot will then consider your mail a report for a regression introduced in the
specified version range. In above case Linux v5.13 still worked fine and Linux
v5.14-rc1 was the first version where you encountered the issue. If you
performed a bisection to find the commit that caused the regression, specify the
culprit's commit-id instead::
#regzbot introduced: 1f2e3d4c5d
Placing such a "regzbot command" is in your interest, as it will ensure the
report won't fall through the cracks unnoticed. If you omit this, the Linux
kernel's regressions tracker will take care of telling regzbot about your
regression, as long as you send a copy to the regressions mailing lists. But the
regression tracker is just one human which sometimes has to rest or occasionally
might even enjoy some time away from computers (as crazy as that might sound).
Relying on this person thus will result in an unnecessary delay before the
regressions becomes mentioned `on the list of tracked and unresolved Linux
kernel regressions <https://linux-regtracking.leemhuis.info/regzbot/>`_ and the
weekly regression reports sent by regzbot. Such delays can result in Linus
Torvalds being unaware of important regressions when deciding between "continue
development or call this finished and release the final?".
Are really all regressions fixed?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Nearly all of them are, as long as the change causing the regression (the
"culprit commit") is reliably identified. Some regressions can be fixed without
this, but often it's required.
Who needs to find the root cause of a regression?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Developers of the affected code area should try to locate the culprit on their
own. But for them that's often impossible to do with reasonable effort, as quite
a lot of issues only occur in a particular environment outside the developer's
reach -- for example, a specific hardware platform, firmware, Linux distro,
system's configuration, or application. That's why in the end it's often up to
the reporter to locate the culprit commit; sometimes users might even need to
run additional tests afterwards to pinpoint the exact root cause. Developers
should offer advice and reasonably help where they can, to make this process
relatively easy and achievable for typical users.
How can I find the culprit?
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Perform a bisection, as roughly outlined in
Documentation/admin-guide/reporting-issues.rst and described in more detail by
Documentation/admin-guide/bug-bisect.rst. It might sound like a lot of work, but
in many cases finds the culprit relatively quickly. If it's hard or
time-consuming to reliably reproduce the issue, consider teaming up with other
affected users to narrow down the search range together.
Who can I ask for advice when it comes to regressions?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
issue might better be dealt with in private, feel free to omit the list.
Additional details about regressions
------------------------------------
What is the goal of the "no regressions rule"?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Users should feel safe when updating kernel versions and not have to worry
something might break. This is in the interest of the kernel developers to make
updating attractive: they don't want users to stay on stable or longterm Linux
series that are either abandoned or more than one and a half years old. That's
in everybody's interest, as `those series might have known bugs, security
issues, or other problematic aspects already fixed in later versions
<http://www.kroah.com/log/blog/2018/08/24/what-stable-kernel-should-i-use/>`_.
Additionally, the kernel developers want to make it simple and appealing for
users to test the latest pre-release or regular release. That's also in
everybody's interest, as it's a lot easier to track down and fix problems, if
they are reported shortly after being introduced.
Is the "no regressions" rule really adhered in practice?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's taken really seriously, as can be seen by many mailing list posts from
Linux creator and lead developer Linus Torvalds, some of which are quoted in
Documentation/process/handling-regressions.rst.
Exceptions to this rule are extremely rare; in the past developers almost always
turned out to be wrong when they assumed a particular situation was warranting
an exception.
Who ensures the "no regressions" is actually followed?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The subsystem maintainers should take care of that, which are watched and
supported by the tree maintainers -- e.g. Linus Torvalds for mainline and
Greg Kroah-Hartman et al. for various stable/longterm series.
All of them are helped by people trying to ensure no regression report falls
through the cracks. One of them is Thorsten Leemhuis, who's currently acting as
the Linux kernel's "regressions tracker"; to facilitate this work he relies on
regzbot, the Linux kernel regression tracking bot. That's why you want to bring
your report on the radar of these people by CCing or forwarding each report to
the regressions mailing list, ideally with a "regzbot command" in your mail to
get it tracked immediately.
Is it a regression, if the issue can be avoided by updating some software?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Almost always: yes. If a developer tells you otherwise, ask the regression
tracker for advice as outlined above.
Is it a regression, if a newer kernel works slower or consumes more energy?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Yes, but the difference has to be significant. A five percent slow-down in a
micro-benchmark thus is unlikely to qualify as regression, unless it also
influences the results of a broad benchmark by more than one percent. If in
doubt, ask for advice.
Is it a regression, if an external kernel module breaks when updating Linux?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No, as the "no regression" rule is about interfaces and services the Linux
kernel provides to the userland. It thus does not cover building or running
externally developed kernel modules, as they run in kernel-space and hook into
the kernel using internal interfaces occasionally changed.
How are regressions handled that are caused by security fixes?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In extremely rare situations security issues can't be fixed without causing
regressions; those fixes are given way, as they are the lesser evil in the end.
Luckily this middling almost always can be avoided, as key developers for the
affected area and often Linus Torvalds himself try very hard to fix security
issues without causing regressions.
If you nevertheless face such a case, check the mailing list archives if people
tried their best to avoid the regression. If not, report it; if in doubt, ask
for advice as outlined above.
What happens if fixing a regression is impossible without causing another?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sadly these things happen, but luckily not very often; if they occur, expert
developers of the affected code area should look into the issue to find a fix
that avoids regressions or at least their impact. If you run into such a
situation, do what was outlined already for regressions caused by security
fixes: check earlier discussions if people already tried their best and ask for
advice if in doubt.
A quick note while at it: these situations could be avoided, if people would
regularly give mainline pre-releases (say v5.15-rc1 or -rc3) from each
development cycle a test run. This is best explained by imagining a change
integrated between Linux v5.14 and v5.15-rc1 which causes a regression, but at
the same time is a hard requirement for some other improvement applied for
5.15-rc1. All these changes often can simply be reverted and the regression thus
solved, if someone finds and reports it before 5.15 is released. A few days or
weeks later this solution can become impossible, as some software might have
started to rely on aspects introduced by one of the follow-up changes: reverting
all changes would then cause a regression for users of said software and thus is
out of the question.
Is it a regression, if some feature I relied on was removed months ago?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It is, but often it's hard to fix such regressions due to the aspects outlined
in the previous section. It hence needs to be dealt with on a case-by-case
basis. This is another reason why it's in everybody's interest to regularly test
mainline pre-releases.
Does the "no regression" rule apply if I seem to be the only affected person?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It does, but only for practical usage: the Linux developers want to be free to
remove support for hardware only to be found in attics and museums anymore.
Note, sometimes regressions can't be avoided to make progress -- and the latter
is needed to prevent Linux from stagnation. Hence, if only very few users seem
to be affected by a regression, it for the greater good might be in their and
everyone else's interest to lettings things pass. Especially if there is an
easy way to circumvent the regression somehow, for example by updating some
software or using a kernel parameter created just for this purpose.
Does the regression rule apply for code in the staging tree as well?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Not according to the `help text for the configuration option covering all
staging code <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/staging/Kconfig>`_,
which since its early days states::
Please note that these drivers are under heavy development, may or
may not work, and may contain userspace interfaces that most likely
will be changed in the near future.
The staging developers nevertheless often adhere to the "no regressions" rule,
but sometimes bend it to make progress. That's for example why some users had to
deal with (often negligible) regressions when a WiFi driver from the staging
tree was replaced by a totally different one written from scratch.
Why do later versions have to be "compiled with a similar configuration"?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because the Linux kernel developers sometimes integrate changes known to cause
regressions, but make them optional and disable them in the kernel's default
configuration. This trick allows progress, as the "no regressions" rule
otherwise would lead to stagnation.
Consider for example a new security feature blocking access to some kernel
interfaces often abused by malware, which at the same time are required to run a
few rarely used applications. The outlined approach makes both camps happy:
people using these applications can leave the new security feature off, while
everyone else can enable it without running into trouble.
How to create a configuration similar to the one of an older kernel?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Start your machine with a known-good kernel and configure the newer Linux
version with ``make olddefconfig``. This makes the kernel's build scripts pick
up the configuration file (the ".config" file) from the running kernel as base
for the new one you are about to compile; afterwards they set all new
configuration options to their default value, which should disable new features
that might cause regressions.
Can I report a regression I found with pre-compiled vanilla kernels?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You need to ensure the newer kernel was compiled with a similar configuration
file as the older one (see above), as those that built them might have enabled
some known-to-be incompatible feature for the newer kernel. If in doubt, report
the matter to the kernel's provider and ask for advice.
More about regression tracking with "regzbot"
---------------------------------------------
What is regression tracking and why should I care about it?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Rules like "no regressions" need someone to ensure they are followed, otherwise
they are broken either accidentally or on purpose. History has shown this to be
true for Linux kernel development as well. That's why Thorsten Leemhuis, the
Linux Kernel's regression tracker, and some people try to ensure all regression
are fixed by keeping an eye on them until they are resolved. Neither of them are
paid for this, that's why the work is done on a best effort basis.
Why and how are Linux kernel regressions tracked using a bot?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tracking regressions completely manually has proven to be quite hard due to the
distributed and loosely structured nature of Linux kernel development process.
That's why the Linux kernel's regression tracker developed regzbot to facilitate
the work, with the long term goal to automate regression tracking as much as
possible for everyone involved.
Regzbot works by watching for replies to reports of tracked regressions.
Additionally, it's looking out for posted or committed patches referencing such
reports with "Link:" tags; replies to such patch postings are tracked as well.
Combined this data provides good insights into the current state of the fixing
process.
How to see which regressions regzbot tracks currently?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Check out `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_.
What kind of issues are supposed to be tracked by regzbot?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The bot is meant to track regressions, hence please don't involve regzbot for
regular issues. But it's okay for the Linux kernel's regression tracker if you
involve regzbot to track severe issues, like reports about hangs, corrupted
data, or internal errors (Panic, Oops, BUG(), warning, ...).
How to change aspects of a tracked regression?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By using a 'regzbot command' in a direct or indirect reply to the mail with the
report. The easiest way to do that: find the report in your "Sent" folder or the
mailing list archive and reply to it using your mailer's "Reply-all" function.
In that mail, use one of the following commands in a stand-alone paragraph (IOW:
use blank lines to separate one or multiple of these commands from the rest of
the mail's text).
* Update when the regression started to happen, for example after performing a
bisection::
#regzbot introduced: 1f2e3d4c5d
* Set or update the title::
#regzbot title: foo
* Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
the issue or a fix are discussed:::
#regzbot monitor: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
#regzbot monitor: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
* Point to a place with further details of interest, like a mailing list post
or a ticket in a bug tracker that are slightly related, but about a different
topic::
#regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
* Mark a regression as invalid::
#regzbot invalid: wasn't a regression, problem has always existed
Regzbot supports a few other commands primarily used by developers or people
tracking regressions. They and more details about the aforementioned regzbot
commands can be found in the `getting started guide
<https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_ and
the `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
for regzbot.
..
end-of-content
..
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 developers" for author attribution and link
this as source:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/reporting-regressions.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.

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@ -0,0 +1,659 @@
.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
.. See the bottom of this file for additional redistribution information.
Handling regressions
++++++++++++++++++++
*We don't cause regressions* -- this document describes what this "first rule of
Linux kernel development" means in practice for developers. It complements
Documentation/admin-guide/reporting-regressions.rst, which covers the topic from a
user's point of view; if you never read that text, go and at least skim over it
before continuing here.
The important bits (aka "The TL;DR")
====================================
#. Ensure subscribers of the `regression mailing list <https://lore.kernel.org/regressions/>`_
(regressions@lists.linux.dev) quickly become aware of any new regression
report:
* When receiving a mailed report that did not CC the list, bring it into the
loop by immediately sending at least a brief "Reply-all" with the list
CCed.
* Forward or bounce any reports submitted in bug trackers to the list.
#. Make the Linux kernel regression tracking bot "regzbot" track the issue (this
is optional, but recommended):
* For mailed reports, check if the reporter included a line like ``#regzbot
introduced v5.13..v5.14-rc1``. If not, send a reply (with the regressions
list in CC) containing a paragraph like the following, which tells regzbot
when the issue started to happen::
#regzbot ^introduced 1f2e3d4c5b6a
* When forwarding reports from a bug tracker to the regressions list (see
above), include a paragraph like the following::
#regzbot introduced: v5.13..v5.14-rc1
#regzbot from: Some N. Ice Human <some.human@example.com>
#regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
#. When submitting fixes for regressions, add "Link:" tags to the patch
description pointing to all places where the issue was reported, as
mandated by Documentation/process/submitting-patches.rst and
:ref:`Documentation/process/5.Posting.rst <development_posting>`.
All the details on Linux kernel regressions relevant for developers
===================================================================
The important basics in more detail
-----------------------------------
What to do when receiving regression reports
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ensure the Linux kernel's regression tracker and others subscribers of the
`regression mailing list <https://lore.kernel.org/regressions/>`_
(regressions@lists.linux.dev) become aware of any newly reported regression:
* When you receive a report by mail that did not CC the list, immediately bring
it into the loop by sending at least a brief "Reply-all" with the list CCed;
try to ensure it gets CCed again in case you reply to a reply that omitted
the list.
* If a report submitted in a bug tracker hits your Inbox, forward or bounce it
to the list. Consider checking the list archives beforehand, if the reporter
already forwarded the report as instructed by
Documentation/admin-guide/reporting-issues.rst.
When doing either, consider making the Linux kernel regression tracking bot
"regzbot" immediately start tracking the issue:
* For mailed reports, check if the reporter included a "regzbot command" like
``#regzbot introduced 1f2e3d4c5b6a``. If not, send a reply (with the
regressions list in CC) with a paragraph like the following:::
#regzbot ^introduced: v5.13..v5.14-rc1
This tells regzbot the version range in which the issue started to happen;
you can specify a range using commit-ids as well or state a single commit-id
in case the reporter bisected the culprit.
Note the caret (^) before the "introduced": it tells regzbot to treat the
parent mail (the one you reply to) as the initial report for the regression
you want to see tracked; that's important, as regzbot will later look out
for patches with "Link:" tags pointing to the report in the archives on
lore.kernel.org.
* When forwarding a regressions reported to a bug tracker, include a paragraph
with these regzbot commands::
#regzbot introduced: 1f2e3d4c5b6a
#regzbot from: Some N. Ice Human <some.human@example.com>
#regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
Regzbot will then automatically associate patches with the report that
contain "Link:" tags pointing to your mail or the mentioned ticket.
What's important when fixing regressions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You don't need to do anything special when submitting fixes for regression, just
remember to do what Documentation/process/submitting-patches.rst,
:ref:`Documentation/process/5.Posting.rst <development_posting>`, and
Documentation/process/stable-kernel-rules.rst already explain in more detail:
* Point to all places where the issue was reported using "Link:" tags::
Link: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
Link: https://bugzilla.kernel.org/show_bug.cgi?id=1234567890
* Add a "Fixes:" tag to specify the commit causing the regression.
* If the culprit was merged in an earlier development cycle, explicitly mark
the fix for backporting using the ``Cc: stable@vger.kernel.org`` tag.
All this is expected from you and important when it comes to regression, as
these tags are of great value for everyone (you included) that might be looking
into the issue weeks, months, or years later. These tags are also crucial for
tools and scripts used by other kernel developers or Linux distributions; one of
these tools is regzbot, which heavily relies on the "Link:" tags to associate
reports for regression with changes resolving them.
More aspects regarding regressions developers should be aware of
----------------------------------------------------------------
How to deal with changes where a risk of regression is known
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Evaluate how big the risk of regressions is, for example by performing a code
search in Linux distributions and Git forges. Also consider asking other
developers or projects likely to be affected to evaluate or even test the
proposed change; if problems surface, maybe some solution acceptable for all
can be found.
If the risk of regressions in the end seems to be relatively small, go ahead
with the change, but let all involved parties know about the risk. Hence, make
sure your patch description makes this aspect obvious. Once the change is
merged, tell the Linux kernel's regression tracker and the regressions mailing
list about the risk, so everyone has the change on the radar in case reports
trickle in. Depending on the risk, you also might want to ask the subsystem
maintainer to mention the issue in his mainline pull request.
What else is there to known about regressions?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Check out Documentation/admin-guide/reporting-regressions.rst, it covers a lot
of other aspects you want might want to be aware of:
* the purpose of the "no regressions rule"
* what issues actually qualify as regression
* who's in charge for finding the root cause of a regression
* how to handle tricky situations, e.g. when a regression is caused by a
security fix or when fixing a regression might cause another one
Whom to ask for advice when it comes to regressions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
issue might better be dealt with in private, feel free to omit the list.
More about regression tracking and regzbot
------------------------------------------
Why the Linux kernel has a regression tracker, and why is regzbot used?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Rules like "no regressions" need someone to ensure they are followed, otherwise
they are broken either accidentally or on purpose. History has shown this to be
true for the Linux kernel as well. That's why Thorsten Leemhuis volunteered to
keep an eye on things as the Linux kernel's regression tracker, who's
occasionally helped by other people. Neither of them are paid to do this,
that's why regression tracking is done on a best effort basis.
Earlier attempts to manually track regressions have shown it's an exhausting and
frustrating work, which is why they were abandoned after a while. To prevent
this from happening again, Thorsten developed regzbot to facilitate the work,
with the long term goal to automate regression tracking as much as possible for
everyone involved.
How does regression tracking work with regzbot?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The bot watches for replies to reports of tracked regressions. Additionally,
it's looking out for posted or committed patches referencing such reports
with "Link:" tags; replies to such patch postings are tracked as well.
Combined this data provides good insights into the current state of the fixing
process.
Regzbot tries to do its job with as little overhead as possible for both
reporters and developers. In fact, only reporters are burdened with an extra
duty: they need to tell regzbot about the regression report using the ``#regzbot
introduced`` command outlined above; if they don't do that, someone else can
take care of that using ``#regzbot ^introduced``.
For developers there normally is no extra work involved, they just need to make
sure to do something that was expected long before regzbot came to light: add
"Link:" tags to the patch description pointing to all reports about the issue
fixed.
Do I have to use regzbot?
~~~~~~~~~~~~~~~~~~~~~~~~~
It's in the interest of everyone if you do, as kernel maintainers like Linus
Torvalds partly rely on regzbot's tracking in their work -- for example when
deciding to release a new version or extend the development phase. For this they
need to be aware of all unfixed regression; to do that, Linus is known to look
into the weekly reports sent by regzbot.
Do I have to tell regzbot about every regression I stumble upon?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ideally yes: we are all humans and easily forget problems when something more
important unexpectedly comes up -- for example a bigger problem in the Linux
kernel or something in real life that's keeping us away from keyboards for a
while. Hence, it's best to tell regzbot about every regression, except when you
immediately write a fix and commit it to a tree regularly merged to the affected
kernel series.
How to see which regressions regzbot tracks currently?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Check `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_
for the latest info; alternatively, `search for the latest regression report
<https://lore.kernel.org/lkml/?q=%22Linux+regressions+report%22+f%3Aregzbot>`_,
which regzbot normally sends out once a week on Sunday evening (UTC), which is a
few hours before Linus usually publishes new (pre-)releases.
What places is regzbot monitoring?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Regzbot is watching the most important Linux mailing lists as well as the git
repositories of linux-next, mainline, and stable/longterm.
What kind of issues are supposed to be tracked by regzbot?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The bot is meant to track regressions, hence please don't involve regzbot for
regular issues. But it's okay for the Linux kernel's regression tracker if you
use regzbot to track severe issues, like reports about hangs, corrupted data,
or internal errors (Panic, Oops, BUG(), warning, ...).
Can I add regressions found by CI systems to regzbot's tracking?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Feel free to do so, if the particular regression likely has impact on practical
use cases and thus might be noticed by users; hence, please don't involve
regzbot for theoretical regressions unlikely to show themselves in real world
usage.
How to interact with regzbot?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By using a 'regzbot command' in a direct or indirect reply to the mail with the
regression report. These commands need to be in their own paragraph (IOW: they
need to be separated from the rest of the mail using blank lines).
One such command is ``#regzbot introduced <version or commit>``, which makes
regzbot consider your mail as a regressions report added to the tracking, as
already described above; ``#regzbot ^introduced <version or commit>`` is another
such command, which makes regzbot consider the parent mail as a report for a
regression which it starts to track.
Once one of those two commands has been utilized, other regzbot commands can be
used in direct or indirect replies to the report. You can write them below one
of the `introduced` commands or in replies to the mail that used one of them
or itself is a reply to that mail:
* Set or update the title::
#regzbot title: foo
* Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
the issue or a fix are discussed -- for example the posting of a patch fixing
the regression::
#regzbot monitor: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
Monitoring only works for lore.kernel.org and bugzilla.kernel.org; regzbot
will consider all messages in that thread or ticket as related to the fixing
process.
* Point to a place with further details of interest, like a mailing list post
or a ticket in a bug tracker that are slightly related, but about a different
topic::
#regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
* Mark a regression as fixed by a commit that is heading upstream or already
landed::
#regzbot fixed-by: 1f2e3d4c5d
* Mark a regression as a duplicate of another one already tracked by regzbot::
#regzbot dup-of: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
* Mark a regression as invalid::
#regzbot invalid: wasn't a regression, problem has always existed
Is there more to tell about regzbot and its commands?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
More detailed and up-to-date information about the Linux
kernel's regression tracking bot can be found on its
`project page <https://gitlab.com/knurd42/regzbot>`_, which among others
contains a `getting started guide <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_
and `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
which both cover more details than the above section.
Quotes from Linus about regression
----------------------------------
Find below a few real life examples of how Linus Torvalds expects regressions to
be handled:
* From `2017-10-26 (1/2)
<https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
If you break existing user space setups THAT IS A REGRESSION.
It's not ok to say "but we'll fix the user space setup".
Really. NOT OK.
[...]
The first rule is:
- we don't cause regressions
and the corollary is that when regressions *do* occur, we admit to
them and fix them, instead of blaming user space.
The fact that you have apparently been denying the regression now for
three weeks means that I will revert, and I will stop pulling apparmor
requests until the people involved understand how kernel development
is done.
* From `2017-10-26 (2/2)
<https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
People should basically always feel like they can update their kernel
and simply not have to worry about it.
I refuse to introduce "you can only update the kernel if you also
update that other program" kind of limitations. If the kernel used to
work for you, the rule is that it continues to work for you.
There have been exceptions, but they are few and far between, and they
generally have some major and fundamental reasons for having happened,
that were basically entirely unavoidable, and people _tried_hard_ to
avoid them. Maybe we can't practically support the hardware any more
after it is decades old and nobody uses it with modern kernels any
more. Maybe there's a serious security issue with how we did things,
and people actually depended on that fundamentally broken model. Maybe
there was some fundamental other breakage that just _had_ to have a
flag day for very core and fundamental reasons.
And notice that this is very much about *breaking* peoples environments.
Behavioral changes happen, and maybe we don't even support some
feature any more. There's a number of fields in /proc/<pid>/stat that
are printed out as zeroes, simply because they don't even *exist* in
the kernel any more, or because showing them was a mistake (typically
an information leak). But the numbers got replaced by zeroes, so that
the code that used to parse the fields still works. The user might not
see everything they used to see, and so behavior is clearly different,
but things still _work_, even if they might no longer show sensitive
(or no longer relevant) information.
But if something actually breaks, then the change must get fixed or
reverted. And it gets fixed in the *kernel*. Not by saying "well, fix
your user space then". It was a kernel change that exposed the
problem, it needs to be the kernel that corrects for it, because we
have a "upgrade in place" model. We don't have a "upgrade with new
user space".
And I seriously will refuse to take code from people who do not
understand and honor this very simple rule.
This rule is also not going to change.
And yes, I realize that the kernel is "special" in this respect. I'm
proud of it.
I have seen, and can point to, lots of projects that go "We need to
break that use case in order to make progress" or "you relied on
undocumented behavior, it sucks to be you" or "there's a better way to
do what you want to do, and you have to change to that new better
way", and I simply don't think that's acceptable outside of very early
alpha releases that have experimental users that know what they signed
up for. The kernel hasn't been in that situation for the last two
decades.
We do API breakage _inside_ the kernel all the time. We will fix
internal problems by saying "you now need to do XYZ", but then it's
about internal kernel API's, and the people who do that then also
obviously have to fix up all the in-kernel users of that API. Nobody
can say "I now broke the API you used, and now _you_ need to fix it
up". Whoever broke something gets to fix it too.
And we simply do not break user space.
* From `2020-05-21
<https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
The rules about regressions have never been about any kind of
documented behavior, or where the code lives.
The rules about regressions are always about "breaks user workflow".
Users are literally the _only_ thing that matters.
No amount of "you shouldn't have used this" or "that behavior was
undefined, it's your own fault your app broke" or "that used to work
simply because of a kernel bug" is at all relevant.
Now, reality is never entirely black-and-white. So we've had things
like "serious security issue" etc that just forces us to make changes
that may break user space. But even then the rule is that we don't
really have other options that would allow things to continue.
And obviously, if users take years to even notice that something
broke, or if we have sane ways to work around the breakage that
doesn't make for too much trouble for users (ie "ok, there are a
handful of users, and they can use a kernel command line to work
around it" kind of things) we've also been a bit less strict.
But no, "that was documented to be broken" (whether it's because the
code was in staging or because the man-page said something else) is
irrelevant. If staging code is so useful that people end up using it,
that means that it's basically regular kernel code with a flag saying
"please clean this up".
The other side of the coin is that people who talk about "API
stability" are entirely wrong. API's don't matter either. You can make
any changes to an API you like - as long as nobody notices.
Again, the regression rule is not about documentation, not about
API's, and not about the phase of the moon.
It's entirely about "we caused problems for user space that used to work".
* From `2017-11-05
<https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/>`_::
And our regression rule has never been "behavior doesn't change".
That would mean that we could never make any changes at all.
For example, we do things like add new error handling etc all the
time, which we then sometimes even add tests for in our kselftest
directory.
So clearly behavior changes all the time and we don't consider that a
regression per se.
The rule for a regression for the kernel is that some real user
workflow breaks. Not some test. Not a "look, I used to be able to do
X, now I can't".
* From `2018-08-03
<https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
YOU ARE MISSING THE #1 KERNEL RULE.
We do not regress, and we do not regress exactly because your are 100% wrong.
And the reason you state for your opinion is in fact exactly *WHY* you
are wrong.
Your "good reasons" are pure and utter garbage.
The whole point of "we do not regress" is so that people can upgrade
the kernel and never have to worry about it.
> Kernel had a bug which has been fixed
That is *ENTIRELY* immaterial.
Guys, whether something was buggy or not DOES NOT MATTER.
Why?
Bugs happen. That's a fact of life. Arguing that "we had to break
something because we were fixing a bug" is completely insane. We fix
tens of bugs every single day, thinking that "fixing a bug" means that
we can break something is simply NOT TRUE.
So bugs simply aren't even relevant to the discussion. They happen,
they get found, they get fixed, and it has nothing to do with "we
break users".
Because the only thing that matters IS THE USER.
How hard is that to understand?
Anybody who uses "but it was buggy" as an argument is entirely missing
the point. As far as the USER was concerned, it wasn't buggy - it
worked for him/her.
Maybe it worked *because* the user had taken the bug into account,
maybe it worked because the user didn't notice - again, it doesn't
matter. It worked for the user.
Breaking a user workflow for a "bug" is absolutely the WORST reason
for breakage you can imagine.
It's basically saying "I took something that worked, and I broke it,
but now it's better". Do you not see how f*cking insane that statement
is?
And without users, your program is not a program, it's a pointless
piece of code that you might as well throw away.
Seriously. This is *why* the #1 rule for kernel development is "we
don't break users". Because "I fixed a bug" is absolutely NOT AN
ARGUMENT if that bug fix broke a user setup. You actually introduced a
MUCH BIGGER bug by "fixing" something that the user clearly didn't
even care about.
And dammit, we upgrade the kernel ALL THE TIME without upgrading any
other programs at all. It is absolutely required, because flag-days
and dependencies are horribly bad.
And it is also required simply because I as a kernel developer do not
upgrade random other tools that I don't even care about as I develop
the kernel, and I want any of my users to feel safe doing the same
time.
So no. Your rule is COMPLETELY wrong. If you cannot upgrade a kernel
without upgrading some other random binary, then we have a problem.
* From `2021-06-05
<https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/>`_::
THERE ARE NO VALID ARGUMENTS FOR REGRESSIONS.
Honestly, security people need to understand that "not working" is not
a success case of security. It's a failure case.
Yes, "not working" may be secure. But security in that case is *pointless*.
* From `2011-05-06 (1/3)
<https://lore.kernel.org/all/BANLkTim9YvResB+PwRp7QTK-a5VNg2PvmQ@mail.gmail.com/>`_::
Binary compatibility is more important.
And if binaries don't use the interface to parse the format (or just
parse it wrongly - see the fairly recent example of adding uuid's to
/proc/self/mountinfo), then it's a regression.
And regressions get reverted, unless there are security issues or
similar that makes us go "Oh Gods, we really have to break things".
I don't understand why this simple logic is so hard for some kernel
developers to understand. Reality matters. Your personal wishes matter
NOT AT ALL.
If you made an interface that can be used without parsing the
interface description, then we're stuck with the interface. Theory
simply doesn't matter.
You could help fix the tools, and try to avoid the compatibility
issues that way. There aren't that many of them.
From `2011-05-06 (2/3)
<https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/>`_::
it's clearly NOT an internal tracepoint. By definition. It's being
used by powertop.
From `2011-05-06 (3/3)
<https://lore.kernel.org/all/BANLkTinazaXRdGovYL7rRVp+j6HbJ7pzhg@mail.gmail.com/>`_::
We have programs that use that ABI and thus it's a regression if they break.
* From `2012-07-06 <https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/>`_::
> Now this got me wondering if Debian _unstable_ actually qualifies as a
> standard distro userspace.
Oh, if the kernel breaks some standard user space, that counts. Tons
of people run Debian unstable
* From `2019-09-15
<https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/>`_::
One _particularly_ last-minute revert is the top-most commit (ignoring
the version change itself) done just before the release, and while
it's very annoying, it's perhaps also instructive.
What's instructive about it is that I reverted a commit that wasn't
actually buggy. In fact, it was doing exactly what it set out to do,
and did it very well. In fact it did it _so_ well that the much
improved IO patterns it caused then ended up revealing a user-visible
regression due to a real bug in a completely unrelated area.
The actual details of that regression are not the reason I point that
revert out as instructive, though. It's more that it's an instructive
example of what counts as a regression, and what the whole "no
regressions" kernel rule means. The reverted commit didn't change any
API's, and it didn't introduce any new bugs. But it ended up exposing
another problem, and as such caused a kernel upgrade to fail for a
user. So it got reverted.
The point here being that we revert based on user-reported _behavior_,
not based on some "it changes the ABI" or "it caused a bug" concept.
The problem was really pre-existing, and it just didn't happen to
trigger before. The better IO patterns introduced by the change just
happened to expose an old bug, and people had grown to depend on the
previously benign behavior of that old issue.
And never fear, we'll re-introduce the fix that improved on the IO
patterns once we've decided just how to handle the fact that we had a
bad interaction with an interface that people had then just happened
to rely on incidental behavior for before. It's just that we'll have
to hash through how to do that (there are no less than three different
patches by three different developers being discussed, and there might
be more coming...). In the meantime, I reverted the thing that exposed
the problem to users for this release, even if I hope it will be
re-introduced (perhaps even backported as a stable patch) once we have
consensus about the issue it exposed.
Take-away from the whole thing: it's not about whether you change the
kernel-userspace ABI, or fix a bug, or about whether the old code
"should never have worked in the first place". It's about whether
something breaks existing users' workflow.
Anyway, that was my little aside on the whole regression thing. Since
it's that "first rule of kernel programming", I felt it is perhaps
worth just bringing it up every once in a while
..
end-of-content
..
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please use "The Linux kernel developers" for author attribution and link
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..
Note: Only the content of this RST file as found in the Linux kernel sources
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1
Documentation/process/index.rst
View File

@ -25,6 +25,7 @@ Below are the essential guides that every developer should read.
code-of-conduct-interpretation
development-process
submitting-patches
handling-regressions
programming-language
coding-style
maintainer-handbooks

2
MAINTAINERS
View File

@ -10438,6 +10438,8 @@ KERNEL REGRESSIONS
M: Thorsten Leemhuis <linux@leemhuis.info>
L: regressions@lists.linux.dev
S: Supported
F: Documentation/admin-guide/reporting-regressions.rst
F: Documentation/process/handling-regressions.rst
KERNEL SELFTEST FRAMEWORK
M: Shuah Khan <shuah@kernel.org>