Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/Documentation/sched-domains.txt
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Tag kernel26-2.6.12-alx-r9
1 | Each CPU has a "base" scheduling domain (struct sched_domain). These are |
2 | accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain |
3 | hierarchy is built from these base domains via the ->parent pointer. ->parent |
4 | MUST be NULL terminated, and domain structures should be per-CPU as they |
5 | are locklessly updated. |
6 | |
7 | Each scheduling domain spans a number of CPUs (stored in the ->span field). |
8 | A domain's span MUST be a superset of it child's span (this restriction could |
9 | be relaxed if the need arises), and a base domain for CPU i MUST span at least |
10 | i. The top domain for each CPU will generally span all CPUs in the system |
11 | although strictly it doesn't have to, but this could lead to a case where some |
12 | CPUs will never be given tasks to run unless the CPUs allowed mask is |
13 | explicitly set. A sched domain's span means "balance process load among these |
14 | CPUs". |
15 | |
16 | Each scheduling domain must have one or more CPU groups (struct sched_group) |
17 | which are organised as a circular one way linked list from the ->groups |
18 | pointer. The union of cpumasks of these groups MUST be the same as the |
19 | domain's span. The intersection of cpumasks from any two of these groups |
20 | MUST be the empty set. The group pointed to by the ->groups pointer MUST |
21 | contain the CPU to which the domain belongs. Groups may be shared among |
22 | CPUs as they contain read only data after they have been set up. |
23 | |
24 | Balancing within a sched domain occurs between groups. That is, each group |
25 | is treated as one entity. The load of a group is defined as the sum of the |
26 | load of each of its member CPUs, and only when the load of a group becomes |
27 | out of balance are tasks moved between groups. |
28 | |
29 | In kernel/sched.c, rebalance_tick is run periodically on each CPU. This |
30 | function takes its CPU's base sched domain and checks to see if has reached |
31 | its rebalance interval. If so, then it will run load_balance on that domain. |
32 | rebalance_tick then checks the parent sched_domain (if it exists), and the |
33 | parent of the parent and so forth. |
34 | |
35 | *** Implementing sched domains *** |
36 | The "base" domain will "span" the first level of the hierarchy. In the case |
37 | of SMT, you'll span all siblings of the physical CPU, with each group being |
38 | a single virtual CPU. |
39 | |
40 | In SMP, the parent of the base domain will span all physical CPUs in the |
41 | node. Each group being a single physical CPU. Then with NUMA, the parent |
42 | of the SMP domain will span the entire machine, with each group having the |
43 | cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example, |
44 | might have just one domain covering its one NUMA level. |
45 | |
46 | The implementor should read comments in include/linux/sched.h: |
47 | struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of |
48 | the specifics and what to tune. |
49 | |
50 | For SMT, the architecture must define CONFIG_SCHED_SMT and provide a |
51 | cpumask_t cpu_sibling_map[NR_CPUS], where cpu_sibling_map[i] is the mask of |
52 | all "i"'s siblings as well as "i" itself. |
53 | |
54 | Architectures may retain the regular override the default SD_*_INIT flags |
55 | while using the generic domain builder in kernel/sched.c if they wish to |
56 | retain the traditional SMT->SMP->NUMA topology (or some subset of that). This |
57 | can be done by #define'ing ARCH_HASH_SCHED_TUNE. |
58 | |
59 | Alternatively, the architecture may completely override the generic domain |
60 | builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your |
61 | arch_init_sched_domains function. This function will attach domains to all |
62 | CPUs using cpu_attach_domain. |
63 | |
64 | Implementors should change the line |
65 | #undef SCHED_DOMAIN_DEBUG |
66 | to |
67 | #define SCHED_DOMAIN_DEBUG |
68 | in kernel/sched.c as this enables an error checking parse of the sched domains |
69 | which should catch most possible errors (described above). It also prints out |
70 | the domain structure in a visual format. |