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+Each CPU has a "base" scheduling domain (struct sched_domain). These are
+accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain
+hierarchy is built from these base domains via the ->parent pointer. ->parent
+MUST be NULL terminated, and domain structures should be per-CPU as they
+are locklessly updated.
+Each scheduling domain spans a number of CPUs (stored in the ->span field).
+A domain's span MUST be a superset of it child's span (this restriction could
+be relaxed if the need arises), and a base domain for CPU i MUST span at least
+i. The top domain for each CPU will generally span all CPUs in the system
+although strictly it doesn't have to, but this could lead to a case where some
+CPUs will never be given tasks to run unless the CPUs allowed mask is
+explicitly set. A sched domain's span means "balance process load among these
+Each scheduling domain must have one or more CPU groups (struct sched_group)
+which are organised as a circular one way linked list from the ->groups
+pointer. The union of cpumasks of these groups MUST be the same as the
+domain's span. The intersection of cpumasks from any two of these groups
+MUST be the empty set. The group pointed to by the ->groups pointer MUST
+contain the CPU to which the domain belongs. Groups may be shared among
+CPUs as they contain read only data after they have been set up.
+Balancing within a sched domain occurs between groups. That is, each group
+is treated as one entity. The load of a group is defined as the sum of the
+load of each of its member CPUs, and only when the load of a group becomes
+out of balance are tasks moved between groups.
+In kernel/sched.c, rebalance_tick is run periodically on each CPU. This
+function takes its CPU's base sched domain and checks to see if has reached
+its rebalance interval. If so, then it will run load_balance on that domain.
+rebalance_tick then checks the parent sched_domain (if it exists), and the
+parent of the parent and so forth.
+*** Implementing sched domains ***
+The "base" domain will "span" the first level of the hierarchy. In the case
+of SMT, you'll span all siblings of the physical CPU, with each group being
+a single virtual CPU.
+In SMP, the parent of the base domain will span all physical CPUs in the
+node. Each group being a single physical CPU. Then with NUMA, the parent
+of the SMP domain will span the entire machine, with each group having the
+cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example,
+might have just one domain covering its one NUMA level.
+The implementor should read comments in include/linux/sched.h:
+struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of
+the specifics and what to tune.
+For SMT, the architecture must define CONFIG_SCHED_SMT and provide a
+cpumask_t cpu_sibling_map[NR_CPUS], where cpu_sibling_map[i] is the mask of
+all "i"'s siblings as well as "i" itself.
+Architectures may retain the regular override the default SD_*_INIT flags
+while using the generic domain builder in kernel/sched.c if they wish to
+retain the traditional SMT->SMP->NUMA topology (or some subset of that). This
+can be done by #define'ing ARCH_HASH_SCHED_TUNE.
+Alternatively, the architecture may completely override the generic domain
+builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your
+arch_init_sched_domains function. This function will attach domains to all
+CPUs using cpu_attach_domain.
+Implementors should change the line
+in kernel/sched.c as this enables an error checking parse of the sched domains
+which should catch most possible errors (described above). It also prints out
+the domain structure in a visual format.

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