hyperthreading not detected by linux

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http://unix.stackexchange.com – I have a Dell server with two cpus, each cpu has 6 cores, and each core is hyperthreaded (i.e. should be equivalent to two virtual cores). I installed CentOS 6.2 on this server and it seems to detect only 12 cores (although there should be 24 virtual cores altogether). When I look at /proc/cpuinfo I get for each cpu: cpu cores: 6 siblings: 6 Which seems to indicate that number of cores equals number of virtual cores, or hyperthreading not detected/enabled. When I run dmidecode I can see that HT flag is turned on and I do see the following, which seem to indicate that the bios is configure (HowTos)