Here are the uperf results of the various patchsets. Note, that disabling smt is better for these tests and that that presumably reflects the overall overhead of core scheduling which went from bad to really bad. The primary focus in this email is to start to understand what happened within core sched itself.
patchset smt=on/cs=off smt=off smt=on/cs=on
--------------------------------------------------------
v5-v5.6.y : 1.78Gb/s 1.57Gb/s 1.07Gb/s
pre-v6-v5.6.y : 1.75Gb/s 1.55Gb/s 822.16Mb/s
v6-5.7 : 1.87Gs/s 1.56Gb/s 561.6Mb/s
v6-5.7-hotplug : 1.75Gb/s 1.58Gb/s 438.21Mb/s
v7 : 1.80Gb/s 1.61Gb/s 440.44Mb/s
I haven't had a chance to play with v7, but I got something different.
branch smt=on/cs=on
coresched/v5-v5.6.y 1.09Gb/s
coresched/v6-v5.7.y 1.05Gb/s
I attached my kernel config in case you want to make a comparison, or you
can send yours, I'll try to see I can replicate your result.