Ubuntu 15.10 unable to detect all CPUs

Question

lscpu lists the following

 venki@venki-H8DCL:~$ lscpu
 Architecture:          x86_64
 CPU op-mode(s):        32-bit, 64-bit
 Byte Order:            Little Endian
 CPU(s):                12
 On-line CPU(s) list:   0-11
 Thread(s) per core:    2
 Core(s) per socket:    3   <------ I have 6 cores
 Socket(s):             2
 NUMA node(s):          2
 Vendor ID:             AuthenticAMD
 CPU family:            21
 Model:                 2
 Model name:            AMD Opteron(tm) Processor 4334
 Stepping:              0
 CPU MHz:               1400.000
 CPU max MHz:           3100.0000
 CPU min MHz:           1400.0000
 BogoMIPS:              6199.90
 Virtualization:        AMD-V
 L1d cache:             16K
 L1i cache:             64K
 L2 cache:              2048K
 L3 cache:              6144K
 NUMA node0 CPU(s):     0-5
 NUMA node1 CPU(s):     6-11

Answer 1

AMD's specs indicate that CPU is six cores and six threads, but you're seeing three cores with two threads each.

Combine that with what appears to be a dual-socket board and your total "CPU count" (4th line), and it does appear to be detecting them all, it's just not displaying the totals the way you expect.

Answer 2

According to the 4th line you have 12 CPU(s).

Maybe you are looking for this information:

cat /proc/cpuinfo