Re: Article about "git bisect run" on LWN

[Christian Couder <chriscool@tuxfamily.org>]

Le jeudi 5 février 2009, Ingo Molnar a écrit :
> * Christian Couder <chriscool@tuxfamily.org> wrote:
> > Hi,
> >
> > For information, an article from me, 'Fully automated bisecting with
> > "git bisect run"' has been published in today's edition of LWN on the
> > development page:
> >
> > http://lwn.net/Articles/317154/
>
> Nice article!
>
> In terms of possible future enhancements of git bisect, here's a couple
> of random ideas that would help my auto-bisection efforts:
>
>  - Feature: support "Bisection Redundancy"
>
>    This feature helps developers realize if a bug is sporadic. This
> happens quite often in the kernel space: a bug looks deterministic, but
> down the line it becomes sporadic. Sometimes a boot crash only occurs
> with a 75% probability - and if one is unlucky it can cause a _lot_ of
> wasted bisection time. The wrong commit gets blamed and the wrong set of
> developers start scratching their heads. It's a reoccuring theme on lkml.
>
>    What git could do here is to allow testers to inject a bit of extra
>    "redundancy" automatically, and use the redundant test-points to
> detect conflicts in good/bad constraints.
>
>    It would work like this:
>
>       git bisect start --redundancy=33%
>
>    It would mean that for every third bisection points, Git would
>    _not_ chose the ideal (estimated) 'middle point' from the set of
> "unknown quality" changes that are still outstanding - but would
> intentionally "weer outside" and select one commit from the _known_ set
> of commits.
>
>    If such a redundant re-test of the known-good or known-bad set yields
> a nonsensical result then Git aborts the bisection with a "logic
> inconsistency detected" kind of message - and people could at this point
> realize the non-determinism of the test.
>
>    ( Git can do this when a "redundant" test point is marked as 'bad' -
>      despite an earlier bisection already categorizing that test point as
>      'good' - or if it's the other way around. Git will only continue
> with the bisection if the test point has the expected quality. )
>
>    This essentially means an automated re-test - but it's much better
> than just a repeated bisection - i've often met non-deterministic bugs
> that yield the _exact same_ nonsensical commit even on repeat bisections.
> That happens when a timing bug depends on the exact kernel layout, or a
> miscompilation or linker bug depends on the exact kernel layout, etc.
>
>    It's also faster than a re-done bisection: 33% more testpoints is
> better than twice as many test-points. Also, auto-bisection can deal with
> redundancy just fine - it does not really matter whether i have to wait
> 20 or 30 minutes for a test result since there's no manual intervention
> needed - but it _very_ much matters whether i can trust the validity of
> the bisection result.

I see. With the current code it seems difficult to have a "weer outside" 
algorithm, but if the "git bisect skip" code is ported (from shell 
in "git-bisect.sh") to C in "builtin-rev-list.c", that might be doable. And 
anyway moving the bisect skip code to C is needed for other improvements.

> - Feature: better "git bisect next" support.

You probably mean "git bisect skip" here.

>   Sometimes a commit wont build. In that case we have "git bisect next",
> but last i checked that only jumps a single commit - and build breakages 
> often have a large scope - full trees that got merged upstream, etc. Most
> of the time those build breakages are uninteresting and the build-broken
> window does not contain the bad commit.
>
>   So it would be nice to have a "git bisect next --left=20%" type of
>   feature. This would jump 20% commits to the "left" from the bisection
>   point, towards the 'known bad' set of commits, but still within the
>   bisection window.
>
>   Similarly, "git bisect next --right=20%" would jump towards the
> known-good edge of the bisection window (but still within the bisection
> window).

In the following thread, H. Peter Anvin suggested an algorithm to deal with 
this kind of problem:

http://thread.gmane.org/gmane.comp.version-control.git/98164/

And I suggested a simpler one, that might be implemented without having to 
port "git bisect skip" code to C first, but I did not work on it yet.

>   Currently when i hit a build error during auto-bisection, it aborts and
> i have to intervene manually. But with a bigger jump distance i could use
> git-bisect-next reliably in scripts too.
>
>   Likewise, users too hit build breakages often, and find it hard to get
> out of the window of breakage. With the high-order tree structure of the
> kernel repository that is rather non-intuitive to do as well, and often
> people make mistakes and test the wrong commit.

I am working slowly on "git replace" these days and, if everything goes 
well, it should make it possible to use "replace" refs when bisecting, so 
that people could bisect on commit trees where many breakages have been 
removed. And as refs can be shared, this means that users and developers 
should be able to easily share these improved trees.

Another way to work around breakages could be to have a list of commits and 
ranges of commits that should always be skipped and always pass them 
to "git bisect skip" before using "git bisect run". Something like that 
perhaps:

$ git bisect start <bad> <good>
$ git bisect skip $(cat always_skipped.txt)
$ git bisect run ./my_test_script.sh

But I agree that it would also be a good thing to have an improved "git 
bisect skip" that could jump out of breakage windows.

> - Feature: detect "redundant" and "inconsistent" test points
>
>   This is a variation of the redunant testing theme, but from a different
>   angle: often newbies when they bisect the kernel weer outside of the
>   bisection window without realizing it. It would be nice if Git printed
> a friendly notifier that:
>
>      git bisect good 12341234
>      info: bisection point 12341234 was already in the 'good' range

I agree that it would be nice. And it might be easy to implement.

>   Or, if the redunant test point is conflicting, print:
>
>      git bisect good 12341234
>      fatal: bisection point 12341234 was already in the 'bad' range!
>
>   And give an error return as well, so that scripts can abort.
>
>   Currently Git seems to be very forgiving and accepts all bisection
> points that we feed it, without checking them for consistency. (this
> might have changed in current development versions, i dont know.)

I don't think conflicting test points are accepted. An error should be 
reported (with exit code 1), like this:

$ git bisect start HEAD~5 HEAD
Some good revs are not ancestor of the bad rev.
git bisect cannot work properly in this case.
Maybe you mistake good and bad revs?

> - User friendliness: give an estimation about how many steps are
> remaining
>
>   Right now git prints this when a bisection session begins:
>
>      aldebaran:~/tip> git bisect start
>      aldebaran:~/tip> git bisect bad linus
>      aldebaran:~/tip> git bisect good v2.6.28
>      Bisecting: 5449 revisions left to test after this
>      [e0b685d39a0404e7f87fb7b7808c3b37a115fe11] Updated contact info for
> CREDITS file
>
>   It would be nice if Git estimated the expected number of bisection
> points. Something like this would be helpful:
>
>      aldebaran:~/tip> git bisect good v2.6.28
>      Bisecting: 5449 revisions left to test after this
>                 About ~16 test steps left [approximated]
>      [e0b685d39a0404e7f87fb7b7808c3b37a115fe11] Updated contact info for
> CREDITS file
>
>   The real number of test points might be higher than this, if the tree
>   layout is unlucky, or it might be less than this if the user manually
>   narrows the bisection window to a suspected set of commits - but that's
>   OK - most kernel testers use the default variant and the message is
> clear enough that it's only an estimation.

I will have a look at that. It might be easy to do.

Thanks,
Christian.