Node assignments pessimized in multitargeted->multitargeted refs with `BuildProjectReferences=false`

[rainersigwald]

Given a project that itself multitargets and references several multitargeted projects, like pessimized_node_assignments.zip, the scheduler does a terrible job spreading the work around among nodes when the referencing project (here Aggregate\Aggregate.csproj) is built with -p:BuildProjectReferences=false -m:3 -nr:false.

The true optimal node count and scheduling for such a system may not be knowable, but number_of_nodes = number_of_TargetFrameworks seems like a very reasonable guess.

From my debugging, what's going wrong is that we have these dependencies (filtered to a single reference):

graph TD
subgraph Aggregate
    Aggregate_outer
    Aggregate_net5
    Aggregate_net6
    Aggregate_net7
end
subgraph Lib1
    Lib1_outer
    Lib1_net5
    Lib1_net6
    Lib1_net7
end
Aggregate_outer -->|Build| Aggregate_net5
Aggregate_outer -->|Build| Aggregate_net6
Aggregate_outer -->|Build| Aggregate_net7
Aggregate_net5 -->|GetTargetFrameworks| Lib1_outer
Aggregate_net6 -->|GetTargetFrameworks| Lib1_outer
Aggregate_net7 -->|GetTargetFrameworks| Lib1_outer

Lib1_outer -->|GetTargetFrameworksWithPlatformForSingleTargetFramework| Lib1_net5
Lib1_outer -->|GetTargetFrameworksWithPlatformForSingleTargetFramework| Lib1_net6
Lib1_outer -->|GetTargetFrameworksWithPlatformForSingleTargetFramework| Lib1_net7

Aggregate_net5 -->|GetTargetPath| Lib1_net5
Aggregate_net6 -->|GetTargetPath| Lib1_net6
Aggregate_net7 -->|GetTargetPath| Lib1_net7

Loading

Unfortunately, the calls to GetTargetFrameworksWithPlatformForSingleTargetFramework are all getting assigned to node 1, locking the inner builds for all referenced projects to that node. That node is also used to do actual build work for one of the inner builds of Aggregate, which blocks work that should be able to run in parallel for the other inner builds:

@dfederm has observed worse cascading where all of the inner builds were serialized on real-world projects.