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It's been broken forever (i.e. it's not scheduling in a power
aware fashion), as reported by Suresh and others sending
patches, and nobody cares enough to fix it properly ...
so remove it to make space free for something better.
There's various problems with the code as it stands today, first
and foremost the user interface which is bound to topology
levels and has multiple values per level. This results in a
state explosion which the administrator or distro needs to
master and almost nobody does.
Furthermore large configuration state spaces aren't good, it
means the thing doesn't just work right because it's either
under so many impossibe to meet constraints, or even if
there's an achievable state workloads have to be aware of
it precisely and can never meet it for dynamic workloads.
So pushing this kind of decision to user-space was a bad idea
even with a single knob - it's exponentially worse with knobs
on every node of the topology.
There is a proposal to replace the user interface with a single
3 state knob:
sched_balance_policy := { performance, power, auto }
where 'auto' would be the preferred default which looks at things
like Battery/AC mode and possible cpufreq state or whatever the hw
exposes to show us power use expectations - but there's been no
progress on it in the past many months.
Aside from that, the actual implementation of the various knobs
is known to be broken. There have been sporadic attempts at
fixing things but these always stop short of reaching a mergable
state.
Therefore this wholesale removal with the hopes of spurring
people who care to come forward once again and work on a
coherent replacement.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1326104915.2442.53.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>
78 lines
4.3 KiB
Plaintext
78 lines
4.3 KiB
Plaintext
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.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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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.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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