Neat: but isn't this a type-1 hypervisor?

I do seem to recall that it was for "locking complexity" reasons. If I recall correctly, around this time, there was the BKL and relatively fewer locks. With even just a BKL, it could scale to 2 to 4 cores/cpus with a lot of typical workloads. There was too much contention for the kernel to scale up to even the 12 to 16 core and beyond range effectively. Several people were of the opinion that Sun Solaris and others had their locks too fine-grained. For this reason, I think they tried to be very cautious in breaking up coarse-grained locks for finer-grained locks; they tried requiring that there were measurements on realistic loads that a lock was having contention or latency issues before they accepted patches to break it up. They tried to avoid too much locking complexity and over-head.

I don't know enough to have an opinion on how Linux kernel was able to scale as successfully as it has. There were certainly doubts in the past. If I recall correctly, RCU was being used in other kernels before it was introduced in Linux, but I don't recall which ones.