To support task performance boosting, the usage of a single knob has the
advantage to be a simple solution, both from the implementation and the
usability standpoint. However, on a real system it can be difficult to
identify a single value for the knob which fits the needs of multiple
different tasks. For example, some kernel threads and/or user-space
background services should be better managed the "standard" way while we
still want to be able to boost the performance of specific workloads.
In order to improve the flexibility of the task boosting mechanism this
patch is the first of a small series which extends the previous
implementation to introduce a "per task group" support.
This first patch introduces just the basic CGroups support, a new
"schedtune" CGroups controller is added which allows to configure
different boost value for different groups of tasks.
To keep the implementation simple but still effective for a boosting
strategy, the new controller:
1. allows only a two layer hierarchy
2. supports only a limited number of boost groups
A two layer hierarchy allows to place each task either:
a) in the root control group
thus being subject to a system-wide boosting value
b) in a child of the root group
thus being subject to the specific boost value defined by that
"boost group"
The limited number of "boost groups" supported is mainly motivated by
the observation that in a real system it could be useful to have only
few classes of tasks which deserve different treatment.
For example, background vs foreground or interactive vs low-priority.
As an additional benefit, a limited number of boost groups allows also
to have a simpler implementation especially for the code required to
compute the boost value for CPUs which have runnable tasks belonging to
different boost groups.
cc: Tejun Heo <tj@kernel.org>
cc: Li Zefan <lizefan@huawei.com>
cc: Johannes Weiner <hannes@cmpxchg.org>
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The CPU usage signal is used by the scheduler as an estimation of the
overall bandwidth currently allocated on a CPU. When SchedDVFS is in
use, this signal affects the selection of the operating points (OPP)
required to accommodate all the workload allocated in a CPU.
A convenient way to boost the performance of tasks running on a CPU,
which is also little intrusive, is to boost the CPU usage signal each
time it is used to select an OPP.
This patch introduces a new function:
get_boosted_cpu_usage(cpu)
to return a boosted value for the usage of a specified CPU.
The margin added to the original usage is:
1. computed based on the "boosting strategy" in use
2. proportional to the system-wide boost value defined by provided
user-space interface
The boosted signal is used by SchedDVFS (transparently) each time it
requires to get an estimation of the capacity required for a CPU.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The basic idea of the boost knob is to "artificially inflate" a signal
to make a task or logical CPU appears more demanding than it actually
is. Independently from the specific signal, a consistent and possibly
simple semantic for the concept of "signal boosting" must define:
1. how we translate the boost percentage into a "margin" value to be added
to the original signal to inflate
2. what is the meaning of a boost value from a user-space perspective
This patch provides the implementation of a possible boost semantic,
named "Signal Proportional Compensation" (SPC), where the boost
percentage (BP) is used to compute a margin (M) which is proportional to
the complement of the original signal (OS):
M = BP * (SCHED_LOAD_SCALE - OS)
The computed margin then added to the OS to obtain the Boosted Signal (BS)
BS = OS + M
The proposed boost semantic has these main features:
- each signal gets a boost which is proportional to its delta with respect
to the maximum available capacity in the system (i.e. SCHED_LOAD_SCALE)
- a 100% boosting has a clear understanding from a user-space perspective,
since it means simply to run (possibly) "all" tasks at the max OPP
- each boosting value means to improve the task performance by a quantity
which is proportional to the maximum achievable performance on that
system
Thus this semantics is somehow forcing a behaviour which is:
50% boosting means to run at half-way between the current and the
maximum performance which a task could achieve on that system
This patch provides the code to implement a fast integer division to
convert a boost percentage (BP) value into a margin (M).
NOTE: this code is suitable for all signals operating in range
[0..SCHED_LOAD_SCALE]
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The current (CFS) scheduler implementation does not allow "to boost"
tasks performance by running them at a higher OPP compared to the
minimum required to meet their workload demands.
To support tasks performance boosting the scheduler should provide a
"knob" which allows to tune how much the system is going to be optimised
for energy efficiency vs performance.
This patch is the first of a series which provides a simple interface to
define a tuning knob. One system-wide "boost" tunable is exposed via:
/proc/sys/kernel/sched_cfs_boost
which can be configured in the range [0..100], to define a percentage
where:
- 0% boost requires to operate in "standard" mode by scheduling
tasks at the minimum capacities required by the workload demand
- 100% boost requires to push at maximum the task performances,
"regardless" of the incurred energy consumption
A boost value in between these two boundaries is used to bias the
power/performance trade-off, the higher the boost value the more the
scheduler is biased toward performance boosting instead of energy
efficiency.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The topic of a single simple power-performance tunable, that is wholly
scheduler centric, and has well defined and predictable properties has
come up on several occasions in the past. With techniques such as a
scheduler driven DVFS, we now have a good framework for implementing
such a tunable.
This patch provides a detailed description of the motivations and design
decisions behind the implementation of the SchedTune.
cc: Jonathan Corbet <corbet@lwn.net>
cc: linux-doc@vger.kernel.org
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
RT tasks don't provide any running constraints like deadline ones
except their running priority. The only current usable input to
estimate the capacity needed by RT tasks is the rt_avg metric. We use
it to estimate the CPU capacity needed for the RT scheduler class.
In order to monitor the evolution for RT task load, we must
peridiocally check it during the tick.
Then, we use the estimated capacity of the last activity to estimate
the next one which can not be that accurate but is a good starting
point without any impact on the wake up path of RT tasks.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Instead of monitoring the exec time of deadline tasks to evaluate the
CPU capacity consumed by deadline scheduler class, we can directly
calculate it thanks to the sum of utilization of deadline tasks on the
CPU. We can remove deadline tasks from rt_avg metric and directly use
the average bandwidth of deadline scheduler in scale_rt_capacity.
Based in part on a similar patch from Luca Abeni <luca.abeni@unitn.it>.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
rt_avg is only used to scale the available CPU's capacity for CFS
tasks. As the update of this scaling is done during periodic load
balance, we only have to ensure that sched_avg_update has been called
before any periodic load balancing. This requirement is already
fulfilled by __update_cpu_load so the call in sched_rt_avg_update,
which is part of the hotpath, is useless.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Since the true utilization of a long running task is not detectable
while it is running and might be bigger than the current cpu capacity,
create the maximum cpu capacity head room by requesting the maximum
cpu capacity once the cpu usage plus the capacity margin exceeds the
current capacity. This is also done to try to harm the performance of
a task the least.
Original fair-class only version authored by Juri Lelli
<juri.lelli@arm.com>.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
As we don't trigger freq changes from {en,de}queue_task_fair() during load
balancing, we need to do explicitly so on load balancing paths.
[smuckle@linaro.org: move update_capacity_of calls so rq lock is held]
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Patch "sched/fair: add triggers for OPP change requests" introduced OPP
change triggers for enqueue_task_fair(), but the trigger was operating only
for wakeups. Fact is that it makes sense to consider wakeup_new also (i.e.,
fork()), as we don't know anything about a newly created task and thus we
most certainly want to jump to max OPP to not harm performance too much.
However, it is not currently possible (or at least it wasn't evident to me
how to do so :/) to tell new wakeups from other (non wakeup) operations.
This patch introduces an additional flag in sched.h that is only set at
fork() time and it is then consumed in enqueue_task_fair() for our purpose.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Each time a task is {en,de}queued we might need to adapt the current
frequency to the new usage. Add triggers on {en,de}queue_task_fair() for
this purpose. Only trigger a freq request if we are effectively waking up
or going to sleep. Filter out load balancing related calls to reduce the
number of triggers.
[smuckle@linaro.org: resolve merge conflicts, define task_new,
use renamed static key sched_freq]
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Scheduler-driven CPU frequency selection hopes to exploit both
per-task and global information in the scheduler to improve frequency
selection policy, achieving lower power consumption, improved
responsiveness/performance, and less reliance on heuristics and
tunables. For further discussion on the motivation of this integration
see [0].
This patch implements a shim layer between the Linux scheduler and the
cpufreq subsystem. The interface accepts capacity requests from the
CFS, RT and deadline sched classes. The requests from each sched class
are summed on each CPU with a margin applied to the CFS and RT
capacity requests to provide some headroom. Deadline requests are
expected to be precise enough given their nature to not require
headroom. The maximum total capacity request for a CPU in a frequency
domain drives the requested frequency for that domain.
Policy is determined by both the sched classes and this shim layer.
Note that this algorithm is event-driven. There is no polling loop to
check cpu idle time nor any other method which is unsynchronized with
the scheduler, aside from a throttling mechanism to ensure frequency
changes are not attempted faster than the hardware can accommodate them.
Thanks to Juri Lelli <juri.lelli@arm.com> for contributing design ideas,
code and test results, and to Ricky Liang <jcliang@chromium.org>
for initialization and static key inc/dec fixes.
[0] http://article.gmane.org/gmane.linux.kernel/1499836
[smuckle@linaro.org: various additions and fixes, revised commit text]
CC: Ricky Liang <jcliang@chromium.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Some architectures and platforms perform CPU frequency transitions
through a non-blocking method, while some might block or sleep. Even
when frequency transitions do not block or sleep they may be very slow.
This distinction is important when trying to change frequency from
a non-interruptible context in a scheduler hot path.
Describe this distinction with a cpufreq driver flag,
CPUFREQ_DRIVER_FAST. The default is to not have this flag set,
thus erring on the side of caution.
cpufreq_driver_is_slow() is also introduced in this patch. Setting
the above flag will allow this function to return false.
[smuckle@linaro.org: change flag/API to include drivers that are too
slow for scheduler hot paths, in addition to those that block/sleep]
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
With the new group_misfit_task load-balancing scenario additional policy
conditions are needed when load-balancing. Misfit task balancing only
makes sense between source group with lower capacity than the target
group. If capacities are the same, fallback to normal group_other
balancing. The aim is to balance tasks such that no task has its
throughput hindered by compute capacity if a cpu with more capacity is
available. Load-balancing is generally based on average load in the
sched_groups, but for misfitting tasks it is necessary to introduce
exceptions to migrate tasks against usual metrics and optimize
throughput.
This patch ensures the following load-balance for mixed capacity systems
(e.g. ARM big.LITTLE) for always-running tasks:
1. Place a task on each cpu starting in order from cpus with highest
capacity to lowest until all cpus are in use (i.e. one task on each
cpu).
2. Once all cpus are in use balance according to compute capacity such
that load per capacity is approximately the same regardless of the
compute capacity (i.e. big cpus get more tasks than little cpus).
Necessary changes are introduced in find_busiest_group(),
calculate_imbalance(), and find_busiest_queue(). This includes passing
the group_type on to find_busiest_queue() through struct lb_env, which
is currently only considers imbalance and not the imbalance situation
(group_type).
To avoid taking remote rq locks to examine source sched_groups for
misfit tasks, each cpu is responsible for tracking misfit tasks
themselves and update the rq->misfit_task flag. This means checking task
utilization when tasks are scheduled and on sched_tick.
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
To maximize throughput in systems with reduced capacity cpus (e.g.
high RT/IRQ load and/or ARM big.LITTLE) load-balancing has to consider
task and cpu utilization as well as per-cpu compute capacity when
load-balancing in addition to the current average load based
load-balancing policy. Tasks that are scheduled on a reduced capacity
cpu need to be identified and migrated to a higher capacity cpu if
possible.
To implement this additional policy an additional group_type
(load-balance scenario) is added: group_misfit_task. This represents
scenarios where a sched_group has tasks that are not suitable for its
per-cpu capacity. group_misfit_task is only considered if the system is
not overloaded in any other way (group_imbalanced or group_overloaded).
Identifying misfit tasks requires the rq lock to be held. To avoid
taking remote rq locks to examine source sched_groups for misfit tasks,
each cpu is responsible for tracking misfit tasks themselves and update
the rq->misfit_task flag. This means checking task utilization when
tasks are scheduled and on sched_tick.
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
struct sched_group_capacity currently represents the compute capacity
sum of all cpus in the sched_group. Unless it is divided by the
group_weight to get the average capacity per cpu it hides differences in
cpu capacity for mixed capacity systems (e.g. high RT/IRQ utilization or
ARM big.LITTLE). But even the average may not be sufficient if the group
covers cpus of different capacities. Instead, by extending struct
sched_group_capacity to indicate max per-cpu capacity in the group a
suitable group for a given task utilization can easily be found such
that cpus with reduced capacity can be avoided for tasks with high
utilization (not implemented by this patch).
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
EAS relies on idle balance to migrate a misfit task towards a cpu with
higher capacity.
When such a cpu becomes idle, idle balance should happen even if the rq
avg idle is smaller than the sched migration cost (default 500us).
The rq avg idle is updated during the wakeup of a task in case the rq has
a non-null idle_stamp. This value stays unchanged and valid until the next
task wakes up on this cpu after an idle period.
So rq avg idle could be smaller than sched migration cost preventing the
idle balance from happening. In this case we would be at the mercy of
wakeup, periodic or nohz-idle load balancing to put another task on this
cpu.
To break this dependency towards rq avg idle make EAS idle balance
independent from this rq avg idle has to be larger than sched migration
cost.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Maximum Frequency Invariance has to be part of Cpu Invariance because
Frequency Invariance deals only with differences in load-tracking
introduces by Dynamic Frequency Scaling and not with limiting the
possible range of cpu frequency.
By placing Maximum Frequency Invariance into Cpu Invariance,
load-tracking is scaled via arch_scale_cpu_capacity()
in __update_load_avg() and cpu capacity is scaled via
arch_scale_cpu_capacity() in update_cpu_capacity().
To be able to save the extra multiplication in the scheduler hotpath
(__update_load_avg()) we could:
1 Inform cpufreq about base cpu capacity at boot and let it handle
scale_cpu_capacity() as well.
2 Use the cpufreq policy callback which would update a per-cpu current
cpu_scale and this value would be return in scale_cpu_capacity().
3 Use per-cpu current max_freq_scale and current cpu_scale with the
current patch.
Including <linux/cpufreq.h> in topology.h like for the arm arch doesn't
work because of CONFIG_COMPAT=y (Kernel support for 32-bit EL0).
That's why cpufreq_scale_max_freq_capacity() has to be declared extern
in topology.h.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Maximum Frequency Invariance has to be part of Cpu Invariance because
Frequency Invariance deals only with differences in load-tracking
introduces by Dynamic Frequency Scaling and not with limiting the
possible range of cpu frequency.
By placing Maximum Frequency Invariance into Cpu Invariance,
load-tracking is scaled via arch_scale_cpu_capacity()
in __update_load_avg() and cpu capacity is scaled via
arch_scale_cpu_capacity() in update_cpu_capacity().
To be able to save the extra multiplication in the scheduler hotpath
(__update_load_avg()) we could:
1 Inform cpufreq about base cpu capacity at boot and let it handle
scale_cpu_capacity() as well.
2 Use the cpufreq policy callback which would update a per-cpu current
cpu_scale and this value would be return in scale_cpu_capacity().
3 Use per-cpu current max_freq_scale and current cpu_scale with the
current patch.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Wakeup balancing uses cpu capacity awareness and needs to know the
system-wide maximum cpu capacity.
Patch "sched: Store system-wide maximum cpu capacity in root domain"
finds the system-wide maximum cpu capacity during scheduler domain
hierarchy setup. This is sufficient as long as maximum frequency
invariance is not enabled.
If it is enabled, the system-wide maximum cpu capacity can change
between scheduler domain hierarchy setups due to frequency capping.
The cpu capacity is changed in update_cpu_capacity() which is called in
load balance on the lowest scheduler domain hierarchy level. To be able
to know if a change in cpu capacity for a certain cpu also has an effect
on the system-wide maximum cpu capacity it is normally necessary to
iterate over all cpus. This would be way too costly. That's why this
patch follows a different approach.
The unsigned long max_cpu_capacity value in struct root_domain is
replaced with a struct max_cpu_capacity, containing value (the
max_cpu_capacity) and cpu (the cpu index of the cpu providing the
maximum cpu_capacity).
Changes to the system-wide maximum cpu capacity and the cpu index are
made if:
1 System-wide maximum cpu capacity < cpu capacity
2 System-wide maximum cpu capacity > cpu capacity and cpu index == cpu
There are no changes to the system-wide maximum cpu capacity in all
other cases.
Atomic read and write access to the pair (max_cpu_capacity.val,
max_cpu_capacity.cpu) is enforced by max_cpu_capacity.lock.
The access to max_cpu_capacity.val in task_fits_max() is still performed
without taking the max_cpu_capacity.lock.
The code to set max cpu capacity in build_sched_domains() has been
removed because the whole functionality is now provided by
update_cpu_capacity() instead.
This approach can introduce errors temporarily, e.g. in case the cpu
currently providing the max cpu capacity has its cpu capacity lowered
due to frequency capping and calls update_cpu_capacity() before any cpu
which might provide the max cpu now.
There is also an outstanding question:
Should the cpu capacity of a cpu going idle be set to a very small
value?
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Implements cpufreq_scale_max_freq_capacity() to provide the scheduler
with a maximum frequency scaling correction factor for more accurate
load-tracking and cpu capacity handling by being able to deal with
frequency capping.
This scaling factor describes the influence of running a cpu with a
current maximum frequency lower than the absolute possible maximum
frequency on load tracking and cpu capacity.
The factor is:
current_max_freq(cpu) << SCHED_CAPACITY_SHIFT / max_freq(cpu)
In fact, max_freq_scale should be a struct cpufreq_policy data member.
But this would require that the scheduler hot path (__update_load_avg())
would have to grab the cpufreq lock. This can be avoided by using per-cpu
data initialized to SCHED_CAPACITY_SCALE for max_freq_scale.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
With the bindings and the associated accessors to extract data from the
bindings in place, remove the static hard-coded data from topology.c and
use the accesors instead.
Signed-off-by: Robin Randhawa <robin.randhawa@arm.com>
This patch implements support for extracting energy cost data from DT.
The data should conform to the DT bindings for energy cost data needed
by EAS (energy aware scheduling).
Signed-off-by: Robin Randhawa <robin.randhawa@arm.com>
EAS (energy aware scheduling) provides the scheduler with an alternative
objective - energy efficiency - as opposed to it's current performance
oriented objectives. EAS relies on a simple platform energy cost model
to guide scheduling decisions. The model only considers the CPU
subsystem.
This patch adds documentation describing DT bindings that should be used to
supply the scheduler with an energy cost model.
Signed-off-by: Robin Randhawa <robin.randhawa@arm.com>
With energy-aware scheduling enabled nohz_kick_needed() generates many
nohz idle-balance kicks which lead to nothing when multiple tasks get
packed on a single cpu to save energy. This causes unnecessary wake-ups
and hence wastes energy. Make these conditions depend on !energy_aware()
for now until the energy-aware nohz story gets sorted out.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
In case the system operates below the tipping point indicator,
introduced in ("sched: Add over-utilization/tipping point
indicator"), bail out in find_busiest_group after the dst and src
group statistics have been checked.
There is simply no need to move usage around because all involved
cpus still have spare cycles available.
For an energy-aware system below its tipping point, we rely on the
task placement of the wakeup path. This works well for short running
tasks.
The existence of long running tasks on one of the involved cpus lets
the system operate over its tipping point. To be able to move such
a task (whose load can't be used to average the load among the cpus)
from a src cpu with lower capacity than the dst_cpu, an additional
rule has to be implemented in need_active_balance.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Let available compute capacity and estimated energy impact select
wake-up target cpu when energy-aware scheduling is enabled and the
system in not over-utilized (above the tipping point).
energy_aware_wake_cpu() attempts to find group of cpus with sufficient
compute capacity to accommodate the task and find a cpu with enough spare
capacity to handle the task within that group. Preference is given to
cpus with enough spare capacity at the current OPP. Finally, the energy
impact of the new target and the previous task cpu is compared to select
the wake-up target cpu.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
To estimate the energy consumption of a sched_group in
sched_group_energy() it is necessary to know which idle-state the group
is in when it is idle. For now, it is assumed that this is the current
idle-state (though it might be wrong). Based on the individual cpu
idle-states group_idle_state() finds the group idle-state.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
The idle-state of each cpu is currently pointed to by rq->idle_state but
there isn't any information in the struct cpuidle_state that can used to
look up the idle-state energy model data stored in struct
sched_group_energy. For this purpose is necessary to store the idle
state index as well. Ideally, the idle-state data should be unified.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Energy-aware scheduling is only meant to be active while the system is
_not_ over-utilized. That is, there are spare cycles available to shift
tasks around based on their actual utilization to get a more
energy-efficient task distribution without depriving any tasks. When
above the tipping point task placement is done the traditional way based
on load_avg, spreading the tasks across as many cpus as possible based
on priority scaled load to preserve smp_nice. Below the tipping point we
want to use util_avg instead. We need to define a criteria for when we
make the switch.
The util_avg for each cpu converges towards 100% (1024) regardless of
how many task additional task we may put on it. If we define
over-utilized as:
sum_{cpus}(rq.cfs.avg.util_avg) + margin > sum_{cpus}(rq.capacity)
some individual cpus may be over-utilized running multiple tasks even
when the above condition is false. That should be okay as long as we try
to spread the tasks out to avoid per-cpu over-utilization as much as
possible and if all tasks have the _same_ priority. If the latter isn't
true, we have to consider priority to preserve smp_nice.
For example, we could have n_cpus nice=-10 util_avg=55% tasks and
n_cpus/2 nice=0 util_avg=60% tasks. Balancing based on util_avg we are
likely to end up with nice=-10 tasks sharing cpus and nice=0 tasks
getting their own as we 1.5*n_cpus tasks in total and 55%+55% is less
over-utilized than 55%+60% for those cpus that have to be shared. The
system utilization is only 85% of the system capacity, but we are
breaking smp_nice.
To be sure not to break smp_nice, we have defined over-utilization
conservatively as when any cpu in the system is fully utilized at it's
highest frequency instead:
cpu_rq(any).cfs.avg.util_avg + margin > cpu_rq(any).capacity
IOW, as soon as one cpu is (nearly) 100% utilized, we switch to load_avg
to factor in priority to preserve smp_nice.
With this definition, we can skip periodic load-balance as no cpu has an
always-running task when the system is not over-utilized. All tasks will
be periodic and we can balance them at wake-up. This conservative
condition does however mean that some scenarios that could benefit from
energy-aware decisions even if one cpu is fully utilized would not get
those benefits.
For system where some cpus might have reduced capacity on some cpus
(RT-pressure and/or big.LITTLE), we want periodic load-balance checks as
soon a just a single cpu is fully utilized as it might one of those with
reduced capacity and in that case we want to migrate it.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Adds a generic energy-aware helper function, energy_diff(), that
calculates energy impact of adding, removing, and migrating utilization
in the system.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Extended sched_group_energy() to support energy prediction with usage
(tasks) added/removed from a specific cpu or migrated between a pair of
cpus. Useful for load-balancing decision making.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
For energy-aware load-balancing decisions it is necessary to know the
energy consumption estimates of groups of cpus. This patch introduces a
basic function, sched_group_energy(), which estimates the energy
consumption of the cpus in the group and any resources shared by the
members of the group.
NOTE: The function has five levels of identation and breaks the 80
character limit. Refactoring is necessary.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Add another member to the family of per-cpu sched_domain shortcut
pointers. This one, sd_ea, points to the highest level at which energy
model is provided. At this level and all levels below all sched_groups
have energy model data attached.
Partial energy model information is possible but restricted to providing
energy model data for lower level sched_domains (sd_ea and below) and
leaving load-balancing on levels above to non-energy-aware
load-balancing. For example, it is possible to apply energy-aware
scheduling within each socket on a multi-socket system and let normal
scheduling handle load-balancing between sockets.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Move cpu_util() to an earlier position in fair.c and change return
type to unsigned long as negative usage doesn't make much sense. All
other load and capacity related functions use unsigned long including
the caller of cpu_util().
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
capacity_orig_of() returns the max available compute capacity of a cpu.
For scale-invariant utilization tracking and energy-aware scheduling
decisions it is useful to know the compute capacity available at the
current OPP of a cpu.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Provides the scheduler with a cpu scaling correction factor for more
accurate load-tracking and cpu capacity handling.
The Energy Model (EM) (in fact the capacity value of the last element
of the capacity states vector of the core (MC) level sched_group_energy
structure) is used as the source for this cpu scaling factor.
The cpu capacity value depends on the micro-architecture and the
maximum frequency of the cpu.
The maximum frequency part should not be confused with the frequency
invariant scheduler load-tracking support which deals with frequency
related scaling due to DFVS functionality.
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Provides the scheduler with a cpu scaling correction factor for more
accurate load-tracking and cpu capacity handling.
The Energy Model (EM) (in fact the capacity value of the last element
of the capacity states vector of the core (MC) level sched_group_energy
structure) is used instead of the arm arch specific cpu_efficiency and
dtb property 'clock-frequency' values as the source for this cpu
scaling factor.
The cpu capacity value depends on the micro-architecture and the
maximum frequency of the cpu.
The maximum frequency part should not be confused with the frequency
invariant scheduler load-tracking support which deals with frequency
related scaling due to DFVS functionality.
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
cpufreq is currently keeping it a secret which cpus are sharing
clock source. The scheduler needs to know about clock domains as well
to become more energy aware. The SD_SHARE_CAP_STATES domain flag
indicates whether cpus belonging to the sched_domain share capacity
states (P-states).
There is no connection with cpufreq (yet). The flag must be set by
the arch specific topology code.
cc: Russell King <linux@arm.linux.org.uk>
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
The sched_group_energy (sge) pointer of the first sched_group (sg) in
the sched_domain (sd) is initialized to point to the appropriate (in
terms of sd level and cpu) sge data defined in the arch and so to the
correct part of the Energy Model (EM).
Energy-aware scheduling allows that a system has only EM data up to a
certain sd level (so called highest energy aware balancing sd level).
A check in init_sched_energy() enforces that all sd's below this sd
level contain EM data.
The 'int cpu' parameter of sched_domain_energy_f requires that
check_sched_energy_data() makes sure that all cpus spanned by a sg
are provisioned with the same EM data.
This patch has also been tested with feature FORCE_SD_OVERLAP enabled.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
The struct sched_group_energy represents the per sched_group related
data which is needed for energy aware scheduling. It contains:
(1) number of elements of the idle state array
(2) pointer to the idle state array which comprises 'power consumption'
for each idle state
(3) number of elements of the capacity state array
(4) pointer to the capacity state array which comprises 'compute
capacity and power consumption' tuples for each capacity state
The struct sched_group obtains a pointer to a struct sched_group_energy.
The function pointer sched_domain_energy_f is introduced into struct
sched_domain_topology_level which will allow the arch to pass a particular
struct sched_group_energy from the topology shim layer into the scheduler
core.
The function pointer sched_domain_energy_f has an 'int cpu' parameter
since the folding of two adjacent sd levels via sd degenerate doesn't work
for all sd levels. I.e. it is not possible for example to use this feature
to provide per-cpu energy in sd level DIE on ARM's TC2 platform.
It was discussed that the folding of sd levels approach is preferable
over the cpu parameter approach, simply because the user (the arch
specifying the sd topology table) can introduce less errors. But since
it is not working, the 'int cpu' parameter is the only way out. It's
possible to use the folding of sd levels approach for
sched_domain_flags_f and the cpu parameter approach for the
sched_domain_energy_f at the same time though. With the use of the
'int cpu' parameter, an extra check function has to be provided to make
sure that all cpus spanned by a sched group are provisioned with the same
energy data.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
This patch introduces the ENERGY_AWARE sched feature, which is
implemented using jump labels when SCHED_DEBUG is defined. It is
statically set false when SCHED_DEBUG is not defined. Hence this doesn't
allow energy awareness to be enabled without SCHED_DEBUG. This
sched_feature knob will be replaced later with a more appropriate
control knob when things have matured a bit.
ENERGY_AWARE is based on per-entity load-tracking hence FAIR_GROUP_SCHED
must be enable. This dependency isn't checked at compile time yet.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
This documentation patch provides an overview of the experimental
scheduler energy costing model, associated data structures, and a
reference recipe on how platforms can be characterized to derive energy
models.
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Scenarios with the busiest group having just one task and the local
being idle on topologies with sched groups with different numbers of
cpus manage to dodge all load-balance bailout conditions resulting the
nr_balance_failed counter to be incremented. This eventually causes a
pointless active migration of the task. This patch prevents this by not
incrementing the counter when the busiest group only has one task.
ASYM_PACKING migrations and migrations due to reduced capacity should
still take place as these are explicitly captured by
need_active_balance().
A better solution would be to not attempt the load-balance in the first
place, but that requires significant changes to the order of bailout
conditions and statistics gathering.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
We do not want to miss out on the ability to pull a single remaining
task from a potential source cpu towards an idle destination cpu. Add an
extra criteria to need_active_balance() to kick off active load balance
if the source cpu is over-utilized and has lower capacity than the
destination cpu.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
find_idlest_group() selects the wake-up target group purely
based on group load which leads to suboptimal choices in low load
scenarios. An idle group with reduced capacity (due to RT tasks or
different cpu type) isn't necessarily a better target than a lightly
loaded group with higher capacity.
The patch adds spare capacity as an additional group selection
parameter. The target group is now selected based on the following
criteria:
1. Return the group with the cpu with most spare capacity and this
capacity is significant if such group exists. Significant spare capacity
is currently at least 20% to spare.
2. Return the group with the lowest load, unless it is the local group
in which case NULL is returned and the search is continued at the next
(lower) level.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
