|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.
|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.