Hint to enable biasing of tasks towards idle cpus, even when a given
task is negatively boosted. The mechanism allows upto 20% reduction in
camera power without hurting performance.
bug: 28312446
Change-Id: I97ea5671aa1e6bcb165408b41e17bc82e41c2c9e
Contains:
sched/tune: fix accounting for runnable tasks (1/5)
The accounting for tasks into boost groups of different CPUs is currently
broken mainly because:
a) we do not properly track the change of boost group of a RUNNABLE task
b) there are race conditions between migration code and accounting code
This patch provides a fixes to ensure enqueue/dequeue
accounting also for throttled tasks.
Without this patch is can happen that a task is enqueued into a throttled
RQ thus not being accounted for the boosting of the corresponding RQ.
We could argue that a throttled task should not boost a CPU, however:
a) properly implementing CPU boosting considering throttled tasks will
increase a lot the complexity of the solution
b) it's not easy to quantify the benefits introduced by such a more
complex solution
Since task throttling requires the usage of the CFS bandwidth controller,
which is not widely used on mobile systems (at least not by Android kernels
so far), for the time being we go for the simple solution and boost also
for throttled RQs.
sched/tune: fix accounting for runnable tasks (2/5)
This patch provides the code required to enforce proper locking.
A per boost group spinlock has been added to grant atomic
accounting of tasks as well as to serialise enqueue/dequeue operations,
triggered by tasks migrations, with cgroups's attach/detach operations.
sched/tune: fix accounting for runnable tasks (3/5)
This patch adds cgroups {allow,can,cancel}_attach callbacks.
Since a task can be migrated between boost groups while it's running,
the CGroups's attach callbacks have been added to properly migrate
boost contributions of RUNNABLE tasks.
The RQ's lock is used to serialise enqueue/dequeue operations, triggered
by tasks migrations, with cgroups's attach/detach operations. While the
SchedTune's CPU lock is used to grant atrocity of the accounting within
the CPU.
NOTE: the current implementation does not allows a concurrent CPU migration
and CGroups change.
sched/tune: fix accounting for runnable tasks (4/5)
This fixes accounting for exiting tasks by adding a dedicated call early
in the do_exit() syscall, which disables SchedTune accounting as soon as a
task is flagged PF_EXITING.
This flag is set before the multiple dequeue/enqueue dance triggered
by cgroup_exit() which is useful only to inject useless tasks movements
thus increasing possibilities for race conditions with the migration code.
The schedtune_exit_task() call does the last dequeue of a task from its
current boost group. This is a solution more aligned with what happens in
mainline kernels (>v4.4) where the exit_cgroup does not move anymore a dying
task to the root control group.
sched/tune: fix accounting for runnable tasks (5/5)
To avoid accounting issues at startup, this patch disable the SchedTune
accounting until the required data structures have been properly
initialized.
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[jstultz: fwdported to 4.4]
Signed-off-by: John Stultz <john.stultz@linaro.org>
The usage of conditional compiled code is discouraged in fair.c.
This patch clean up a bit fair.c by moving schedtune_{cpu.task}_boost
definitions into tune.h.
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The energy normalization function is required to get the proper values
for the P-E space filtering function to work.
That normalization is part of the hot wakeup path and currently implemented
with a function call.
Moving the normalization function into fair.c allows the compiler to
further optimize that code by reducing overheads in the wakeup hot path.
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[jstultz: fwdported to 4.4]
Signed-off-by: John Stultz <john.stultz@linaro.org>
The current EAS implementation considers only energy variations, while it
disregards completely the impact on performance for the selection of
a certain schedule candidate. Moreover, it also makes its decision based
on the "absolute" value of expected energy variations.
In order to properly define a trade-off strategy between increased energy
consumption and performances benefits it is required to compare energy
variations with performance variations.
Thus, both performance and energy metrics must be expressed in comparable
units. While the performance variations are expressed in terms of capacity
deltas, which are defined in the range [0..SCHED_LOAD_SCALE], the same
scale is not used for energy variations.
This patch introduces the function:
schedtune_normalize_energy(energy_diff)
which returns a normalized value in the same range of capacity variations,
i.e. [0..SCHED_LOAD_SCALE].
A proper set of energy normalization constants are required to provide
a fast division by a constant during the normalziation of the energy_diff.
The value of these constants depends on the specific energy model and
topology of a target device.
Thus, this patch provides also the required support for the computation
at boot time of this set of variables.
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
The task utilization signal, which is derived from PELT signals and
properly scaled to be architecture and frequency invariant, is used by
EAS as an estimation of the task requirements in terms of CPU bandwidth.
When the energy aware scheduler is in use, this signal affects the CPU
selection. Thus, a convenient way to bias that decision, which is also
little intrusive, is to boost the task utilization signal each time it
is required to support them.
This patch introduces the new function:
boosted_task_util(task)
which returns a boosted value for the utilization of the specified task.
The margin added to the original utilization is:
1. computed based on the "boosting strategy" in use
2. proportional to boost value defined either by the sysctl interface,
when global boosting is in use, or the "taskgroup" value, when
per-task boosting is enabled.
The boosted signal is used by EAS
a. transparently, via its integration into the task_fits() function
b. explicitly, in the energy-aware wakeup path
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
When per-task boosting is enabled, every time a task enters/exits a CPU
its boost value could impact the currently selected OPP for that CPU.
Thus, the "aggregated" boost value for that CPU potentially needs to
be updated to match the current maximum boost value among all the tasks
currently RUNNABLE on that CPU.
This patch introduces the required support to keep track of which boost
groups are impacting a CPU. Each time a task is enqueued/dequeued to/from
a CPU its boost group is used to increment a per-cpu counter of RUNNABLE
tasks on that CPU.
Only when the number of runnable tasks for a specific boost group
becomes 1 or 0 the corresponding boost group changes its effects on
that CPU, specifically:
a) boost_group::tasks == 1: this boost group starts to impact the CPU
b) boost_group::tasks == 0: this boost group stops to impact the CPU
In each of these two conditions the aggregation function:
sched_cpu_update(cpu)
could be required to run in order to identify the new maximum boost
value required for the CPU.
The proposed patch minimizes the number of times the aggregation
function is executed while still providing the required support to
always boost a CPU to the maximum boost value required by all its
currently RUNNABLE tasks.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
