FreeBSD/Linux Kernel Cross Reference
sys/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:
     
             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:
    
            internel kernel map of cpuX's hardware threads within the same
            core as cpuX
    
    5) /sys/devices/system/cpu/cpuX/topology/core_siblings:
    
            internal kernel map of cpuX's hardware threads within the same
            physical_package_id.
    
    6) /sys/devices/system/cpu/cpuX/topology/book_siblings:
    
            internal kernel map 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 4 or 6 attributes. The two 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_thread_cpumask(cpu)
    #define topology_core_cpumask(cpu)
    #define topology_book_cpumask(cpu)
    
    The type of **_id is int.
    The type of siblings is (const) struct cpumask *.
    
    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
    
    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.

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