[llvm-dev] LLVM Releases: Upstream vs. Downstream / Distros

Renato Golin via llvm-dev llvm-dev at lists.llvm.org
Wed May 11 07:08:30 PDT 2016


There has been enough discussion about keeping development downstream
and how painful that is. Even more so, I think we all agree, is having
downstream releases while tracking upstream releases, trunk and other
branches (ex. Android).

I have proposed "en passant" a few times already, but now I'm going to
do it to a wider audience:

Shall we sync our upstream release with the bulk of other downstream
ones as well as OS distributions?

This work involves a *lot* of premises that are not encoded yet, so
we'll need a lot of work from all of us. But from the recent problems
with GCC abi_tag and the arduous job of downstream release managers to
know which patches to pick, I think there has been a lot of wasted
effort by everyone, and that generates stress, conflicts, etc.

I'd also like to reinforce the basic idea of software engineering: to
use the same pattern for the same problem. So, if we have one way to
link sequences of patches and merge them upstream, we ought to use the
same pattern (preferably the same scripts) downstream, too. Of course
there will be differences, but we should treat them as the exception,
not the rule.

So, a list of things will need to be solved to get to a low waste
release process:

  1. Timing

Many downstream release managers, as well as distro maintainers have
complained about the timing of our releases, and how unreliable they
are, and how that makes it hard for them to plan their own branches,
cherry-picks and merges. If we release too early, they miss out
important optimisations, if we do too late, they'll have to branch
"just before" and risk having to back-port late fixes to their own
modified trees.

Products that rely on LLVM already have their own life cycles and we
can't change that. Nor we can make all downstream products align to
our quasi-chaotic release process. However, the important of the
upstream release for upstream developers is *a lot* lower than for the
downstream maintainers, so unless the latter group puts their weight
(and effort) in the upstream process, little is going to happen to
help them.

A few (random) ideas:

 * Do an average on all product cycles, pick the least costly time to
release. This would marginalise those beyond the first sigma and we'd
make their lives much harder than those within one sigma.
 * Do the same average on the projects that are willing to lend a
serious hand to the upstream release process. This has the same
problem, but it's based on actual effort. It does concentrate bias on
the better funded projects, but it's also easier for low key projects
to change their release schedules.
 * Try to release more often. The current cost of a release is high,
but if we managed to lower it (by having more people, more automation,
shared efforts), than it could be feasible and much fairer than
weighted averages.

  2. Process

Our release process is *very* lean, and that's what makes it
quasi-chaotic. In the beginning, not many people / companies wanted to
help or cared about the releases, so the process was what whomever was
doing, did. The major release process is now better defined, but the
same happened to the minor releases.

For example, we have no defined date to start, or to end. We have no
assigned people to do the official releases, or test the supported
targets. We still rely on voluntary work from all parties. That's ok
when the release is just "a point in time", but if downstream releases
and OS distributions start relying on our releases, we really should
get a bit more professional.

A few (random) ideas:

 * We should have predictable release times, both for starting it and
finishing it. There will be complications, but we should treat them as
the exception, not the rule.
 * We should have appointed members of the community that would be
responsible for those releases, in the same way we have code owners
(volunteers, but no less responsible), so that we can guarantee a
consistent validation across all relevant targets. This goes beyond
x86/ARM/MIPS/PPC and includes the other targets like AMD, NVidia, BPF,
 * The upstream release should be, as much as possible, independent of
which OS they run on. OS specific releases should be done in the
distributions themselves, and people interested should raise the
concern in their own communities.
 * Downstream managers should be an integral part of the upstream
release process. Whenever the release manager sends the email, they
should test on their end and reply with GREEN/RED flags.
 * Downstream managers should also propose back-ports that are
important to them in the upstream release. It's likely that a fix is
important to a number of downstream releases but not many people
upstream (since we're all using trunk). So, unless they tell us, we
won't know.
 * OS distribution managers should test on their builds, too. I know
FreeBSD and Mandriva build by default with Clang. I know that Debian
has an experimental build. I know that RedHat and Ubuntu have LLVM
packages that they do care. All that has to be tested *at least* every
major release, but hopefully on all releases. (those who already do
that, thank you!)
 * A number of upstream groups, or downstream releases that don't
track upstream releases, should *also* test them on their own
workloads. Doing so, will get the upstream release in a much better
quality level, and in turn, allow those projects to use it on their
own internal releases.
 * Every *new* bug found in any of those downstream tests should be
reported in Bugzilla with the appropriate category (critical / major /
minor). All major bugs have to be closed for the release to be out,
etc. (the specific process will have to be agreed and documented).

  3. Automation

As exposed in the timing and process sections, automation is key to
reducing costs for all parties. We should collate the encoded process
we have upstream with the process projects have downstream, and
convert upstream everything that we can / is relevant.

For example, finding which patches revert / fix another one that was
already cherry-picked is a common task that should be identical to
everyone. A script that would sweep the commit logs, looking for
clues, would be useful to everyone.

A few (random) ideas:

 * We should discuss the process, express the consensus on the
official documentation, and encode it in a set of scripts. It's a lot
easier to tell a developer "please do X because it helps our script
back-port your patch" than to say "please do X because it's nice" or
"do X because it's in the 'guideline'".
 * There's no way to force (via git-hook) developers to add a bugzilla
ID or a review number on the commit message (not all commits are
equal), so the scripts that scan commits will have to be smart enough,
but that'll create false-positives, and they can't commit without
human intervention. Showing why a commit wasn't picked up by the
script, or was erroneously picked up, is a good way to educate people.
 * We could have a somewhat-common interface with downstream releases,
so some scripts that they use could be upstreamed, if many of them
used the same entry point for testing, validating, building,
 * We could have the scripts that distros use for building their own
packages in our tree, so they could maintain them locally and we'd
know which changes are happening and would be much easier to warn the
others, common up the interface, etc.

In the end, we're a bunch of people from *very* different communities
doing similar work. In the spirit of open source, I'd like to propose
that we share the work and the responsibility of producing high
quality software with minimal waste.

I don't think anyone receiving this email disagrees with the statement
that we can't just take and not give back, and that being part of this
community means we may have to work harder than our employers would
think brings direct profit, so that they can profit even more
indirectly later, and with that, everyone that uses or depends on our

My personal and very humble opinion is that coalescing the release
process will, in the long term, actually *save* us a lot of work, and
the quality will be increased. Even if we don't reach perfection, and
by no means I think we will, at least we'll have something slightly
better. If anything, at least we tried.

I'd hate to continue doing an inefficient process without even trying
an alternative...



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