84 lines
4.3 KiB
ReStructuredText
84 lines
4.3 KiB
ReStructuredText
=================
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Scheduler Domains
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=================
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Each CPU has a "base" scheduling domain (struct sched_domain). The domain
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hierarchy is built from these base domains via the ->parent pointer. ->parent
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MUST be NULL terminated, and domain structures should be per-CPU as they are
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locklessly updated.
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Each scheduling domain spans a number of CPUs (stored in the ->span field).
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A domain's span MUST be a superset of it child's span (this restriction could
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be relaxed if the need arises), and a base domain for CPU i MUST span at least
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i. The top domain for each CPU will generally span all CPUs in the system
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although strictly it doesn't have to, but this could lead to a case where some
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CPUs will never be given tasks to run unless the CPUs allowed mask is
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explicitly set. A sched domain's span means "balance process load among these
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CPUs".
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Each scheduling domain must have one or more CPU groups (struct sched_group)
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which are organised as a circular one way linked list from the ->groups
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pointer. The union of cpumasks of these groups MUST be the same as the
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domain's span. The intersection of cpumasks from any two of these groups
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MUST be the empty set. The group pointed to by the ->groups pointer MUST
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contain the CPU to which the domain belongs. Groups may be shared among
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CPUs as they contain read only data after they have been set up.
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Balancing within a sched domain occurs between groups. That is, each group
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is treated as one entity. The load of a group is defined as the sum of the
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load of each of its member CPUs, and only when the load of a group becomes
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out of balance are tasks moved between groups.
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In kernel/sched/core.c, trigger_load_balance() is run periodically on each CPU
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through scheduler_tick(). It raises a softirq after the next regularly scheduled
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rebalancing event for the current runqueue has arrived. The actual load
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balancing workhorse, run_rebalance_domains()->rebalance_domains(), is then run
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in softirq context (SCHED_SOFTIRQ).
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The latter function takes two arguments: the current CPU and whether it was idle
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at the time the scheduler_tick() happened and iterates over all sched domains
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our CPU is on, starting from its base domain and going up the ->parent chain.
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While doing that, it checks to see if the current domain has exhausted its
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rebalance interval. If so, it runs load_balance() on that domain. It then checks
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the parent sched_domain (if it exists), and the parent of the parent and so
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forth.
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Initially, load_balance() finds the busiest group in the current sched domain.
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If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in
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that group. If it manages to find such a runqueue, it locks both our initial
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CPU's runqueue and the newly found busiest one and starts moving tasks from it
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to our runqueue. The exact number of tasks amounts to an imbalance previously
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computed while iterating over this sched domain's groups.
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Implementing sched domains
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==========================
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The "base" domain will "span" the first level of the hierarchy. In the case
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of SMT, you'll span all siblings of the physical CPU, with each group being
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a single virtual CPU.
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In SMP, the parent of the base domain will span all physical CPUs in the
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node. Each group being a single physical CPU. Then with NUMA, the parent
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of the SMP domain will span the entire machine, with each group having the
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cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example,
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might have just one domain covering its one NUMA level.
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The implementor should read comments in include/linux/sched.h:
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struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of
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the specifics and what to tune.
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Architectures may retain the regular override the default SD_*_INIT flags
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while using the generic domain builder in kernel/sched/core.c if they wish to
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retain the traditional SMT->SMP->NUMA topology (or some subset of that). This
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can be done by #define'ing ARCH_HASH_SCHED_TUNE.
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Alternatively, the architecture may completely override the generic domain
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builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your
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arch_init_sched_domains function. This function will attach domains to all
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CPUs using cpu_attach_domain.
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The sched-domains debugging infrastructure can be enabled by enabling
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CONFIG_SCHED_DEBUG. This enables an error checking parse of the sched domains
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which should catch most possible errors (described above). It also prints out
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the domain structure in a visual format.
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