linux-next/Documentation/cputopology.txt

Export cpu topology info via sysfs. Items (attributes) are similar
to /proc/cpuinfo.

1) /sys/devices/system/cpu/cpuX/topology/physical_package_id:
represent the physical package id of  cpu X;
2) /sys/devices/system/cpu/cpuX/topology/core_id:
represent the cpu core id to cpu X;
3) /sys/devices/system/cpu/cpuX/topology/thread_siblings:
represent the thread siblings to cpu X in the same core;
4) /sys/devices/system/cpu/cpuX/topology/core_siblings:
represent the thread siblings to cpu X in the same physical package;

To implement it in an architecture-neutral way, a new source file,
drivers/base/topology.c, is to export the 4 attributes.

For an architecture to support this feature, it must define some of
these macros in include/asm-XXX/topology.h:
#define topology_physical_package_id(cpu)
#define topology_core_id(cpu)
#define topology_thread_siblings(cpu)
#define topology_core_siblings(cpu)

The type of **_id is int.
The type of siblings is cpumask_t.

To be consistent on all architectures, include/linux/topology.h
provides default definitions for any of the above macros that are
not defined by include/asm-XXX/topology.h:
1) physical_package_id: -1
2) core_id: 0
3) thread_siblings: just the given CPU
4) core_siblings: just the given CPU

Additionally, cpu topology information is provided under
/sys/devices/system/cpu and includes these files.  The internal
source for the output is in brackets ("[]").

    kernel_max: the maximum cpu index allowed by the kernel configuration.
		[NR_CPUS-1]

    offline:	cpus that are not online because they have been
		HOTPLUGGED off (see cpu-hotplug.txt) or exceed the limit
		of cpus allowed by the kernel configuration (kernel_max
		above). [~cpu_online_mask + cpus >= NR_CPUS]

    online:	cpus that are online and being scheduled [cpu_online_mask]

    possible:	cpus that have been allocated resources and can be
		brought online if they are present. [cpu_possible_mask]

    present:	cpus that have been identified as being present in the
		system. [cpu_present_mask]

The format for the above output is compatible with cpulist_parse()
[see <linux/cpumask.h>].  Some examples follow.

In this example, there are 64 cpus in the system but cpus 32-63 exceed
the kernel max which is limited to 0..31 by the NR_CPUS config option
being 32.  Note also that cpus 2 and 4-31 are not online but could be
brought online as they are both present and possible.

     kernel_max: 31
        offline: 2,4-31,32-63
         online: 0-1,3
       possible: 0-31
        present: 0-31

In this example, the NR_CPUS config option is 128, but the kernel was
started with possible_cpus=144.  There are 4 cpus in the system and cpu2
was manually taken offline (and is the only cpu that can be brought
online.)

     kernel_max: 127
        offline: 2,4-127,128-143
         online: 0-1,3
       possible: 0-127
        present: 0-3

See cpu-hotplug.txt for the possible_cpus=NUM kernel start parameter
as well as more information on the various cpumask's.