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Diffstat (limited to 'Documentation/scheduler/sched-domains.txt')
| -rw-r--r-- | Documentation/scheduler/sched-domains.txt | 77 |
1 files changed, 0 insertions, 77 deletions
diff --git a/Documentation/scheduler/sched-domains.txt b/Documentation/scheduler/sched-domains.txt deleted file mode 100644 index 4af80b1c05aa..000000000000 --- a/Documentation/scheduler/sched-domains.txt +++ /dev/null @@ -1,77 +0,0 @@ -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 -CPUs". - -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/core.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 -forth. - -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/core.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. |