Bryan Steele 
Theo de Raadt has introduced a new hw.blockcpu sysctl to #OpenBSD -current to offer more control over which CPU core types (Performance, Efficiency, and SMT) are available to schedule processes on. Modern Intel (and ARM) CPUs additionally have slower LP-E (low-power) cores that severely hinder system performance.
deraadt@ modified src/sys/*: Some new intel machines have a new 3rd tier of cpus called LP-E which are E-core (Atom) without L3 cache. These cpus are Lethargic, and it sucks when processes migrate to them.
This introduces sysctl hw.blockcpu= which takes a sequence of 4 letters.
S (for SMT), P (regular performance cpu), E (efficient cpu) generally 80% to 50% as fast), and L (lethargic cpu) which are even slower.
By setting this, you can select cpus to kick out of the scheduler. The default is SL.
The hw.smt sysctl remains for now but we will eventually delete it.
hw.smt changes and follows hw.blockcpu=S.
ok kettenis mlarkin
Justine Smithies