mirror of
https://github.com/torvalds/linux.git
synced 2024-11-23 04:31:50 +00:00
4c3e509ea9
Standardize scheduler load-balancing function names on the sched_balance_() prefix. Also load_balance() has become somewhat of a misnomer: historically it was the first and primary load-balancing function that was called, but with the introduction of sched domains, it's become a lower layer function that balances runqueues. Rename it to sched_balance_rq() accordingly. Signed-off-by: Ingo Molnar <mingo@kernel.org> Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com> Link: https://lore.kernel.org/r/20240308111819.1101550-6-mingo@kernel.org
82 lines
4.3 KiB
ReStructuredText
82 lines
4.3 KiB
ReStructuredText
=================
|
|
Scheduler Domains
|
|
=================
|
|
|
|
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 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. The intersection of cpumasks from
|
|
any two of these groups may be non empty. If this is the case the SD_OVERLAP
|
|
flag is set on the corresponding scheduling domain and its groups may not be
|
|
shared between CPUs.
|
|
|
|
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, sched_balance_trigger() is run periodically on each CPU
|
|
through sched_tick(). It raises a softirq after the next regularly scheduled
|
|
rebalancing event for the current runqueue has arrived. The actual load
|
|
balancing workhorse, sched_balance_softirq()->sched_balance_domains(), is then run
|
|
in softirq context (SCHED_SOFTIRQ).
|
|
|
|
The latter function takes two arguments: the runqueue of current CPU and whether
|
|
the CPU was idle at the time the sched_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 sched_balance_rq() on that domain. It then checks
|
|
the parent sched_domain (if it exists), and the parent of the parent and so
|
|
forth.
|
|
|
|
Initially, sched_balance_rq() 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/sd_flags.h:
|
|
SD_* to get an idea of the specifics and what to tune for the SD flags
|
|
of a sched_domain.
|
|
|
|
Architectures may override the generic domain builder and the default SD flags
|
|
for a given topology level by creating a sched_domain_topology_level array and
|
|
calling set_sched_topology() with this array as the parameter.
|
|
|
|
The sched-domains debugging infrastructure can be enabled by enabling
|
|
CONFIG_SCHED_DEBUG and adding 'sched_verbose' to your cmdline. If you
|
|
forgot to tweak your cmdline, you can also flip the
|
|
/sys/kernel/debug/sched/verbose knob. 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.
|