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283 lines
12 KiB
283 lines
12 KiB
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Doing the BK Thing, Penguin-Style
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This set of notes is intended mainly for kernel developers, occasional
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or full-time, but sysadmins and power users may find parts of it useful
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as well. It assumes at least a basic familiarity with CVS, both at a
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user level (use on the cmd line) and at a higher level (client-server model).
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Due to the author's background, an operation may be described in terms
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of CVS, or in terms of how that operation differs from CVS.
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This is -not- intended to be BitKeeper documentation. Always run
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"bk help <command>" or in X "bk helptool <command>" for reference
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documentation.
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BitKeeper Concepts
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------------------
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In the true nature of the Internet itself, BitKeeper is a distributed
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system. When applied to revision control, this means doing away with
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client-server, and changing to a parent-child model... essentially
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peer-to-peer. On the developer's end, this also represents a
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fundamental disruption in the standard workflow of changes, commits,
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and merges. You will need to take a few minutes to think about
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how to best work under BitKeeper, and re-optimize things a bit.
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In some sense it is a bit radical, because it might described as
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tossing changes out into a maelstrom and having them magically
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land at the right destination... but I'm getting ahead of myself.
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Let's start with this progression:
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Each BitKeeper source tree on disk is a repository unto itself.
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Each repository has a parent (except the root/original, of course).
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Each repository contains a set of a changesets ("csets").
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Each cset is one or more changed files, bundled together.
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Each tree is a repository, so all changes are checked into the local
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tree. When a change is checked in, all modified files are grouped
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into a logical unit, the changeset. Internally, BK links these
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changesets in a tree, representing various converging and diverging
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lines of development. These changesets are the bread and butter of
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the BK system.
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After the concept of changesets, the next thing you need to get used
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to is having multiple copies of source trees lying around. This -really-
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takes some getting used to, for some people. Separate source trees
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are the means in BitKeeper by which you delineate parallel lines
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of development, both minor and major. What would be branches in
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CVS become separate source trees, or "clones" in BitKeeper [heh,
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or Star Wars] terminology.
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Clones and changesets are the tools from which most of the power of
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BitKeeper is derived. As mentioned earlier, each clone has a parent,
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the tree used as the source when the new clone was created. In a
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CVS-like setup, the parent would be a remote server on the Internet,
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and the child is your local clone of that tree.
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Once you have established a common baseline between two source trees --
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a common parent -- then you can merge changesets between those two
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trees with ease. Merging changes into a tree is called a "pull", and
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is analagous to 'cvs update'. A pull downloads all the changesets in
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the remote tree you do not have, and merges them. Sending changes in
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one tree to another tree is called a "push". Push sends all changes
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in the local tree the remote does not yet have, and merges them.
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From these concepts come some initial command examples:
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1) bk clone -q http://linux.bkbits.net/linux-2.5 linus-2.5
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Download a 2.5 stock kernel tree, naming it "linus-2.5" in the local dir.
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The "-q" disables listing every single file as it is downloaded.
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2) bk clone -ql linus-2.5 alpha-2.5
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Create a separate source tree for the Alpha AXP architecture.
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The "-l" uses hard links instead of copying data, since both trees are
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on the local disk. You can also replace the above with "bk lclone -q ..."
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You only clone a tree -once-. After cloning the tree lives a long time
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on disk, being updating by pushes and pulls.
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3) cd alpha-2.5 ; bk pull http://gkernel.bkbits.net/alpha-2.5
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Download changes in "alpha-2.5" repository which are not present
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in the local repository, and merge them into the source tree.
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4) bk -r co -q
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Because every tree is a repository, files must be checked out before
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they will be in their standard places in the source tree.
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5) bk vi fs/inode.c # example change...
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bk citool # checkin, using X tool
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bk push bk://gkernel@bkbits.net/alpha-2.5 # upload change
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Typical example of a BK sequence that would replace the analagous CVS
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situation,
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vi fs/inode.c
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cvs commit
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As this is just supposed to be a quick BK intro, for more in-depth
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tutorials, live working demos, and docs, see http://www.bitkeeper.com/
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BK and Kernel Development Workflow
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----------------------------------
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Currently the latest 2.5 tree is available via "bk clone $URL"
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and "bk pull $URL" at http://linux.bkbits.net/linux-2.5
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This should change in a few weeks to a kernel.org URL.
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A big part of using BitKeeper is organizing the various trees you have
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on your local disk, and organizing the flow of changes among those
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trees, and remote trees. If one were to graph the relationships between
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a desired BK setup, you are likely to see a few-many-few graph, like
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this:
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linux-2.5
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merge-to-linus-2.5
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/ | |
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/ | |
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vm-hacks bugfixes filesys personal-hacks
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\ | | /
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\ | | /
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\ | | /
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testing-and-validation
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Since a "bk push" sends all changes not in the target tree, and
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since a "bk pull" receives all changes not in the source tree, you want
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to make sure you are only pushing specific changes to the desired tree,
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not all changes from "peer parent" trees. For example, pushing a change
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from the testing-and-validation tree would probably be a bad idea,
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because it will push all changes from vm-hacks, bugfixes, filesys, and
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personal-hacks trees into the target tree.
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One would typically work on only one "theme" at a time, either
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vm-hacks or bugfixes or filesys, keeping those changes isolated in
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their own tree during development, and only merge the isolated with
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other changes when going upstream (to Linus or other maintainers) or
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downstream (to your "union" trees, like testing-and-validation above).
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It should be noted that some of this separation is not just recommended
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practice, it's actually [for now] -enforced- by BitKeeper. BitKeeper
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requires that changesets maintain a certain order, which is the reason
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that "bk push" sends all local changesets the remote doesn't have. This
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separation may look like a lot of wasted disk space at first, but it
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helps when two unrelated changes may "pollute" the same area of code, or
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don't follow the same pace of development, or any other of the standard
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reasons why one creates a development branch.
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Small development branches (clones) will appear and disappear:
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-------- A --------- B --------- C --------- D -------
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\ /
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-----short-term devel branch-----
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While long-term branches will parallel a tree (or trees), with period
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merge points. In this first example, we pull from a tree (pulls,
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"\") periodically, such as what occurs when tracking changes in a
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vendor tree, never pushing changes back up the line:
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-------- A --------- B --------- C --------- D -------
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\ \ \
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----long-term devel branch-----------------
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And then a more common case in Linux kernel development, a long term
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branch with periodic merges back into the tree (pushes, "/"):
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-------- A --------- B --------- C --------- D -------
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\ \ / \
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----long-term devel branch-----------------
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Submitting Changes to Linus
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---------------------------
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There's a bit of an art, or style, of submitting changes to Linus.
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Since Linus's tree is now (you might say) fully integrated into the
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distributed BitKeeper system, there are several prerequisites to
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properly submitting a BitKeeper change. All these prereq's are just
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general cleanliness of BK usage, so as people become experts at BK, feel
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free to optimize this process further (assuming Linus agrees, of
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course).
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0) Make sure your tree was originally cloned from the linux-2.5 tree
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created by Linus. If your tree does not have this as its ancestor, it
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is impossible to reliably exchange changesets.
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1) Pay attention to your commit text. The commit message that
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accompanies each changeset you submit will live on forever in history,
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and is used by Linus to accurately summarize the changes in each
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pre-patch. Remember that there is no context, so
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"fix for new scheduler changes"
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would be too vague, but
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"fix mips64 arch for new scheduler switch_to(), TIF_xxx semantics"
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would be much better.
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You can and should use the command "bk comment -C<rev>" to update the
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commit text, and improve it after the fact. This is very useful for
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development: poor, quick descriptions during development, which get
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cleaned up using "bk comment" before issuing the "bk push" to submit the
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changes.
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2) Include an Internet-available URL for Linus to pull from, such as
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Pull from: http://gkernel.bkbits.net/net-drivers-2.5
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3) Include a summary and "diffstat -p1" of each changeset that will be
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downloaded, when Linus issues a "bk pull". The author auto-generates
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these summaries using "bk changes -L <parent>", to obtain a listing
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of all the pending-to-send changesets, and their commit messages.
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It is important to show Linus what he will be downloading when he issues
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a "bk pull", to reduce the time required to sift the changes once they
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are downloaded to Linus's local machine.
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IMPORTANT NOTE: One of the features of BK is that your repository does
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not have to be up to date, in order for Linus to receive your changes.
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It is considered a courtesy to keep your repository fairly recent, to
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lessen any potential merge work Linus may need to do.
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4) Split up your changes. Each maintainer<->Linus situation is likely
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to be slightly different here, so take this just as general advice. The
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author splits up changes according to "themes" when merging with Linus.
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Simultaneous pushes from local development go to special trees which
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exist solely to house changes "queued" for Linus. Example of the trees:
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net-drivers-2.5 -- on-going net driver maintenance
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vm-2.5 -- VM-related changes
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fs-2.5 -- filesystem-related changes
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Linus then has much more freedom for pulling changes. He could (for
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example) issue a "bk pull" on vm-2.5 and fs-2.5 trees, to merge their
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changes, but hold off net-drivers-2.5 because of a change that needs
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more discussion.
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Other maintainers may find that a single linus-pull-from tree is
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adequate for passing BK changesets to him.
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Frequently Answered Questions
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-----------------------------
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1) How do I change the e-mail address shown in the changelog?
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A. When you run "bk citool" or "bk commit", set environment
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variables BK_USER and BK_HOST to the desired username
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and host/domain name.
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2) How do I use tags / get a diff between two kernel versions?
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A. Pass the tags Linus uses to 'bk export'.
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ChangeSets are in a forward-progressing order, so it's pretty easy
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to get a snapshot starting and ending at any two points in time.
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Linus puts tags on each release and pre-release, so you could use
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these two examples:
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bk export -tpatch -hdu -rv2.5.4,v2.5.5 | less
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# creates patch-2.5.5 essentially
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bk export -tpatch -du -rv2.5.5-pre1,v2.5.5 | less
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# changes from pre1 to final
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A tag is just an alias for a specific changeset... and since changesets
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are ordered, a tag is thus a marker for a specific point in time (or
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specific state of the tree).
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3) Is there an easy way to generate One Big Patch versus mainline,
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for my long-lived kernel branch?
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A. Yes. This requires BK 3.x, though.
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bk export -tpatch -r`bk repogca bk://linux.bkbits.net/linux-2.5`,+
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