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.