Export CPU topology info via sysfs. Items (attributes) are similar

  to /proc/cpuinfo output of some architectures:

  
  1) /sys/devices/system/cpu/cpuX/topology/physical_package_id:

  
  	physical package id of cpuX. Typically corresponds to a physical
  	socket number, but the actual value is architecture and platform
  	dependent.

  2) /sys/devices/system/cpu/cpuX/topology/core_id:

  
  	the CPU core ID of cpuX. Typically it is the hardware platform's
  	identifier (rather than the kernel's).  The actual value is
  	architecture and platform dependent.

  3) /sys/devices/system/cpu/cpuX/topology/book_id:
  
  	the book ID of cpuX. Typically it is the hardware platform's
  	identifier (rather than the kernel's).	The actual value is
  	architecture and platform dependent.
  
  4) /sys/devices/system/cpu/cpuX/topology/thread_siblings:

  	internal kernel map of cpuX's hardware threads within the same

  	core as cpuX.

  5) /sys/devices/system/cpu/cpuX/topology/thread_siblings_list:
  
  	human-readable list of cpuX's hardware threads within the same
  	core as cpuX.
  
  6) /sys/devices/system/cpu/cpuX/topology/core_siblings:

  
  	internal kernel map of cpuX's hardware threads within the same
  	physical_package_id.

  7) /sys/devices/system/cpu/cpuX/topology/core_siblings_list:
  
  	human-readable list of cpuX's hardware threads within the same
  	physical_package_id.
  
  8) /sys/devices/system/cpu/cpuX/topology/book_siblings:

  
  	internal kernel map of cpuX's hardware threads within the same
  	book_id.

  9) /sys/devices/system/cpu/cpuX/topology/book_siblings_list:
  
  	human-readable list of cpuX's hardware threads within the same
  	book_id.

  To implement it in an architecture-neutral way, a new source file,

  drivers/base/topology.c, is to export the 6 or 9 attributes. The three book

  related sysfs files will only be created if CONFIG_SCHED_BOOK is selected.

  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_book_id(cpu)

  #define topology_sibling_cpumask(cpu)

  #define topology_core_cpumask(cpu)

  #define topology_book_cpumask(cpu)

  The type of **_id macros is int.
  The type of **_cpumask macros is (const) struct cpumask *. The latter
  correspond with appropriate **_siblings sysfs attributes (except for
  topology_sibling_cpumask() which corresponds with thread_siblings).

  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) sibling_cpumask: just the given CPU
  4) core_cpumask: just the given CPU

  For architectures that don't support books (CONFIG_SCHED_BOOK) there are no
  default definitions for topology_book_id() and topology_book_cpumask().

  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 cpumasks.