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Jonathan Corbet75b02142008-09-30 15:15:56 -060012: HOW THE DEVELOPMENT PROCESS WORKS
2
3Linux kernel development in the early 1990's was a pretty loose affair,
4with relatively small numbers of users and developers involved. With a
5user base in the millions and with some 2,000 developers involved over the
6course of one year, the kernel has since had to evolve a number of
7processes to keep development happening smoothly. A solid understanding of
8how the process works is required in order to be an effective part of it.
9
10
112.1: THE BIG PICTURE
12
13The kernel developers use a loosely time-based release process, with a new
14major kernel release happening every two or three months. The recent
15release history looks like this:
16
17 2.6.26 July 13, 2008
18 2.6.25 April 16, 2008
19 2.6.24 January 24, 2008
20 2.6.23 October 9, 2007
21 2.6.22 July 8, 2007
22 2.6.21 April 25, 2007
23 2.6.20 February 4, 2007
24
25Every 2.6.x release is a major kernel release with new features, internal
26API changes, and more. A typical 2.6 release can contain over 10,000
27changesets with changes to several hundred thousand lines of code. 2.6 is
28thus the leading edge of Linux kernel development; the kernel uses a
29rolling development model which is continually integrating major changes.
30
31A relatively straightforward discipline is followed with regard to the
32merging of patches for each release. At the beginning of each development
33cycle, the "merge window" is said to be open. At that time, code which is
34deemed to be sufficiently stable (and which is accepted by the development
35community) is merged into the mainline kernel. The bulk of changes for a
36new development cycle (and all of the major changes) will be merged during
37this time, at a rate approaching 1,000 changes ("patches," or "changesets")
38per day.
39
40(As an aside, it is worth noting that the changes integrated during the
41merge window do not come out of thin air; they have been collected, tested,
42and staged ahead of time. How that process works will be described in
43detail later on).
44
45The merge window lasts for two weeks. At the end of this time, Linus
46Torvalds will declare that the window is closed and release the first of
47the "rc" kernels. For the kernel which is destined to be 2.6.26, for
48example, the release which happens at the end of the merge window will be
49called 2.6.26-rc1. The -rc1 release is the signal that the time to merge
50new features has passed, and that the time to stabilize the next kernel has
51begun.
52
53Over the next six to ten weeks, only patches which fix problems should be
54submitted to the mainline. On occasion a more significant change will be
55allowed, but such occasions are rare; developers who try to merge new
56features outside of the merge window tend to get an unfriendly reception.
57As a general rule, if you miss the merge window for a given feature, the
58best thing to do is to wait for the next development cycle. (An occasional
59exception is made for drivers for previously-unsupported hardware; if they
60touch no in-tree code, they cannot cause regressions and should be safe to
61add at any time).
62
63As fixes make their way into the mainline, the patch rate will slow over
64time. Linus releases new -rc kernels about once a week; a normal series
65will get up to somewhere between -rc6 and -rc9 before the kernel is
66considered to be sufficiently stable and the final 2.6.x release is made.
67At that point the whole process starts over again.
68
69As an example, here is how the 2.6.25 development cycle went (all dates in
702008):
71
72 January 24 2.6.24 stable release
73 February 10 2.6.25-rc1, merge window closes
74 February 15 2.6.25-rc2
75 February 24 2.6.25-rc3
76 March 4 2.6.25-rc4
77 March 9 2.6.25-rc5
78 March 16 2.6.25-rc6
79 March 25 2.6.25-rc7
80 April 1 2.6.25-rc8
81 April 11 2.6.25-rc9
82 April 16 2.6.25 stable release
83
84How do the developers decide when to close the development cycle and create
85the stable release? The most significant metric used is the list of
86regressions from previous releases. No bugs are welcome, but those which
87break systems which worked in the past are considered to be especially
88serious. For this reason, patches which cause regressions are looked upon
89unfavorably and are quite likely to be reverted during the stabilization
90period.
91
92The developers' goal is to fix all known regressions before the stable
93release is made. In the real world, this kind of perfection is hard to
94achieve; there are just too many variables in a project of this size.
95There comes a point where delaying the final release just makes the problem
96worse; the pile of changes waiting for the next merge window will grow
97larger, creating even more regressions the next time around. So most 2.6.x
98kernels go out with a handful of known regressions though, hopefully, none
99of them are serious.
100
101Once a stable release is made, its ongoing maintenance is passed off to the
102"stable team," currently comprised of Greg Kroah-Hartman and Chris Wright.
103The stable team will release occasional updates to the stable release using
104the 2.6.x.y numbering scheme. To be considered for an update release, a
105patch must (1) fix a significant bug, and (2) already be merged into the
106mainline for the next development kernel. Continuing our 2.6.25 example,
107the history (as of this writing) is:
108
109 May 1 2.6.25.1
110 May 6 2.6.25.2
111 May 9 2.6.25.3
112 May 15 2.6.25.4
113 June 7 2.6.25.5
114 June 9 2.6.25.6
115 June 16 2.6.25.7
116 June 21 2.6.25.8
117 June 24 2.6.25.9
118
119Stable updates for a given kernel are made for approximately six months;
120after that, the maintenance of stable releases is solely the responsibility
121of the distributors which have shipped that particular kernel.
122
123
1242.2: THE LIFECYCLE OF A PATCH
125
126Patches do not go directly from the developer's keyboard into the mainline
127kernel. There is, instead, a somewhat involved (if somewhat informal)
128process designed to ensure that each patch is reviewed for quality and that
129each patch implements a change which is desirable to have in the mainline.
130This process can happen quickly for minor fixes, or, in the case of large
131and controversial changes, go on for years. Much developer frustration
132comes from a lack of understanding of this process or from attempts to
133circumvent it.
134
135In the hopes of reducing that frustration, this document will describe how
136a patch gets into the kernel. What follows below is an introduction which
137describes the process in a somewhat idealized way. A much more detailed
138treatment will come in later sections.
139
140The stages that a patch goes through are, generally:
141
142 - Design. This is where the real requirements for the patch - and the way
143 those requirements will be met - are laid out. Design work is often
144 done without involving the community, but it is better to do this work
145 in the open if at all possible; it can save a lot of time redesigning
146 things later.
147
148 - Early review. Patches are posted to the relevant mailing list, and
149 developers on that list reply with any comments they may have. This
150 process should turn up any major problems with a patch if all goes
151 well.
152
153 - Wider review. When the patch is getting close to ready for mainline
154 inclusion, it will be accepted by a relevant subsystem maintainer -
155 though this acceptance is not a guarantee that the patch will make it
156 all the way to the mainline. The patch will show up in the maintainer's
157 subsystem tree and into the staging trees (described below). When the
158 process works, this step leads to more extensive review of the patch and
159 the discovery of any problems resulting from the integration of this
160 patch with work being done by others.
161
162 - Merging into the mainline. Eventually, a successful patch will be
163 merged into the mainline repository managed by Linus Torvalds. More
164 comments and/or problems may surface at this time; it is important that
165 the developer be responsive to these and fix any issues which arise.
166
167 - Stable release. The number of users potentially affected by the patch
168 is now large, so, once again, new problems may arise.
169
170 - Long-term maintenance. While it is certainly possible for a developer
171 to forget about code after merging it, that sort of behavior tends to
172 leave a poor impression in the development community. Merging code
173 eliminates some of the maintenance burden, in that others will fix
174 problems caused by API changes. But the original developer should
175 continue to take responsibility for the code if it is to remain useful
176 in the longer term.
177
178One of the largest mistakes made by kernel developers (or their employers)
179is to try to cut the process down to a single "merging into the mainline"
180step. This approach invariably leads to frustration for everybody
181involved.
182
183
1842.3: HOW PATCHES GET INTO THE KERNEL
185
186There is exactly one person who can merge patches into the mainline kernel
187repository: Linus Torvalds. But, of the over 12,000 patches which went
188into the 2.6.25 kernel, only 250 (around 2%) were directly chosen by Linus
189himself. The kernel project has long since grown to a size where no single
190developer could possibly inspect and select every patch unassisted. The
191way the kernel developers have addressed this growth is through the use of
192a lieutenant system built around a chain of trust.
193
194The kernel code base is logically broken down into a set of subsystems:
195networking, specific architecture support, memory management, video
196devices, etc. Most subsystems have a designated maintainer, a developer
197who has overall responsibility for the code within that subsystem. These
198subsystem maintainers are the gatekeepers (in a loose way) for the portion
199of the kernel they manage; they are the ones who will (usually) accept a
200patch for inclusion into the mainline kernel.
201
202Subsystem maintainers each manage their own version of the kernel source
203tree, usually (but certainly not always) using the git source management
204tool. Tools like git (and related tools like quilt or mercurial) allow
205maintainers to track a list of patches, including authorship information
206and other metadata. At any given time, the maintainer can identify which
207patches in his or her repository are not found in the mainline.
208
209When the merge window opens, top-level maintainers will ask Linus to "pull"
210the patches they have selected for merging from their repositories. If
211Linus agrees, the stream of patches will flow up into his repository,
212becoming part of the mainline kernel. The amount of attention that Linus
213pays to specific patches received in a pull operation varies. It is clear
214that, sometimes, he looks quite closely. But, as a general rule, Linus
215trusts the subsystem maintainers to not send bad patches upstream.
216
217Subsystem maintainers, in turn, can pull patches from other maintainers.
218For example, the networking tree is built from patches which accumulated
219first in trees dedicated to network device drivers, wireless networking,
220etc. This chain of repositories can be arbitrarily long, though it rarely
221exceeds two or three links. Since each maintainer in the chain trusts
222those managing lower-level trees, this process is known as the "chain of
223trust."
224
225Clearly, in a system like this, getting patches into the kernel depends on
226finding the right maintainer. Sending patches directly to Linus is not
227normally the right way to go.
228
229
2302.4: STAGING TREES
231
232The chain of subsystem trees guides the flow of patches into the kernel,
233but it also raises an interesting question: what if somebody wants to look
234at all of the patches which are being prepared for the next merge window?
235Developers will be interested in what other changes are pending to see
236whether there are any conflicts to worry about; a patch which changes a
237core kernel function prototype, for example, will conflict with any other
238patches which use the older form of that function. Reviewers and testers
239want access to the changes in their integrated form before all of those
240changes land in the mainline kernel. One could pull changes from all of
241the interesting subsystem trees, but that would be a big and error-prone
242job.
243
244The answer comes in the form of staging trees, where subsystem trees are
245collected for testing and review. The older of these trees, maintained by
246Andrew Morton, is called "-mm" (for memory management, which is how it got
247started). The -mm tree integrates patches from a long list of subsystem
248trees; it also has some patches aimed at helping with debugging.
249
250Beyond that, -mm contains a significant collection of patches which have
251been selected by Andrew directly. These patches may have been posted on a
252mailing list, or they may apply to a part of the kernel for which there is
253no designated subsystem tree. As a result, -mm operates as a sort of
254subsystem tree of last resort; if there is no other obvious path for a
255patch into the mainline, it is likely to end up in -mm. Miscellaneous
256patches which accumulate in -mm will eventually either be forwarded on to
257an appropriate subsystem tree or be sent directly to Linus. In a typical
258development cycle, approximately 10% of the patches going into the mainline
259get there via -mm.
260
261The current -mm patch can always be found from the front page of
262
263 http://kernel.org/
264
265Those who want to see the current state of -mm can get the "-mm of the
266moment" tree, found at:
267
268 http://userweb.kernel.org/~akpm/mmotm/
269
270Use of the MMOTM tree is likely to be a frustrating experience, though;
271there is a definite chance that it will not even compile.
272
273The other staging tree, started more recently, is linux-next, maintained by
274Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
275the mainline is expected to look like after the next merge window closes.
276Linux-next trees are announced on the linux-kernel and linux-next mailing
277lists when they are assembled; they can be downloaded from:
278
279 http://www.kernel.org/pub/linux/kernel/people/sfr/linux-next/
280
281Some information about linux-next has been gathered at:
282
283 http://linux.f-seidel.de/linux-next/pmwiki/
284
285How the linux-next tree will fit into the development process is still
286changing. As of this writing, the first full development cycle involving
287linux-next (2.6.26) is coming to an end; thus far, it has proved to be a
288valuable resource for finding and fixing integration problems before the
289beginning of the merge window. See http://lwn.net/Articles/287155/ for
290more information on how linux-next has worked to set up the 2.6.27 merge
291window.
292
293Some developers have begun to suggest that linux-next should be used as the
294target for future development as well. The linux-next tree does tend to be
295far ahead of the mainline and is more representative of the tree into which
296any new work will be merged. The downside to this idea is that the
297volatility of linux-next tends to make it a difficult development target.
298See http://lwn.net/Articles/289013/ for more information on this topic, and
299stay tuned; much is still in flux where linux-next is involved.
300
301
3022.5: TOOLS
303
304As can be seen from the above text, the kernel development process depends
305heavily on the ability to herd collections of patches in various
306directions. The whole thing would not work anywhere near as well as it
307does without suitably powerful tools. Tutorials on how to use these tools
308are well beyond the scope of this document, but there is space for a few
309pointers.
310
311By far the dominant source code management system used by the kernel
312community is git. Git is one of a number of distributed version control
313systems being developed in the free software community. It is well tuned
314for kernel development, in that it performs quite well when dealing with
315large repositories and large numbers of patches. It also has a reputation
316for being difficult to learn and use, though it has gotten better over
317time. Some sort of familiarity with git is almost a requirement for kernel
318developers; even if they do not use it for their own work, they'll need git
319to keep up with what other developers (and the mainline) are doing.
320
321Git is now packaged by almost all Linux distributions. There is a home
322page at
323
324 http://git.or.cz/
325
326That page has pointers to documentation and tutorials. One should be
327aware, in particular, of the Kernel Hacker's Guide to git, which has
328information specific to kernel development:
329
330 http://linux.yyz.us/git-howto.html
331
332Among the kernel developers who do not use git, the most popular choice is
333almost certainly Mercurial:
334
335 http://www.selenic.com/mercurial/
336
337Mercurial shares many features with git, but it provides an interface which
338many find easier to use.
339
340The other tool worth knowing about is Quilt:
341
342 http://savannah.nongnu.org/projects/quilt/
343
344Quilt is a patch management system, rather than a source code management
345system. It does not track history over time; it is, instead, oriented
346toward tracking a specific set of changes against an evolving code base.
347Some major subsystem maintainers use quilt to manage patches intended to go
348upstream. For the management of certain kinds of trees (-mm, for example),
349quilt is the best tool for the job.
350
351
3522.6: MAILING LISTS
353
354A great deal of Linux kernel development work is done by way of mailing
355lists. It is hard to be a fully-functioning member of the community
356without joining at least one list somewhere. But Linux mailing lists also
357represent a potential hazard to developers, who risk getting buried under a
358load of electronic mail, running afoul of the conventions used on the Linux
359lists, or both.
360
361Most kernel mailing lists are run on vger.kernel.org; the master list can
362be found at:
363
364 http://vger.kernel.org/vger-lists.html
365
366There are lists hosted elsewhere, though; a number of them are at
367lists.redhat.com.
368
369The core mailing list for kernel development is, of course, linux-kernel.
370This list is an intimidating place to be; volume can reach 500 messages per
371day, the amount of noise is high, the conversation can be severely
372technical, and participants are not always concerned with showing a high
373degree of politeness. But there is no other place where the kernel
374development community comes together as a whole; developers who avoid this
375list will miss important information.
376
377There are a few hints which can help with linux-kernel survival:
378
379- Have the list delivered to a separate folder, rather than your main
380 mailbox. One must be able to ignore the stream for sustained periods of
381 time.
382
383- Do not try to follow every conversation - nobody else does. It is
384 important to filter on both the topic of interest (though note that
385 long-running conversations can drift away from the original subject
386 without changing the email subject line) and the people who are
387 participating.
388
389- Do not feed the trolls. If somebody is trying to stir up an angry
390 response, ignore them.
391
392- When responding to linux-kernel email (or that on other lists) preserve
393 the Cc: header for all involved. In the absence of a strong reason (such
394 as an explicit request), you should never remove recipients. Always make
395 sure that the person you are responding to is in the Cc: list. This
396 convention also makes it unnecessary to explicitly ask to be copied on
397 replies to your postings.
398
399- Search the list archives (and the net as a whole) before asking
400 questions. Some developers can get impatient with people who clearly
401 have not done their homework.
402
403- Avoid top-posting (the practice of putting your answer above the quoted
404 text you are responding to). It makes your response harder to read and
405 makes a poor impression.
406
407- Ask on the correct mailing list. Linux-kernel may be the general meeting
408 point, but it is not the best place to find developers from all
409 subsystems.
410
411The last point - finding the correct mailing list - is a common place for
412beginning developers to go wrong. Somebody who asks a networking-related
413question on linux-kernel will almost certainly receive a polite suggestion
414to ask on the netdev list instead, as that is the list frequented by most
415networking developers. Other lists exist for the SCSI, video4linux, IDE,
416filesystem, etc. subsystems. The best place to look for mailing lists is
417in the MAINTAINERS file packaged with the kernel source.
418
419
4202.7: GETTING STARTED WITH KERNEL DEVELOPMENT
421
422Questions about how to get started with the kernel development process are
423common - from both individuals and companies. Equally common are missteps
424which make the beginning of the relationship harder than it has to be.
425
426Companies often look to hire well-known developers to get a development
427group started. This can, in fact, be an effective technique. But it also
428tends to be expensive and does not do much to grow the pool of experienced
429kernel developers. It is possible to bring in-house developers up to speed
430on Linux kernel development, given the investment of a bit of time. Taking
431this time can endow an employer with a group of developers who understand
432the kernel and the company both, and who can help to train others as well.
433Over the medium term, this is often the more profitable approach.
434
435Individual developers are often, understandably, at a loss for a place to
436start. Beginning with a large project can be intimidating; one often wants
437to test the waters with something smaller first. This is the point where
438some developers jump into the creation of patches fixing spelling errors or
439minor coding style issues. Unfortunately, such patches create a level of
440noise which is distracting for the development community as a whole, so,
441increasingly, they are looked down upon. New developers wishing to
442introduce themselves to the community will not get the sort of reception
443they wish for by these means.
444
445Andrew Morton gives this advice for aspiring kernel developers
446
447 The #1 project for all kernel beginners should surely be "make sure
448 that the kernel runs perfectly at all times on all machines which
449 you can lay your hands on". Usually the way to do this is to work
450 with others on getting things fixed up (this can require
451 persistence!) but that's fine - it's a part of kernel development.
452
453(http://lwn.net/Articles/283982/).
454
455In the absence of obvious problems to fix, developers are advised to look
456at the current lists of regressions and open bugs in general. There is
457never any shortage of issues in need of fixing; by addressing these issues,
458developers will gain experience with the process while, at the same time,
459building respect with the rest of the development community.