It is relatively easy for tasks to occupy all of the s390x builders, causing many hours of delay before a koji build which has finished everywhere else will even pick up a s390x builder. I know koji has a priority mechanism to prefer regular builds over scratch builds, but unfortunately (as Koschei has experienced) it's still far to easy to clog up the buildsys with scratch builds, preventing people from getting work done.

Koschei limits its scratch building to aarch64, x86_64 and ppc64 for basically this reason. While it doesn't result in a 100% complete test, it does provide useful testing without delaying regular builds too much. Koschei also appears to have additional logic to avoid congesting the system by making sure that there will be excess builder capacity even after it submits its builds.

So I'd suggest either limiting the build arch list or adopting what Koschei has done to avoid tying up all of the resources. Or maybe both.