Re: detecting hyperthreading in linux 2.4.19

[Mikael Pettersson <mikpe@csd.uu.se>]

James Cleverdon writes:
 > On Thursday 09 January 2003 12:02 pm, Jason Lunz wrote:
 > > Is there a way for a userspace program running on linux 2.4.19 to tell
 > > the difference between a single hyperthreaded xeon P4 with HT enabled
 > > and a dual hyperthreaded xeon P4 with HT disabled? The /proc/cpuinfos
 > > for the two cases are indistinguishable.
 > >
 > > Jason
 > >
 > > -
 > 
 > In the kernel that's no problem:
 > 
 > A) If the BIOS writers followed Intel's guidelines, just look at the physical 
 > APIC IDs.  HT siblings have odd IDs, the real ones have even.
 > 
 > B) Check the siblings table built up at boot time and used by the scheduler.
 > 
 > I don't know of any way to do this in userland.  The whole point is that the 
 > sibling processors are supposed to look like real ones.

If the kernel has sched_setaffinity() or some other way of binding a process
to a given CPU (as numbered by the kernel, which may or may not be related
to any physical CPU numbers), then this will do it: execute CPUID on each
CPU and check the initial APIC ID field. If you find one that's non-zero,
then HT is enabled.

My performance monitoring counters driver uses this approach in kernel-space
using smp_call_function(). I don't use the siblings tables because they suck :-)
[I don't think they distinguish between logical CPUs #0 and #1, and they aren't
exported to modules. The CPUID check is simple and portable across kernel versions.]