Re: [GIT PULL] KVM updates for the 3.4 merge window

From: Ingo Molnar
Date: Mon Mar 26 2012 - 12:21:05 EST

* Avi Kivity <avi@xxxxxxxxxx> wrote:

> Say a fix comes in which needs to be mainlined during -rc. So
> I put it in some other branch, to be sent off to Linus in a
> few days after maturing a little. Meanwhile developers see an
> incomplete tree, since that patch is not in it.
> Once Linus pulls, I can merge it back (or even before, if I'm
> reasonably certain it's not going to change), but it leaves a
> history of unneeded merges. Or we can do throwaway merges
> like tip.git.

We don't do throwaway merges in the -tip development branches
themselves, i.e. in tip:sched/core, tip:perf/core,
tip:timers/core, etc.

When a fix goes into tip:sched/urgent then until Linus merges it
it's not in tip:sched/core. 99% of the fixes don't *have to* go
into sched/core straight away.

In the odd case where there's some dependency, we can manually
merge it into tip:sched/core ahead of Linus pulling into an -rc.
Those rare merges are not a problem, and I explain the reason in
the merge commit itself.

If you look at:

gll v3.2..v3.3 | grep -E '/urgent.*/core'

you'll see that I only had to do it once in the previous cycle:

d6c1c49de577 Merge branch 'perf/urgent' into perf/core

and the changelog explains the background:

Merge reason: Add these cherry-picked commits so that future changes
on perf/core don't conflict.

it was a rare, oddball situation where we cherry-picked
perf/core changes into perf/urgent. Extra merges are perfectly
fine in that case.

The 'throwaway' tip:master branch you are probably referring to
is basically just a testing branch, a convenient merged tree of
the one dozen maintainer trees that are hosted in -tip. Since we
don't want to force Linus's hand of him being able to reject
individual trees we don't merge them properly - hence the
integrated tree is a throwaway tree in theory.

In practice I tend to throw it away only once per cycle, around
-rc1, once all pending trees went to Linus. tip:master is not
used for any Git based contribution work - it's for testing,
it's for people who want to work with patches - the commits
themselves always go into persistent non-rebasing, append-only
Git trees.

If we mess up bisectability we do a delta fix. When choosing
between somewhat better bisectability and a proper history that
others can rely on then proper history wins hands down.


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