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SubjectBK Kernel Hacking HOWTO
Comments welcome...

[Erik a couple of your suggestions are not yet integrated]
--
Jeff Garzik | XXX FREE! secure AFSPC AK-47 unclassified CDC
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Doing the BK Thing, Penguin-Style




This set of notes is intended mainly for kernel developers, occasional
or full-time, but sysadmins and power users may find parts of it useful
as well. It assumes at least a basic familiarity with CVS, both at a
user level (use on the cmd line) and at a higher level (client-server model).
Due to the author's background, an operation may be described in terms
of CVS, or in terms of how that operation differs from CVS.

This is -not- intended to be BitKeeper documentation. Always run
"bk help <command>" or in X "bk helptool <command>" for reference
documentation.


BitKeeper Concepts
------------------

In the true nature of the Internet itself, BitKeeper is a distributed
system. When applied to revision control, this means doing away with
client-server, and changing to a parent-child model... essentially
peer-to-peer. On the developer's end, this also represents a
fundamental disruption in the standard workflow of changes, commits,
and merges. You will need to take a few minutes to think about
how to best work under BitKeeper, and re-optimize things a bit.
In some sense it is a bit radical, because it might described as
tossing changes out into a maelstrom and having them them magically
land at the right destination... but I'm getting ahead of myself.

Let's start with this progression:
Each BitKeeper source tree on disk is a repository unto itself.
Each repository has a parent.
Each repository contains a set of a changsets ("csets").
Each cset is one or more changed files, bundled together.

Each tree is a repository, so all changes are checked into the local
tree. When a change is checked in, all modified files are grouped
into a logical unit, the changeset. Internally, BK links these
changesets in a tree, representing various converging and diverging
lines of development. These changesets are the bread and butter of
the BK system.

After the concept of changesets, the next thing you need to get used
to having multiple copies of source trees lying around. This -really-
takes some getting used to, for some people. Separate source trees
are the means in BitKeeper by which you delineate parallel lines
of development, both minor and major. What would be branches in
CVS become separate source trees, or "clones" in BitKeeper [heh,
or Star Wars] terminology.

Clones and changesets are the tools from which most of the power of
BitKeeper is derived. As mentioned earlier, each clone has a parent,
the tree used as the source when the new clone was created. In a
CVS-like setup, the parent would be a remote server on the Internet,
and the child is your local clone of that tree.

Once you have established a common baseline between two source trees --
a common parent -- then you can merge changesets between those two
trees with ease. Merging changes into a tree is called a "pull", and
is analagous to 'cvs update'. A pull downloads all the changesets in
the remote tree you do not have, and merges them. Sending changes in
one tree to another tree is called a "push". Push sends all changes
in the local tree the remote does not yet have, and merges them.

From these concepts come some initial command examples:

1) bk clone -q http://linux.bkbits.net/linux-2.5 linus-2.5
Download a 2.5 stock kernel tree, naming it "linus-2.5" in the local dir.
The "-q" disables listing every single file as it is downloaded.

2) bk clone -ql linus-2.5 alpha-2.5
Create a separate source tree for the Alpha AXP architecture.
The "-l" uses hard links instead of copying data, since both trees are
on the local disk. You can also replace the above with "bk lclone -q ..."

You only clone a tree -once-. After cloning the tree lives a long time
on disk, being updating by pushes and pulls.

3) cd alpha-2.5 ; bk pull http://gkernel.bkbits.net/alpha-2.5
Download changes in "alpha-2.5" repository which are not present
in the local repository, and merge them into the source tree.

4) bk -r co -q
Because every tree is a repository, files must be checked out before
they will be in their standard places in the source tree.

5) bk vi fs/inode.c # example change...
bk citool # checkin, using X tool
bk push bk://gkernel@bkbits.net/alpha-2.5 # upload change
Typical example of a BK sequence that would replace the analagous CVS
situation,
vi fs/inode.c
cvs commit

As this is just supposed to be a quick BK intro, for more in-depth
tutorials, live working demos, and docs, see http://www.bitkeeper.com/



BK and Kernel Development Workflow
----------------------------------
Currently the latest 2.5 tree is available via "bk clone $URL"
and "bk pull $URL" at http://linux.bkbits.net/linux-2.5
This should change in a few weeks to a kernel.org URL.


A big part of using BitKeeper is organizing the various trees you have
on your local disk, and organizing the flow of changes among those
trees, and remote trees. If one were to graph the relationships between
a desired BK setup, you are likely to see a few-many-few graph, like
this:

linux-2.5
|
merge-to-linus-2.5
/ | |
/ | |
vm-hacks bugfixes filesys personal-hacks
\ | | /
\ | | /
\ | | /
testing-and-validation

Since a "bk push" sends all changes not in the target tree, and
since a "bk pull" receives all changes not in the source tree, you want
to make sure you are only pushing specific changes to the desired tree,
not all changes from "peer parent" trees. For example, pushing a change
from the testing-and-validation tree would probably be a bad idea,
because it will push all changes from vm-hacks, bugfixes, filesys, and
personal-hacks trees into the target tree.

One would typically work on only one "theme" at a time, either
vm-hacks or bugfixes or filesys, keeping those changes isolated in
their own tree during development, and only merge the isolated with
other changes when going upstream (to Linus or other maintainers) or
downstream (to your "union" trees, like testing-and-validation above).

It should be noted that some of this separation is not just recommended
practice, it's actually [for now] -enforced- by BitKeeper. BitKeeper
requires that changesets maintain a certain order, which is the reason
that "bk push" sends all local changesets the remote doesn't have. This
separation may look like a lot of wasted disk space at first, but it
helps when two unrelated changes may "pollute" the same area of code, or
don't follow the same pace of development, or any other of the standard
reasons why one creates a development branch.

Small development branches (clones) will appear and disappear:

-------- A --------- B --------- C --------- D -------
\ /
-----short-term devel branch-----

While long-term branches will parallel a tree (or trees), with period
merge points. In this first example, we pull from a tree (pulls,
"\") periodically, such a what occurs when tracking changes in a
vendor tree, never pushing changes back up the line:

-------- A --------- B --------- C --------- D -------
\ \ \
----long-term devel branch-----------------

And then a more common case in Linux kernel development, a long term
branch with periodic merges back into the tree (pushes, "/"):

-------- A --------- B --------- C --------- D -------
\ \ / \
----long-term devel branch-----------------





Submitting Changes to Linus
---------------------------
There's a bit of an art, or style, of submitting changes to Linus.
Since Linus's tree is now (you might say) fully integrated into the
distributed BitKeeper system, there are several prerequisites to
properly submitting a BitKeeper change. All these prereq's are just
general cleanliness of BK usage, so as people become experts at BK, feel
free to optimize this process further (assuming Linus agrees, of
course).



0) Make sure your tree was originally cloned from the linux-2.5 tree
created by Linus. If your tree does not have this as its ancestor, it
is impossible to reliably exchanges changesets.



1) Pay attention to your commit text. The commit message that
accompanies each changeset you submit will live on forever in history,
and is used by Linus to accurately summarize the changes in each
pre-patch. Remember that there is no context, so
"fix for new scheduler changes"
would be too vague, but
"fix mips64 arch for new scheduler switch_to(), TIF_xxx semantics"
would be much better.

You can and should use the command "bk comment -C<rev>" to update the
commit text, and improve it after the fact. This is very useful for
development: poor, quick descriptions during development, which get
cleaned up using "bk comment" before issuing the "bk push" to submit the
changes.



2) Include an Internet-available URL for Linus to pull from, such as

Pull from: http://gkernel.bkbits.net/net-drivers-2.5



3) Include a summary and "diffstat -p1" of each changeset that will be
downloaded, when Linus issues a "bk pull". The author auto-generates
these summaries using "bk push -nl <parent> 2>&1", to obtain a listing
of all the pending-to-send changesets, and their commit messages.

It is important to show Linus what he will be downloading when he issues
a "bk pull", to reduce the time required to sift the changes once they
are downloaded to Linus's local machine.

IMPORTANT NOTE: One of the features of BK is that your repository does
not have to be up to date, in order for Linus to receive your changes.
It is considered a courtesy to keep your repository fairly recent, to
lessen any potential merge work Linus may need to do.


4) Split up your changes. Each maintainer<->Linus situation is likely
to be slightly different here, so take this just as general advice. The
author splits up changes according to "themes" when merging with Linus.
Simultaneous pushes from local development to goes special trees which
exist solely to house changes "queued" for Linus. Example of the trees:

net-drivers-2.5 -- on-going net driver maintenance
vm-2.5 -- VM-related changes
fs-2.5 -- filesystem-related changes

Linus then has much more freedom for pulling changes. He could (for
example) issue a "bk pull" on vm-2.5 and fs-2.5 trees, to merge their
changes, but hold off net-drivers-2.5 because of a change that needs
more discussion.

Other maintainers may find that a single linus-pull-from tree is
adequate for passing BK changesets to him.



Frequently Answered Questions
-----------------------------
1) How do I change the e-mail address shown in the changelog?
A. When you run "bk citool" or "bk commit", set environment
variables BK_USER and BK_HOST to the desired username
and host/domain name.


2) How do I use tags / get a diff between two kernel versions?
A. Pass the tags Linus uses to 'bk export'.

ChangeSets are in a forward-progressing order, so it's pretty easy
to get a snapshot starting and ending at any two points in time.
Linus puts tags on each release and pre-release, so you could use
these two examples:

bk export -tpatch -hdu -rv2.5.4,v2.5.5 | less
# creates patch-2.5.5 essentially
bk export -tpatch -du -rv2.5.5-pre1,v2.5.5 | less
# changes from pre1 to final

A tag is just an alias for a specific changeset... and since changesets
are ordered, a tag is thus a marker for a specific point in time (or
specific state of the tree).
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