[llvm-dev] False positive notifications around commit notifications

[Philip Reames via llvm-dev]

On 9/10/21 11:36 AM, Mehdi AMINI wrote:
>
>
> On Thu, Sep 9, 2021 at 3:18 PM Philip Reames via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     I've been noticing a trend where there is more and more false
>     positive email notifications sent out on valid commits.  This is
>     getting really problematic as real signal is being lost in the
>     noise.  I've had several cases in the last few weeks where I did
>     not see a "real" failure notice because it was buried in a bunch
>     of false positives.
>
>     Let me run through a few sources of what I consider false
>     positives, and suggest a couple things we could do to clean these
>     up.  Note that the recommendations here are entirely independent
>     and we can adopt any subset.
>
>     *Slow Try Bots*
>
>     ex: "This revision was landed with ongoing or failed builds." on
>     https://reviews.llvm.org/D109091 <https://reviews.llvm.org/D109091>
>
>     Someone - I'm not really sure who - enabled builds for all
>     reviews, and this notice on landed commits.  Given it's utterly
>     routine to make a last few style fixes before landing an LGTMed change
>
>
> I do such "few style fixes", but I don't re-upload a revision before 
> landing, so I don't see this "false positive" in general.
I don't explicit upload the final patch either, but something in the 
close automation does.
> What I frequently see is that the pre-merge config is broken for some 
> other reason, and that's quite annoying. One aspect of the issue is 
> that the is no buildbot tracking the pre-merge configuration so it can 
> be broken without notification (there is a buildkite job tracking it, 
> but buildkite does not support blamelist notifications).

Hm, maybe I misinterpreted the cause of these entirely?  Your 
explanation sounds plausible as well.

If your explanation is correct, that would lean strongly to the "just 
disable" option.

Do you know who to contact about this?  (i.e. Who owns the automation 
here?  Or where is the appropriate code to adjust?)

>
>     , I consider this notice complete noise.  In practice, almost
>     review gets tagged this way.  To be clear, there is value in being
>     told about changes which don't build. The false positive part is
>     only around the "ongoing" builds.
>
>     Recommendation: Disable this message for the "ongoing" build case,
>     and if we can't, disable them entirely.
>
>     *Flaky Builders*
>
>     ex: https://lab.llvm.org/buildbot/#/builders/68/builds/18250
>     <https://lab.llvm.org/buildbot/#/builders/68/builds/18250>
>
>     We have many build bots which are not entirely stable. It's gotten
>     to the point where I *expect* failure notifications on literally
>     every change I land.  I've been trying to reach out to individual
>     build bot owners to get issues resolved, and to their credit, most
>     owners have been very responsive.  However, we have enough
>     builders that the situation isn't getting meaningful better.
>
>     Recommendation: Introduce specific "test commits" whose only
>     purpose is to run the CI infrastructure.  Any builder which
>     notifies of failure on such a commit (and only said commit) is
>     disabled without discussion until human action is taken by the bot
>     owner to re-enable. The idea here is to a) automate the process,
>     and b) shift the responsibility of action to the bot owner for any
>     flaky bot.
>
>     Note: By "disabled", I specifically mean that *notification* is
>     disabled.  Leaving it in the waterfall view is fine, as long as
>     we're not sending out email about it.
>
>     Aside: It's really tempting to attempt to separate builders which
>     are "still failing" (e.g. a rare configuration which has been
>     broken for a few days) from "flaky" ones.  I'd argue any bot
>     notifying on a "still failing" case is buggy, and thus it's fine
>     to treat them the same as a "flaky" bot.
>
>     *Slow Builders and Redundant Notices
>     *
>
>     ex: https://lab.llvm.org/buildbot#builders/67/builds/4128
>     <https://lab.llvm.org/buildbot#builders/67/builds/4128>
>
>     Occasionally, we have a bad commit land which breaks every (or
>     nearly every) builder.  That happens.  If you happen to land a
>     change just before or after it, you then get on the blame list for
>     every slow running builder we have (since they tend to have large
>     commit windows) if they happen to cycle before the fix is
>     committed.  This is particularly annoying since the root issue is
>     likely fixed quickly, but due to cycle times on the builders, you
>     may be getting emails for 24 hours to come.
>
>     Recommendation: Introduce a new requirement for "slow" builders
>     (say cycle time of > 30 minutes) either a) have a maximum commit
>     window of ~15 commits, or b) use a staged builder model. 
>     Personally, I'd prefer the staged model, but the max commit window
>     at least helps to limit the damage.
>
>     By "staged builder model", I mean that slow builders only build
>     points in the history which have already been successfully build
>     by one of the fast builders.  This eliminates redundant build
>     failures, at the cost of delaying the slow builder slightly.  As
>     long as the slow builder uses the "last good commit" as opposed to
>     waiting until the current fast builder finishes, the delay should
>     be very minimal for most commits.
>
>
> Does buildbot support staged builders? That would really be ideal indeed!
> If we could also disable notification to the blamelist when it is 
> larger than 5, that'd be great!

I'll be honest here and say I don't know what buildbot natively 
supports.  Even if it doesn't, there are "easy" process workarounds to 
achieve the same effect.  Just as an example (i.e. definitely not 
proposing this as technical solution to be implemented right now), we 
could introduce a new branch in git called e.g. "buildbot-tracking-slow" 
and have a specific fast builder do a fast forward merge from main into 
this branch.  All "slow" builders would simply follow this branch and 
not main.

If we get consensus that this is the right approach, I am willing to put 
some of my own time to figuring out how to implement this. For my own 
volunteer time, I'd probably start with the flaky bot test commit piece 
just because that's much easier to do manually first and then automate, 
and because I find them personally more annoying.

Your point about disabling notification on a blamelist larger than 5 
seems reasonable to me, but I'd definitely consider "build chunks of no 
more than N commits" and "build arbitrary sets, but only notify if less 
than M people" as distinct possibilities to be evaluated independently.

>
> Cheers,
>
> -- 
> Mehdi
>
>     Philip
>
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