Re: [GIT pull] x86 fixes for 2.6.26

From: Ingo Molnar
Date: Sat May 17 2008 - 15:40:01 EST

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> > If users, or even you, want to get a bug fix from Linus's tree or
> > check if there will be merge conflicts, just create a test branch
> > and play with such things there.
> Yes. I think you guys already have a test branch for the "join it all
> together" case, don't you?

yep, we have such a branch in -tip that is the integration of all
currently stable topic branches, and which also embedds misc temporary
convenience fixes that we pick up for ourselves anyway. It's basically
the -rt model.

[ Our 'latest' branch is in essence a _daily_ stable release of the
kernel - stable as far as our own testsystems go and as far as
contributor boxes go. (We dont "release" it without having at least 10
successful bootups of the tree and we immediate resolve regressions
one way or another.) ]

We've now solved the merge-commit pollution problem that you noticed (we
still are git newbies ;-) and you shouldnt see unnecessary merge commits
in the future anymore. Holler if you see anything else in -tip that
looks non-Git-ish ...

We've solved it by still doing the merges (we _need_ the information
about potential upcoming merge trouble), but we dont commit them to the
topic branches - we throw them away. We only do a permanent, externally
visible merge if we (after the fact) detect that a merge was non-trivial
[because an upstream change interfered with the topic].

We had a couple of rather strong technical reasons to keep the topic
branches merged on a daily basis:

- Spreading out maintenance and risks: people interested in a particular
topic can just pick up a topic branch and _usually_ it should just
work and should be fairly recent. We encourage contributors to advance
individual topics themselves and [now with the -tip tree] send pull
requests against trees with that specific topic branch modified,
instead of having to come back to our integration branches all the

- Testing: we auto-test each topic branch in isolation - so keeping them
uptodate against upstream is important.

- Automation of conflict resolution: with 30+ topics we cannot detect
interactions manually in a reliable way, we detect them automatically.
We deal with popular, high-flux areas of the kernel where we often get
"outside" interference and where we in fact _encourage_ outside
interference (even if we could ask for strict maintenance boundaries)
because we dont think strict maintenance boundaries are particularly
good for Linux. Other subsystem trees have it "easier" because they
have a strict hierarchy that controls essentially all changes to that
area of code. So they can do conflict resolution by basically doing
conflict _avoidance_. (The flip side is that conflict avoidance tends
to teach people to fight "outside" interference instead of learning
how to interact intelligently.)

- Lower cost of fixes, lower latency of conflict resolution: to detect
conflicts and interaction between continuing work in each topic, and
with work that goes on upstream. It's much easier to resolve such
interactions on a clean, per topic basis than on a 'whole tree' basis.
The sooner we detect trouble, the faster we can act. Also, we can
often detect interaction right when it happens: when the other party
is still "active" in pursuing that other, interacting topic - this is
the time when fixes are the cheapest and where any change of plans is
the cheapest as well.

- Inability to predict future conflicts: we cannot predict the future,
so we dont know it _in advance_ when conflicts or interactions occur.
So we kept topics rolled forward on a daily basis. This spreads out
our workload quite a bit, and also makes any trouble easier to
repair: a same-day bugreport or reverts are much easier for all
parties involved than a revert 1 or 2 weeks later, when everyone has
already forgotten the details of that change and is busy in another

- Stability: our experience is that after the merge window, the
stability of the upstream kernel increases near monotonically. By
keeping a topic branch merged daily, the practical stability of each
topic branch is 1-7 days better than if we merged to it weekly.

We've done tons of non-trivial conflict resolutions in the past ~5 years
against a 'hostile' (well, indifferent) upstream tree. So we've learned
how to do this rather efficiently in the long run [ while still staying
sane and productive :) ], and one of the key things was to have a daily
merge cycle.

The reason is that the best way to do conflict resolution IMO is either
to do hard-core conflict avoidance, or to use our "finegrained roll
forward" method: to have a topic merged cleanly to just before the point
where the conflict happens, and then to resolve the conflict by also
merging to just after the conflict. This means there's the least
possible amount of unrelated flux to look at.

The reasons you cited for not having that many merge commits are of
course all valid (graphical output and bisectability) - and they trump
our concerns because they affect the 'end result' tree which might not
care about our sub-tree and development concerns. (I wish we extended
such thinking to make the tree even more bisectable than it is today -
but that attacks the append-only nature of the tree.)

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