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-Each CPU has a "base" scheduling domain (struct sched_domain). 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, trigger_load_balance() is run periodically on each CPU
-through scheduler_tick(). It raises a softirq after the next regularly scheduled
-rebalancing event for the current runqueue has arrived. The actual load
-balancing workhorse, run_rebalance_domains()->rebalance_domains(), is then run
-in softirq context (SCHED_SOFTIRQ).
-The latter function takes two arguments: the current CPU and whether it was idle
-at the time the scheduler_tick() happened and iterates over all sched domains
-our CPU is on, starting from its base domain and going up the ->parent chain.
-While doing that, it checks to see if the current domain has exhausted its
-rebalance interval. If so, it runs load_balance() on that domain. It then checks
-the parent sched_domain (if it exists), and the parent of the parent and so
-Initially, load_balance() finds the busiest group in the current sched domain.
-If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in
-that group. If it manages to find such a runqueue, it locks both our initial
-CPU's runqueue and the newly found busiest one and starts moving tasks from it
-to our runqueue. The exact number of tasks amounts to an imbalance previously
-computed while iterating over this sched domain's groups.
-*** 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.
-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.
-The sched-domains debugging infrastructure can be enabled by enabling
-CONFIG_SCHED_DEBUG. 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.