Based on kernel version 4.8. Page generated on 2016-10-06 23:18 EST.
1 Each CPU has a "base" scheduling domain (struct sched_domain). The domain 2 hierarchy is built from these base domains via the ->parent pointer. ->parent 3 MUST be NULL terminated, and domain structures should be per-CPU as they are 4 locklessly updated. 5 6 Each scheduling domain spans a number of CPUs (stored in the ->span field). 7 A domain's span MUST be a superset of it child's span (this restriction could 8 be relaxed if the need arises), and a base domain for CPU i MUST span at least 9 i. The top domain for each CPU will generally span all CPUs in the system 10 although strictly it doesn't have to, but this could lead to a case where some 11 CPUs will never be given tasks to run unless the CPUs allowed mask is 12 explicitly set. A sched domain's span means "balance process load among these 13 CPUs". 14 15 Each scheduling domain must have one or more CPU groups (struct sched_group) 16 which are organised as a circular one way linked list from the ->groups 17 pointer. The union of cpumasks of these groups MUST be the same as the 18 domain's span. The intersection of cpumasks from any two of these groups 19 MUST be the empty set. The group pointed to by the ->groups pointer MUST 20 contain the CPU to which the domain belongs. Groups may be shared among 21 CPUs as they contain read only data after they have been set up. 22 23 Balancing within a sched domain occurs between groups. That is, each group 24 is treated as one entity. The load of a group is defined as the sum of the 25 load of each of its member CPUs, and only when the load of a group becomes 26 out of balance are tasks moved between groups. 27 28 In kernel/sched/core.c, trigger_load_balance() is run periodically on each CPU 29 through scheduler_tick(). It raises a softirq after the next regularly scheduled 30 rebalancing event for the current runqueue has arrived. The actual load 31 balancing workhorse, run_rebalance_domains()->rebalance_domains(), is then run 32 in softirq context (SCHED_SOFTIRQ). 33 34 The latter function takes two arguments: the current CPU and whether it was idle 35 at the time the scheduler_tick() happened and iterates over all sched domains 36 our CPU is on, starting from its base domain and going up the ->parent chain. 37 While doing that, it checks to see if the current domain has exhausted its 38 rebalance interval. If so, it runs load_balance() on that domain. It then checks 39 the parent sched_domain (if it exists), and the parent of the parent and so 40 forth. 41 42 Initially, load_balance() finds the busiest group in the current sched domain. 43 If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in 44 that group. If it manages to find such a runqueue, it locks both our initial 45 CPU's runqueue and the newly found busiest one and starts moving tasks from it 46 to our runqueue. The exact number of tasks amounts to an imbalance previously 47 computed while iterating over this sched domain's groups. 48 49 *** Implementing sched domains *** 50 The "base" domain will "span" the first level of the hierarchy. In the case 51 of SMT, you'll span all siblings of the physical CPU, with each group being 52 a single virtual CPU. 53 54 In SMP, the parent of the base domain will span all physical CPUs in the 55 node. Each group being a single physical CPU. Then with NUMA, the parent 56 of the SMP domain will span the entire machine, with each group having the 57 cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example, 58 might have just one domain covering its one NUMA level. 59 60 The implementor should read comments in include/linux/sched.h: 61 struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of 62 the specifics and what to tune. 63 64 Architectures may retain the regular override the default SD_*_INIT flags 65 while using the generic domain builder in kernel/sched/core.c if they wish to 66 retain the traditional SMT->SMP->NUMA topology (or some subset of that). This 67 can be done by #define'ing ARCH_HASH_SCHED_TUNE. 68 69 Alternatively, the architecture may completely override the generic domain 70 builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your 71 arch_init_sched_domains function. This function will attach domains to all 72 CPUs using cpu_attach_domain. 73 74 The sched-domains debugging infrastructure can be enabled by enabling 75 CONFIG_SCHED_DEBUG. This enables an error checking parse of the sched domains 76 which should catch most possible errors (described above). It also prints out 77 the domain structure in a visual format.