I am a little bit surprised with the following information that I get with dmidecode:

srs@ubuntu:~$ sudo dmidecode -t processor

# dmidecode 2.9

SMBIOS 2.6 present.



Handle 0x0004, DMI type 4, 42 bytes

Processor Information

    Socket Designation: LGA1155

    Type: Central Processor

    Family: <OUT OF SPEC>

    Manufacturer: Intel            

    ID: A7 06 02 00 FF FB EB BF

    Version: Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz       

    Voltage: 1.0 V

    External Clock: 100 MHz

    Max Speed: 3800 MHz

    Current Speed: 3300 MHz

    Status: Populated, Enabled

    Upgrade: Other

    L1 Cache Handle: 0x0005

    L2 Cache Handle: 0x0006

    L3 Cache Handle: 0x0007

    Serial Number: To Be Filled By O.E.M.

    Asset Tag: To Be Filled By O.E.M.

    Part Number: To Be Filled By O.E.M.

    Core Count: 4

    Core Enabled: 1

    Characteristics:

        64-bit capable

However from gnome-system-monitor I can see 4 CPU working at the same time.