Re: linux-next: origin tree build failure

From: Ingo Molnar
Date: Fri Jun 12 2009 - 09:50:21 EST

* Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote:

> > linux-next should not be second-guessing maintainers and should
> > not act as an "approval forum" for controversial features,
> > increasing the (already quite substantial) pressure on
> > maintainers to apply more crap.
> I agree here. That's not the point. The idea is that for things
> that -are- approved by their respective maintainers, to get some
> integration testing and ironing of those mechanical bugs so that
> by the time they hit mainstream, they don't break bisection among
> others.

This is certainly doable for agreeable features - which is the bulk
- and it is being done.

But this is a catch-22 for _controversial_ new features - which
perfcounters clearly was, in case you turned off your lkml
subscription ;-)

And if you hit that build breakage during bisection you can do:

git cherry-pick e14112d

Also, you seem to brush off the notion that far more bugs slip
through linux-next than get caught by it.

So if you think linux-next matters in terms of _regression_ testing,
the numbers dont seem to support that notion. This particular
incident does support that notion though, granted - but it's taken
out of context IMHO:

In terms of test coverage, at least for our trees, less than 1% of
the bugs we handle get reported in a linux-next context - and most
of the bugs that get reported (against say the scheduler tree) are
related to rare architectures.

In fact, i checked, there were _zero_ x86 bugs reported against
linux-next and solved against it between v2.6.30-rc1 and v2.6.30:

git log --grep=next -i v2.6.30-rc1..v2.6.30 arch/x86/

Doing it over the full cycle shows one commit altogether - a Xen
build failure. In fact, i just checked the whole stabilization cycle
for the whole kernel (v2.6.30-rc1..v2.6.30-final), and there were
only 5 linux-next originated patches, most of them build failures.

I did this by looking at all occurances of 'next', in all commit

git log --grep=next -i v2.6.30-rc1..v2.6.30

and then manually checking the context of all 'next' matches and
counting the linux-next related commits.

So lets be generous and say that because some people dont put the
bug report originator into the changelog it was four times as many,
20 - but that's still dwarved by the sheer amount of post-rc1
changes: thousands of changes and hundreds of regressions.

linux-next is mostly useful (to me at least) not for the
cross-builds it does, but in terms of mapping out upcoming conflicts
- which also drives early detection of problematic patches and
problematic conflicts.

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