Abysmal scheduler performance in Linus' tree?

[Andy Lutomirski]

I'm running e7d0c41ecc2e372a81741a30894f556afec24315 from Linus' tree
today, and I'm seeing abysmal scheduler performance.  Running make -j4
ends up with all the tasks on CPU 3 most of the time (on my
4-logical-thread laptop).  taskset -c 0 whatever puts whatever on CPU
0, but plain while true; do true; done puts the infinite loop on CPU 3
right along with the make -j4 tasks.

This is on Fedora 26, and I don't think I'm doing anything weird.
systemd has enabled the cpu controller, but it doesn't seem to have
configured anything or created any non-root cgroups.

Just a heads up.  I haven't tried to diagnose it at all.