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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
Randy Dunlapef0eba42010-05-23 17:02:30 -0700154 inclusion, it should be accepted by a relevant subsystem maintainer -
Jonathan Corbet75b02142008-09-30 15:15:56 -0600155 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
Randy Dunlapef0eba42010-05-23 17:02:30 -0700162- Please note that most maintainers also have day jobs, so merging
163 your patch may not be their highest priority. If your patch is
164 getting feedback about changes that are needed, you should either
165 make those changes or justify why they should not be made. If your
166 patch has no review complaints but is not being merged by its
167 appropriate subsystem or driver maintainer, you should be persistent
168 in updating the patch to the current kernel so that it applies cleanly
169 and keep sending it for review and merging.
170
Jonathan Corbet75b02142008-09-30 15:15:56 -0600171 - Merging into the mainline. Eventually, a successful patch will be
172 merged into the mainline repository managed by Linus Torvalds. More
173 comments and/or problems may surface at this time; it is important that
174 the developer be responsive to these and fix any issues which arise.
175
176 - Stable release. The number of users potentially affected by the patch
177 is now large, so, once again, new problems may arise.
178
179 - Long-term maintenance. While it is certainly possible for a developer
180 to forget about code after merging it, that sort of behavior tends to
181 leave a poor impression in the development community. Merging code
182 eliminates some of the maintenance burden, in that others will fix
183 problems caused by API changes. But the original developer should
184 continue to take responsibility for the code if it is to remain useful
185 in the longer term.
186
187One of the largest mistakes made by kernel developers (or their employers)
188is to try to cut the process down to a single "merging into the mainline"
189step. This approach invariably leads to frustration for everybody
190involved.
191
192
1932.3: HOW PATCHES GET INTO THE KERNEL
194
195There is exactly one person who can merge patches into the mainline kernel
196repository: Linus Torvalds. But, of the over 12,000 patches which went
197into the 2.6.25 kernel, only 250 (around 2%) were directly chosen by Linus
198himself. The kernel project has long since grown to a size where no single
199developer could possibly inspect and select every patch unassisted. The
200way the kernel developers have addressed this growth is through the use of
201a lieutenant system built around a chain of trust.
202
203The kernel code base is logically broken down into a set of subsystems:
204networking, specific architecture support, memory management, video
205devices, etc. Most subsystems have a designated maintainer, a developer
206who has overall responsibility for the code within that subsystem. These
207subsystem maintainers are the gatekeepers (in a loose way) for the portion
208of the kernel they manage; they are the ones who will (usually) accept a
209patch for inclusion into the mainline kernel.
210
211Subsystem maintainers each manage their own version of the kernel source
212tree, usually (but certainly not always) using the git source management
213tool. Tools like git (and related tools like quilt or mercurial) allow
214maintainers to track a list of patches, including authorship information
215and other metadata. At any given time, the maintainer can identify which
216patches in his or her repository are not found in the mainline.
217
218When the merge window opens, top-level maintainers will ask Linus to "pull"
219the patches they have selected for merging from their repositories. If
220Linus agrees, the stream of patches will flow up into his repository,
221becoming part of the mainline kernel. The amount of attention that Linus
222pays to specific patches received in a pull operation varies. It is clear
223that, sometimes, he looks quite closely. But, as a general rule, Linus
224trusts the subsystem maintainers to not send bad patches upstream.
225
226Subsystem maintainers, in turn, can pull patches from other maintainers.
227For example, the networking tree is built from patches which accumulated
228first in trees dedicated to network device drivers, wireless networking,
229etc. This chain of repositories can be arbitrarily long, though it rarely
230exceeds two or three links. Since each maintainer in the chain trusts
231those managing lower-level trees, this process is known as the "chain of
232trust."
233
234Clearly, in a system like this, getting patches into the kernel depends on
235finding the right maintainer. Sending patches directly to Linus is not
236normally the right way to go.
237
238
2392.4: STAGING TREES
240
241The chain of subsystem trees guides the flow of patches into the kernel,
242but it also raises an interesting question: what if somebody wants to look
243at all of the patches which are being prepared for the next merge window?
244Developers will be interested in what other changes are pending to see
245whether there are any conflicts to worry about; a patch which changes a
246core kernel function prototype, for example, will conflict with any other
247patches which use the older form of that function. Reviewers and testers
248want access to the changes in their integrated form before all of those
249changes land in the mainline kernel. One could pull changes from all of
250the interesting subsystem trees, but that would be a big and error-prone
251job.
252
253The answer comes in the form of staging trees, where subsystem trees are
254collected for testing and review. The older of these trees, maintained by
255Andrew Morton, is called "-mm" (for memory management, which is how it got
256started). The -mm tree integrates patches from a long list of subsystem
257trees; it also has some patches aimed at helping with debugging.
258
259Beyond that, -mm contains a significant collection of patches which have
260been selected by Andrew directly. These patches may have been posted on a
261mailing list, or they may apply to a part of the kernel for which there is
262no designated subsystem tree. As a result, -mm operates as a sort of
263subsystem tree of last resort; if there is no other obvious path for a
264patch into the mainline, it is likely to end up in -mm. Miscellaneous
265patches which accumulate in -mm will eventually either be forwarded on to
266an appropriate subsystem tree or be sent directly to Linus. In a typical
267development cycle, approximately 10% of the patches going into the mainline
268get there via -mm.
269
Randy Dunlape6a591e2010-05-23 17:02:30 -0700270The current -mm patch is available in the "mmotm" (-mm of the moment)
271directory at:
Jonathan Corbet75b02142008-09-30 15:15:56 -0600272
273 http://userweb.kernel.org/~akpm/mmotm/
274
275Use of the MMOTM tree is likely to be a frustrating experience, though;
276there is a definite chance that it will not even compile.
277
278The other staging tree, started more recently, is linux-next, maintained by
279Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
280the mainline is expected to look like after the next merge window closes.
281Linux-next trees are announced on the linux-kernel and linux-next mailing
282lists when they are assembled; they can be downloaded from:
283
284 http://www.kernel.org/pub/linux/kernel/people/sfr/linux-next/
285
286Some information about linux-next has been gathered at:
287
288 http://linux.f-seidel.de/linux-next/pmwiki/
289
290How the linux-next tree will fit into the development process is still
291changing. As of this writing, the first full development cycle involving
292linux-next (2.6.26) is coming to an end; thus far, it has proved to be a
293valuable resource for finding and fixing integration problems before the
294beginning of the merge window. See http://lwn.net/Articles/287155/ for
295more information on how linux-next has worked to set up the 2.6.27 merge
296window.
297
298Some developers have begun to suggest that linux-next should be used as the
299target for future development as well. The linux-next tree does tend to be
300far ahead of the mainline and is more representative of the tree into which
301any new work will be merged. The downside to this idea is that the
302volatility of linux-next tends to make it a difficult development target.
303See http://lwn.net/Articles/289013/ for more information on this topic, and
304stay tuned; much is still in flux where linux-next is involved.
305
Randy Dunlape6a591e2010-05-23 17:02:30 -0700306Besides the mmotm and linux-next trees, the kernel source tree now contains
307the drivers/staging/ directory and many sub-directories for drivers or
308filesystems that are on their way to being added to the kernel tree
309proper, but they remain in drivers/staging/ while they still need more
310work.
311
Jonathan Corbet75b02142008-09-30 15:15:56 -0600312
3132.5: TOOLS
314
315As can be seen from the above text, the kernel development process depends
316heavily on the ability to herd collections of patches in various
317directions. The whole thing would not work anywhere near as well as it
318does without suitably powerful tools. Tutorials on how to use these tools
319are well beyond the scope of this document, but there is space for a few
320pointers.
321
322By far the dominant source code management system used by the kernel
323community is git. Git is one of a number of distributed version control
324systems being developed in the free software community. It is well tuned
325for kernel development, in that it performs quite well when dealing with
326large repositories and large numbers of patches. It also has a reputation
327for being difficult to learn and use, though it has gotten better over
328time. Some sort of familiarity with git is almost a requirement for kernel
329developers; even if they do not use it for their own work, they'll need git
330to keep up with what other developers (and the mainline) are doing.
331
332Git is now packaged by almost all Linux distributions. There is a home
Randy Dunlapef0eba42010-05-23 17:02:30 -0700333page at:
Jonathan Corbet75b02142008-09-30 15:15:56 -0600334
Randy Dunlapef0eba42010-05-23 17:02:30 -0700335 http://git-scm.com/
Jonathan Corbet75b02142008-09-30 15:15:56 -0600336
337That page has pointers to documentation and tutorials. One should be
338aware, in particular, of the Kernel Hacker's Guide to git, which has
339information specific to kernel development:
340
341 http://linux.yyz.us/git-howto.html
342
343Among the kernel developers who do not use git, the most popular choice is
344almost certainly Mercurial:
345
346 http://www.selenic.com/mercurial/
347
348Mercurial shares many features with git, but it provides an interface which
349many find easier to use.
350
351The other tool worth knowing about is Quilt:
352
353 http://savannah.nongnu.org/projects/quilt/
354
355Quilt is a patch management system, rather than a source code management
356system. It does not track history over time; it is, instead, oriented
357toward tracking a specific set of changes against an evolving code base.
358Some major subsystem maintainers use quilt to manage patches intended to go
359upstream. For the management of certain kinds of trees (-mm, for example),
360quilt is the best tool for the job.
361
362
3632.6: MAILING LISTS
364
365A great deal of Linux kernel development work is done by way of mailing
366lists. It is hard to be a fully-functioning member of the community
367without joining at least one list somewhere. But Linux mailing lists also
368represent a potential hazard to developers, who risk getting buried under a
369load of electronic mail, running afoul of the conventions used on the Linux
370lists, or both.
371
372Most kernel mailing lists are run on vger.kernel.org; the master list can
373be found at:
374
375 http://vger.kernel.org/vger-lists.html
376
377There are lists hosted elsewhere, though; a number of them are at
378lists.redhat.com.
379
380The core mailing list for kernel development is, of course, linux-kernel.
381This list is an intimidating place to be; volume can reach 500 messages per
382day, the amount of noise is high, the conversation can be severely
383technical, and participants are not always concerned with showing a high
384degree of politeness. But there is no other place where the kernel
385development community comes together as a whole; developers who avoid this
386list will miss important information.
387
388There are a few hints which can help with linux-kernel survival:
389
390- Have the list delivered to a separate folder, rather than your main
391 mailbox. One must be able to ignore the stream for sustained periods of
392 time.
393
394- Do not try to follow every conversation - nobody else does. It is
395 important to filter on both the topic of interest (though note that
396 long-running conversations can drift away from the original subject
397 without changing the email subject line) and the people who are
398 participating.
399
400- Do not feed the trolls. If somebody is trying to stir up an angry
401 response, ignore them.
402
403- When responding to linux-kernel email (or that on other lists) preserve
404 the Cc: header for all involved. In the absence of a strong reason (such
405 as an explicit request), you should never remove recipients. Always make
406 sure that the person you are responding to is in the Cc: list. This
407 convention also makes it unnecessary to explicitly ask to be copied on
408 replies to your postings.
409
410- Search the list archives (and the net as a whole) before asking
411 questions. Some developers can get impatient with people who clearly
412 have not done their homework.
413
414- Avoid top-posting (the practice of putting your answer above the quoted
415 text you are responding to). It makes your response harder to read and
416 makes a poor impression.
417
418- Ask on the correct mailing list. Linux-kernel may be the general meeting
419 point, but it is not the best place to find developers from all
420 subsystems.
421
422The last point - finding the correct mailing list - is a common place for
423beginning developers to go wrong. Somebody who asks a networking-related
424question on linux-kernel will almost certainly receive a polite suggestion
425to ask on the netdev list instead, as that is the list frequented by most
426networking developers. Other lists exist for the SCSI, video4linux, IDE,
427filesystem, etc. subsystems. The best place to look for mailing lists is
428in the MAINTAINERS file packaged with the kernel source.
429
430
4312.7: GETTING STARTED WITH KERNEL DEVELOPMENT
432
433Questions about how to get started with the kernel development process are
434common - from both individuals and companies. Equally common are missteps
435which make the beginning of the relationship harder than it has to be.
436
437Companies often look to hire well-known developers to get a development
438group started. This can, in fact, be an effective technique. But it also
439tends to be expensive and does not do much to grow the pool of experienced
440kernel developers. It is possible to bring in-house developers up to speed
441on Linux kernel development, given the investment of a bit of time. Taking
442this time can endow an employer with a group of developers who understand
443the kernel and the company both, and who can help to train others as well.
444Over the medium term, this is often the more profitable approach.
445
446Individual developers are often, understandably, at a loss for a place to
447start. Beginning with a large project can be intimidating; one often wants
448to test the waters with something smaller first. This is the point where
449some developers jump into the creation of patches fixing spelling errors or
450minor coding style issues. Unfortunately, such patches create a level of
451noise which is distracting for the development community as a whole, so,
452increasingly, they are looked down upon. New developers wishing to
453introduce themselves to the community will not get the sort of reception
454they wish for by these means.
455
456Andrew Morton gives this advice for aspiring kernel developers
457
458 The #1 project for all kernel beginners should surely be "make sure
459 that the kernel runs perfectly at all times on all machines which
460 you can lay your hands on". Usually the way to do this is to work
461 with others on getting things fixed up (this can require
462 persistence!) but that's fine - it's a part of kernel development.
463
464(http://lwn.net/Articles/283982/).
465
466In the absence of obvious problems to fix, developers are advised to look
467at the current lists of regressions and open bugs in general. There is
468never any shortage of issues in need of fixing; by addressing these issues,
469developers will gain experience with the process while, at the same time,
470building respect with the rest of the development community.