Core ids are not consecutive

I encountered a weird situation on a 28-core Intel Skylake system (kernel: 4.4.0-121-generic #145-Ubuntu SMP, hyperthreading is disabled):

cat /proc/cpuinfo |grep "core id"
core id     : 0
core id     : 1
core id     : 2
core id     : 3
core id     : 4
core id     : 5
core id     : 6
core id     : 8
core id     : 9
core id     : 10
core id     : 11
core id     : 12
core id     : 13
core id     : 14
core id     : 16
core id     : 17
core id     : 18
core id     : 19
core id     : 20
core id     : 21
core id     : 22
core id     : 24
core id     : 25
core id     : 26
core id     : 27
core id     : 28
core id     : 29
core id     : 30

Core ids 7, 15 and 23 are missing. Do you have any idea?