[ Please note the cross-post and respect the Reply-To... ] Hi folks, This has taken a while in coming, for which I apologise. There's a lot of work involved in rebuilding the whole Debian archive, and many many hours spent analysing the results. You learn quite a lot, too! :-) I promised way back before DC18 that I'd publish the results of the rebuilds that I'd just started. Here they are, after a few false starts. I've been rebuilding the archive *specifically* to check if we would have any problems building our 32-bit Arm ports (armel and armhf) using 64-bit arm64 hardware. I might have found other issues too, but that was my goal. The logs for all my builds are online at https://www.einval.com/debian/arm/rebuild-logs/ for reference. See in particular https://www.einval.com/debian/arm/rebuild-logs/armel/FAIL/FAIL.html https://www.einval.com/debian/arm/rebuild-logs/armhf/FAIL/FAIL.html for automated analysis of the build logs that I've used as the basis for the stats below. Executive summary ================= As far as I can see we're basically fine to use arm64 hosts for building armel and armhf, *so long as* those hosts include hardware support for the 32-bit A32 instruction set. As I've mentioned before  that's not a given on *all* arm64 machines, but there are sufficient machine types available that I think we should be fine. There are a couple of things we need to do in terms of setup - see "Machine configuration" below.  https://lists.debian.org/debian-arm/2018/06/msg00062.html Methodology =========== I (naïvely) just attempted to rebuild all the source packages in unstable main, at first using pbuilder to control the build process and then later using sbuild instead. I didn't think to check on the stated architectures listed for the source packages, which was a mistake - I would do it differently if redoing this test. That will have contributed quite a large number of failures in the stats below, but I believe I have accounted for them in my analysis. I built lots of packages, using a range of machines in a small build farm at home: * Macchiatobin * Seattle * Synquacer * Multiple Mustangs using my local mirror for improved performance when fetching build-deps etc. I started off with a fixed list of packages that were in unstable when I started each rebuild, for the sake of simplicity. That's one reason why I have two different numbers of source packages attempted for each arch below. If packages failed due to no longer being available, I simply requeued using the latest version in unstable at that point. I then developed a script to scan the logs of failed builds to pick up on patterns that matched with obvious causes. Once that was done, I worked through all the failures to (a) verify those patterns, and (b) identify any other failures. I've classified many of the failures to make sense of the results. I've also scanned the BTS for existing bugs matching my failed builds (and linked to them), or filed new bugs where I could not find matches. I did *not* investigate fully every build failure. For example, where a package has never been built before on armel or armhf and failed here I simply noted that fact. Many of those are probably real bugs, but beyond the scope of my testing. For reference, all my scripts and config are in git at https://git.einval.com/cgi-bin/gitweb.cgi?p=buildd-scripts.git armel results ============= Total source packages attempted: 28457 Successfully built: 25827 Failed: 2630 Almost half of the failed builds were simply due to the lack of a single desired build dependency (nodejs:armel, 1289). There were a smattering of other notable causes: * 100 log(s) showing build failures (java/javadoc) Java build failures seem particularly opaque (to me!), and in many cases I couldn't ascertain if it was a real build problem or just maven being flaky. :-( * 15 log(s) showing Go 32-bit integer overflow Quite a number of go packages are blindly assuming sizes for 64-bit hosts. That's probably fair, but seems unfortunate. * 8 log(s) showing Sbuild build timeout I was using quite a generous timeout (12h) with sbuild, but still a very small number of packages failed. I'd earlier abandoned pbuilder for sbuild as I could not get it to behave sensibly with timeouts. The stats that matter are the arch-specific failures for armel: * 13 log(s) showing Alignment problem * 5 log(s) showing Segmentation fault * 1 log showing Illegal instruction and the new bugs I filed: * 3 bugs for arch misdetection * 8 bugs for alignment problems * 4 bugs for arch-specific test failures * 3 bugs for arch-specific misc failures Considering the number of package builds here, I think these numbers are basically noise. The vast majority of the failures I found were either already known in the BTS (260), unrelated to what I was looking for, or both. See below for more details about host configuration for armel builds. armhf results ============= Total source packages attempted: 28056 Successfully built: 26772 Failed: 1284 FTAOD: I attempted fewer package builds for armhf as we just had a smaller number of packages when I started that rebuild. A few weeks later, it seems we had a few hundred more source packages for the armel rebuild. The armhf rebuild showed broadly the same percentage of failures, if you take into account the nodejs difference - it exists in the armhf archive, so many hundreds more packages could build using it. In a similar vein for notable failures: * 89 log(s) showing build failures (java/javadoc) Similar problems, I guess... * 15 log(s) showing Go 32-bit integer overflow That's the same as for armel, I'm assuming they're the same packages without checking! * 4 log(s) showing Sbuild build timeout Only 4 timeouts compared to the 8 for armel. *Maybe* a sign that armhf will be slightly quicker in build time, so less likely to hit a timeout? Total guesswork on small-number stats! :-) Arch-specific failures found for armhf: * 11 log(s) showing Alignment problem * 4 log(s) showing Segmentation fault * 1 log(s) showing Illegal instruction and the new bugs I filed: • 1 bugs for arch misdetection • 8 bugs for alignment problems • 10 bugs for arch-specific test failures • 3 bugs for arch-specific misc failures Again, these small numbers tell me that we're fine. I liked to 139 existing bugs in the BTS here. Machine configuration ===================== To be able to support 32-bit builds on arm64 hardware, there are a few specific hardware support issues to consider: * Our 32-bit Arm kernels are configured to fix up userspace alignment faults, which hides lazy programming at the cost of a (sometimes massive) slowdown in performance when this triggers. The arm64 kernel *cannot* be configured to do this - if a userspace program triggers an alignment exception, it will simply be handed a SIGBUS from the kernel. This was one of the main things I was looking for here, common to both armel and armhf. In the end, I only found a very small number of problems. Given that, I think we should *immediately* turn off the alignment fixups on our existing 32-bit Arm buildd machines. Let's flush out any more problems early, and I don't expect to see many. To give credit here: Ubuntu have been using arm64 machines for building 32-bit Arm packages for a while now, and have already been filing bugs with patches which will have helped reduce this problem. Thanks! * In theory(!), that's all we should need to worry about for armhf, but our armel software baseline needs two additional pieces of configuration to make things work, enabling emulation for: + SWP (low-level locking primitive, deprecated since ARMv6 AFAIK) + CP15 barriers (low-level barrier primitives, deprecated since ARMv7) Again, there is quite a performance cost to enabling these but they are at least possible! In my initial testing for rebuilding armhf only, I did not enable either of these. I was then finding *lots* of "Illegal Instruction" crashes due to CP15 barrier usage in armhf Haskell and Mono programs. This suggests that the baseline architecture in these toolchains is incorrectly set to target ARMv6 rather than ARMv7. That should be fixed and all those packages rebuilt at some point. Bug highlights ============== * In the glibc build, we found an arm64 kernel bug (#904385) which has since been fixed upstream thanks to Will Deacon at Arm. I've backported the fix into 4.9-stable so the fix will be in our Stretch kernels soon. * There's something really weird happening with Vim (#917859). It FTBFS for me with an odd test failure for both armel-on-arm64 and armhf-on-arm64 *using sbuild*, but in a porter box chroot or directly on my hardware using debuild it works just fine. Confusing... * I've filed quite a number of bugs over the last few weeks. Many are generic new FTBFS reports for old packages that haven't been rebuilt in a while, and some of them look unmaintained. However, quite a few of my bugs are arch-specific ones in better-maintained packages and several have already been fixed * Yesterday, I filed a slew of identical-looking reports for packages using MPI and all failing tests. It seems that we have a real problem hitting openmpi-based packages across the archive at the moment (#918157 in libpmix2). I'm going to verify that on my systems shortly. Thanks ====== I've spent a lot of time looking at existing FTBFS bugs over the last weeks, to compare results against what I've been seeing. Much kudos to people who have been finding and filing those bugs ahead of me, in particular Adrian Bunk and Matthias Klose who have filed *many* such bugs. Also thanks to Helmut Grohne for his script to pull down a summary of FTBFS bugs from UDD - that saved many hours of effort! Please let me know if you think you've found a problem in what I've done, or how I've analysed the results here. I still have my machines set up for easy rebuilds, so reproducing things and testing fixes is quite easy - just ask! -- Steve McIntyre, Cambridge, UK. email@example.com Armed with "Valor": "Centurion" represents quality of Discipline, Honor, Integrity and Loyalty. Now you don't have to be a Caesar to concord the digital world while feeling safe and proud.
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