To implement it in an architecture-neutral way, a new source file,
drivers/base/topology.c, is to export the 4 attributes.
-If one architecture wants to support this feature, it just needs to
-implement 4 defines, typically in file include/asm-XXX/topology.h.
-The 4 defines are:
+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)
+#define topology_thread_cpumask(cpu)
+#define topology_core_cpumask(cpu)
The type of **_id is int.
-The type of siblings is cpumask_t.
+The type of siblings is (const) struct cpumask *.
-To be consistent on all architectures, the 4 attributes should have
-default values if their values are unavailable. Below is the rule.
-1) physical_package_id: If cpu has no physical package id, -1 is the
-default value.
-2) core_id: If cpu doesn't support multi-core, its core id is 0.
-3) thread_siblings: Just include itself, if the cpu doesn't support
-HT/multi-thread.
-4) core_siblings: Just include itself, if the cpu doesn't support
-multi-core and HT/Multi-thread.
+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
-So be careful when declaring the 4 defines in include/asm-XXX/topology.h.
+Additionally, cpu topology information is provided under
+/sys/devices/system/cpu and includes these files. The internal
+source for the output is in brackets ("[]").
-If an attribute isn't defined on an architecture, it won't be exported.
+ 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.
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