742 lines
27 KiB
ReStructuredText
742 lines
27 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
====================
|
|
Utilization Clamping
|
|
====================
|
|
|
|
1. Introduction
|
|
===============
|
|
|
|
Utilization clamping, also known as util clamp or uclamp, is a scheduler
|
|
feature that allows user space to help in managing the performance requirement
|
|
of tasks. It was introduced in v5.3 release. The CGroup support was merged in
|
|
v5.4.
|
|
|
|
Uclamp is a hinting mechanism that allows the scheduler to understand the
|
|
performance requirements and restrictions of the tasks, thus it helps the
|
|
scheduler to make a better decision. And when schedutil cpufreq governor is
|
|
used, util clamp will influence the CPU frequency selection as well.
|
|
|
|
Since the scheduler and schedutil are both driven by PELT (util_avg) signals,
|
|
util clamp acts on that to achieve its goal by clamping the signal to a certain
|
|
point; hence the name. That is, by clamping utilization we are making the
|
|
system run at a certain performance point.
|
|
|
|
The right way to view util clamp is as a mechanism to make request or hint on
|
|
performance constraints. It consists of two tunables:
|
|
|
|
* UCLAMP_MIN, which sets the lower bound.
|
|
* UCLAMP_MAX, which sets the upper bound.
|
|
|
|
These two bounds will ensure a task will operate within this performance range
|
|
of the system. UCLAMP_MIN implies boosting a task, while UCLAMP_MAX implies
|
|
capping a task.
|
|
|
|
One can tell the system (scheduler) that some tasks require a minimum
|
|
performance point to operate at to deliver the desired user experience. Or one
|
|
can tell the system that some tasks should be restricted from consuming too
|
|
much resources and should not go above a specific performance point. Viewing
|
|
the uclamp values as performance points rather than utilization is a better
|
|
abstraction from user space point of view.
|
|
|
|
As an example, a game can use util clamp to form a feedback loop with its
|
|
perceived Frames Per Second (FPS). It can dynamically increase the minimum
|
|
performance point required by its display pipeline to ensure no frame is
|
|
dropped. It can also dynamically 'prime' up these tasks if it knows in the
|
|
coming few hundred milliseconds a computationally intensive scene is about to
|
|
happen.
|
|
|
|
On mobile hardware where the capability of the devices varies a lot, this
|
|
dynamic feedback loop offers a great flexibility to ensure best user experience
|
|
given the capabilities of any system.
|
|
|
|
Of course a static configuration is possible too. The exact usage will depend
|
|
on the system, application and the desired outcome.
|
|
|
|
Another example is in Android where tasks are classified as background,
|
|
foreground, top-app, etc. Util clamp can be used to constrain how much
|
|
resources background tasks are consuming by capping the performance point they
|
|
can run at. This constraint helps reserve resources for important tasks, like
|
|
the ones belonging to the currently active app (top-app group). Beside this
|
|
helps in limiting how much power they consume. This can be more obvious in
|
|
heterogeneous systems (e.g. Arm big.LITTLE); the constraint will help bias the
|
|
background tasks to stay on the little cores which will ensure that:
|
|
|
|
1. The big cores are free to run top-app tasks immediately. top-app
|
|
tasks are the tasks the user is currently interacting with, hence
|
|
the most important tasks in the system.
|
|
2. They don't run on a power hungry core and drain battery even if they
|
|
are CPU intensive tasks.
|
|
|
|
.. note::
|
|
**little cores**:
|
|
CPUs with capacity < 1024
|
|
|
|
**big cores**:
|
|
CPUs with capacity = 1024
|
|
|
|
By making these uclamp performance requests, or rather hints, user space can
|
|
ensure system resources are used optimally to deliver the best possible user
|
|
experience.
|
|
|
|
Another use case is to help with **overcoming the ramp up latency inherit in
|
|
how scheduler utilization signal is calculated**.
|
|
|
|
On the other hand, a busy task for instance that requires to run at maximum
|
|
performance point will suffer a delay of ~200ms (PELT HALFIFE = 32ms) for the
|
|
scheduler to realize that. This is known to affect workloads like gaming on
|
|
mobile devices where frames will drop due to slow response time to select the
|
|
higher frequency required for the tasks to finish their work in time. Setting
|
|
UCLAMP_MIN=1024 will ensure such tasks will always see the highest performance
|
|
level when they start running.
|
|
|
|
The overall visible effect goes beyond better perceived user
|
|
experience/performance and stretches to help achieve a better overall
|
|
performance/watt if used effectively.
|
|
|
|
User space can form a feedback loop with the thermal subsystem too to ensure
|
|
the device doesn't heat up to the point where it will throttle.
|
|
|
|
Both SCHED_NORMAL/OTHER and SCHED_FIFO/RR honour uclamp requests/hints.
|
|
|
|
In the SCHED_FIFO/RR case, uclamp gives the option to run RT tasks at any
|
|
performance point rather than being tied to MAX frequency all the time. Which
|
|
can be useful on general purpose systems that run on battery powered devices.
|
|
|
|
Note that by design RT tasks don't have per-task PELT signal and must always
|
|
run at a constant frequency to combat undeterministic DVFS rampup delays.
|
|
|
|
Note that using schedutil always implies a single delay to modify the frequency
|
|
when an RT task wakes up. This cost is unchanged by using uclamp. Uclamp only
|
|
helps picking what frequency to request instead of schedutil always requesting
|
|
MAX for all RT tasks.
|
|
|
|
See :ref:`section 3.4 <uclamp-default-values>` for default values and
|
|
:ref:`3.4.1 <sched-util-clamp-min-rt-default>` on how to change RT tasks
|
|
default value.
|
|
|
|
2. Design
|
|
=========
|
|
|
|
Util clamp is a property of every task in the system. It sets the boundaries of
|
|
its utilization signal; acting as a bias mechanism that influences certain
|
|
decisions within the scheduler.
|
|
|
|
The actual utilization signal of a task is never clamped in reality. If you
|
|
inspect PELT signals at any point of time you should continue to see them as
|
|
they are intact. Clamping happens only when needed, e.g: when a task wakes up
|
|
and the scheduler needs to select a suitable CPU for it to run on.
|
|
|
|
Since the goal of util clamp is to allow requesting a minimum and maximum
|
|
performance point for a task to run on, it must be able to influence the
|
|
frequency selection as well as task placement to be most effective. Both of
|
|
which have implications on the utilization value at CPU runqueue (rq for short)
|
|
level, which brings us to the main design challenge.
|
|
|
|
When a task wakes up on an rq, the utilization signal of the rq will be
|
|
affected by the uclamp settings of all the tasks enqueued on it. For example if
|
|
a task requests to run at UTIL_MIN = 512, then the util signal of the rq needs
|
|
to respect to this request as well as all other requests from all of the
|
|
enqueued tasks.
|
|
|
|
To be able to aggregate the util clamp value of all the tasks attached to the
|
|
rq, uclamp must do some housekeeping at every enqueue/dequeue, which is the
|
|
scheduler hot path. Hence care must be taken since any slow down will have
|
|
significant impact on a lot of use cases and could hinder its usability in
|
|
practice.
|
|
|
|
The way this is handled is by dividing the utilization range into buckets
|
|
(struct uclamp_bucket) which allows us to reduce the search space from every
|
|
task on the rq to only a subset of tasks on the top-most bucket.
|
|
|
|
When a task is enqueued, the counter in the matching bucket is incremented,
|
|
and on dequeue it is decremented. This makes keeping track of the effective
|
|
uclamp value at rq level a lot easier.
|
|
|
|
As tasks are enqueued and dequeued, we keep track of the current effective
|
|
uclamp value of the rq. See :ref:`section 2.1 <uclamp-buckets>` for details on
|
|
how this works.
|
|
|
|
Later at any path that wants to identify the effective uclamp value of the rq,
|
|
it will simply need to read this effective uclamp value of the rq at that exact
|
|
moment of time it needs to take a decision.
|
|
|
|
For task placement case, only Energy Aware and Capacity Aware Scheduling
|
|
(EAS/CAS) make use of uclamp for now, which implies that it is applied on
|
|
heterogeneous systems only.
|
|
When a task wakes up, the scheduler will look at the current effective uclamp
|
|
value of every rq and compare it with the potential new value if the task were
|
|
to be enqueued there. Favoring the rq that will end up with the most energy
|
|
efficient combination.
|
|
|
|
Similarly in schedutil, when it needs to make a frequency update it will look
|
|
at the current effective uclamp value of the rq which is influenced by the set
|
|
of tasks currently enqueued there and select the appropriate frequency that
|
|
will satisfy constraints from requests.
|
|
|
|
Other paths like setting overutilization state (which effectively disables EAS)
|
|
make use of uclamp as well. Such cases are considered necessary housekeeping to
|
|
allow the 2 main use cases above and will not be covered in detail here as they
|
|
could change with implementation details.
|
|
|
|
.. _uclamp-buckets:
|
|
|
|
2.1. Buckets
|
|
------------
|
|
|
|
::
|
|
|
|
[struct rq]
|
|
|
|
(bottom) (top)
|
|
|
|
0 1024
|
|
| |
|
|
+-----------+-----------+-----------+---- ----+-----------+
|
|
| Bucket 0 | Bucket 1 | Bucket 2 | ... | Bucket N |
|
|
+-----------+-----------+-----------+---- ----+-----------+
|
|
: : :
|
|
+- p0 +- p3 +- p4
|
|
: :
|
|
+- p1 +- p5
|
|
:
|
|
+- p2
|
|
|
|
|
|
.. note::
|
|
The diagram above is an illustration rather than a true depiction of the
|
|
internal data structure.
|
|
|
|
To reduce the search space when trying to decide the effective uclamp value of
|
|
an rq as tasks are enqueued/dequeued, the whole utilization range is divided
|
|
into N buckets where N is configured at compile time by setting
|
|
CONFIG_UCLAMP_BUCKETS_COUNT. By default it is set to 5.
|
|
|
|
The rq has a bucket for each uclamp_id tunables: [UCLAMP_MIN, UCLAMP_MAX].
|
|
|
|
The range of each bucket is 1024/N. For example, for the default value of
|
|
5 there will be 5 buckets, each of which will cover the following range:
|
|
|
|
::
|
|
|
|
DELTA = round_closest(1024/5) = 204.8 = 205
|
|
|
|
Bucket 0: [0:204]
|
|
Bucket 1: [205:409]
|
|
Bucket 2: [410:614]
|
|
Bucket 3: [615:819]
|
|
Bucket 4: [820:1024]
|
|
|
|
When a task p with following tunable parameters
|
|
|
|
::
|
|
|
|
p->uclamp[UCLAMP_MIN] = 300
|
|
p->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
is enqueued into the rq, bucket 1 will be incremented for UCLAMP_MIN and bucket
|
|
4 will be incremented for UCLAMP_MAX to reflect the fact the rq has a task in
|
|
this range.
|
|
|
|
The rq then keeps track of its current effective uclamp value for each
|
|
uclamp_id.
|
|
|
|
When a task p is enqueued, the rq value changes to:
|
|
|
|
::
|
|
|
|
// update bucket logic goes here
|
|
rq->uclamp[UCLAMP_MIN] = max(rq->uclamp[UCLAMP_MIN], p->uclamp[UCLAMP_MIN])
|
|
// repeat for UCLAMP_MAX
|
|
|
|
Similarly, when p is dequeued the rq value changes to:
|
|
|
|
::
|
|
|
|
// update bucket logic goes here
|
|
rq->uclamp[UCLAMP_MIN] = search_top_bucket_for_highest_value()
|
|
// repeat for UCLAMP_MAX
|
|
|
|
When all buckets are empty, the rq uclamp values are reset to system defaults.
|
|
See :ref:`section 3.4 <uclamp-default-values>` for details on default values.
|
|
|
|
|
|
2.2. Max aggregation
|
|
--------------------
|
|
|
|
Util clamp is tuned to honour the request for the task that requires the
|
|
highest performance point.
|
|
|
|
When multiple tasks are attached to the same rq, then util clamp must make sure
|
|
the task that needs the highest performance point gets it even if there's
|
|
another task that doesn't need it or is disallowed from reaching this point.
|
|
|
|
For example, if there are multiple tasks attached to an rq with the following
|
|
values:
|
|
|
|
::
|
|
|
|
p0->uclamp[UCLAMP_MIN] = 300
|
|
p0->uclamp[UCLAMP_MAX] = 900
|
|
|
|
p1->uclamp[UCLAMP_MIN] = 500
|
|
p1->uclamp[UCLAMP_MAX] = 500
|
|
|
|
then assuming both p0 and p1 are enqueued to the same rq, both UCLAMP_MIN
|
|
and UCLAMP_MAX become:
|
|
|
|
::
|
|
|
|
rq->uclamp[UCLAMP_MIN] = max(300, 500) = 500
|
|
rq->uclamp[UCLAMP_MAX] = max(900, 500) = 900
|
|
|
|
As we shall see in :ref:`section 5.1 <uclamp-capping-fail>`, this max
|
|
aggregation is the cause of one of limitations when using util clamp, in
|
|
particular for UCLAMP_MAX hint when user space would like to save power.
|
|
|
|
2.3. Hierarchical aggregation
|
|
-----------------------------
|
|
|
|
As stated earlier, util clamp is a property of every task in the system. But
|
|
the actual applied (effective) value can be influenced by more than just the
|
|
request made by the task or another actor on its behalf (middleware library).
|
|
|
|
The effective util clamp value of any task is restricted as follows:
|
|
|
|
1. By the uclamp settings defined by the cgroup CPU controller it is attached
|
|
to, if any.
|
|
2. The restricted value in (1) is then further restricted by the system wide
|
|
uclamp settings.
|
|
|
|
:ref:`Section 3 <uclamp-interfaces>` discusses the interfaces and will expand
|
|
further on that.
|
|
|
|
For now suffice to say that if a task makes a request, its actual effective
|
|
value will have to adhere to some restrictions imposed by cgroup and system
|
|
wide settings.
|
|
|
|
The system will still accept the request even if effectively will be beyond the
|
|
constraints, but as soon as the task moves to a different cgroup or a sysadmin
|
|
modifies the system settings, the request will be satisfied only if it is
|
|
within new constraints.
|
|
|
|
In other words, this aggregation will not cause an error when a task changes
|
|
its uclamp values, but rather the system may not be able to satisfy requests
|
|
based on those factors.
|
|
|
|
2.4. Range
|
|
----------
|
|
|
|
Uclamp performance request has the range of 0 to 1024 inclusive.
|
|
|
|
For cgroup interface percentage is used (that is 0 to 100 inclusive).
|
|
Just like other cgroup interfaces, you can use 'max' instead of 100.
|
|
|
|
.. _uclamp-interfaces:
|
|
|
|
3. Interfaces
|
|
=============
|
|
|
|
3.1. Per task interface
|
|
-----------------------
|
|
|
|
sched_setattr() syscall was extended to accept two new fields:
|
|
|
|
* sched_util_min: requests the minimum performance point the system should run
|
|
at when this task is running. Or lower performance bound.
|
|
* sched_util_max: requests the maximum performance point the system should run
|
|
at when this task is running. Or upper performance bound.
|
|
|
|
For example, the following scenario have 40% to 80% utilization constraints:
|
|
|
|
::
|
|
|
|
attr->sched_util_min = 40% * 1024;
|
|
attr->sched_util_max = 80% * 1024;
|
|
|
|
When task @p is running, **the scheduler should try its best to ensure it
|
|
starts at 40% performance level**. If the task runs for a long enough time so
|
|
that its actual utilization goes above 80%, the utilization, or performance
|
|
level, will be capped.
|
|
|
|
The special value -1 is used to reset the uclamp settings to the system
|
|
default.
|
|
|
|
Note that resetting the uclamp value to system default using -1 is not the same
|
|
as manually setting uclamp value to system default. This distinction is
|
|
important because as we shall see in system interfaces, the default value for
|
|
RT could be changed. SCHED_NORMAL/OTHER might gain similar knobs too in the
|
|
future.
|
|
|
|
3.2. cgroup interface
|
|
---------------------
|
|
|
|
There are two uclamp related values in the CPU cgroup controller:
|
|
|
|
* cpu.uclamp.min
|
|
* cpu.uclamp.max
|
|
|
|
When a task is attached to a CPU controller, its uclamp values will be impacted
|
|
as follows:
|
|
|
|
* cpu.uclamp.min is a protection as described in :ref:`section 3-3 of cgroup
|
|
v2 documentation <cgroupv2-protections-distributor>`.
|
|
|
|
If a task uclamp_min value is lower than cpu.uclamp.min, then the task will
|
|
inherit the cgroup cpu.uclamp.min value.
|
|
|
|
In a cgroup hierarchy, effective cpu.uclamp.min is the max of (child,
|
|
parent).
|
|
|
|
* cpu.uclamp.max is a limit as described in :ref:`section 3-2 of cgroup v2
|
|
documentation <cgroupv2-limits-distributor>`.
|
|
|
|
If a task uclamp_max value is higher than cpu.uclamp.max, then the task will
|
|
inherit the cgroup cpu.uclamp.max value.
|
|
|
|
In a cgroup hierarchy, effective cpu.uclamp.max is the min of (child,
|
|
parent).
|
|
|
|
For example, given following parameters:
|
|
|
|
::
|
|
|
|
p0->uclamp[UCLAMP_MIN] = // system default;
|
|
p0->uclamp[UCLAMP_MAX] = // system default;
|
|
|
|
p1->uclamp[UCLAMP_MIN] = 40% * 1024;
|
|
p1->uclamp[UCLAMP_MAX] = 50% * 1024;
|
|
|
|
cgroup0->cpu.uclamp.min = 20% * 1024;
|
|
cgroup0->cpu.uclamp.max = 60% * 1024;
|
|
|
|
cgroup1->cpu.uclamp.min = 60% * 1024;
|
|
cgroup1->cpu.uclamp.max = 100% * 1024;
|
|
|
|
when p0 and p1 are attached to cgroup0, the values become:
|
|
|
|
::
|
|
|
|
p0->uclamp[UCLAMP_MIN] = cgroup0->cpu.uclamp.min = 20% * 1024;
|
|
p0->uclamp[UCLAMP_MAX] = cgroup0->cpu.uclamp.max = 60% * 1024;
|
|
|
|
p1->uclamp[UCLAMP_MIN] = 40% * 1024; // intact
|
|
p1->uclamp[UCLAMP_MAX] = 50% * 1024; // intact
|
|
|
|
when p0 and p1 are attached to cgroup1, these instead become:
|
|
|
|
::
|
|
|
|
p0->uclamp[UCLAMP_MIN] = cgroup1->cpu.uclamp.min = 60% * 1024;
|
|
p0->uclamp[UCLAMP_MAX] = cgroup1->cpu.uclamp.max = 100% * 1024;
|
|
|
|
p1->uclamp[UCLAMP_MIN] = cgroup1->cpu.uclamp.min = 60% * 1024;
|
|
p1->uclamp[UCLAMP_MAX] = 50% * 1024; // intact
|
|
|
|
Note that cgroup interfaces allows cpu.uclamp.max value to be lower than
|
|
cpu.uclamp.min. Other interfaces don't allow that.
|
|
|
|
3.3. System interface
|
|
---------------------
|
|
|
|
3.3.1 sched_util_clamp_min
|
|
--------------------------
|
|
|
|
System wide limit of allowed UCLAMP_MIN range. By default it is set to 1024,
|
|
which means that permitted effective UCLAMP_MIN range for tasks is [0:1024].
|
|
By changing it to 512 for example the range reduces to [0:512]. This is useful
|
|
to restrict how much boosting tasks are allowed to acquire.
|
|
|
|
Requests from tasks to go above this knob value will still succeed, but
|
|
they won't be satisfied until it is more than p->uclamp[UCLAMP_MIN].
|
|
|
|
The value must be smaller than or equal to sched_util_clamp_max.
|
|
|
|
3.3.2 sched_util_clamp_max
|
|
--------------------------
|
|
|
|
System wide limit of allowed UCLAMP_MAX range. By default it is set to 1024,
|
|
which means that permitted effective UCLAMP_MAX range for tasks is [0:1024].
|
|
|
|
By changing it to 512 for example the effective allowed range reduces to
|
|
[0:512]. This means is that no task can run above 512, which implies that all
|
|
rqs are restricted too. IOW, the whole system is capped to half its performance
|
|
capacity.
|
|
|
|
This is useful to restrict the overall maximum performance point of the system.
|
|
For example, it can be handy to limit performance when running low on battery
|
|
or when the system wants to limit access to more energy hungry performance
|
|
levels when it's in idle state or screen is off.
|
|
|
|
Requests from tasks to go above this knob value will still succeed, but they
|
|
won't be satisfied until it is more than p->uclamp[UCLAMP_MAX].
|
|
|
|
The value must be greater than or equal to sched_util_clamp_min.
|
|
|
|
.. _uclamp-default-values:
|
|
|
|
3.4. Default values
|
|
-------------------
|
|
|
|
By default all SCHED_NORMAL/SCHED_OTHER tasks are initialized to:
|
|
|
|
::
|
|
|
|
p_fair->uclamp[UCLAMP_MIN] = 0
|
|
p_fair->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
That is, by default they're boosted to run at the maximum performance point of
|
|
changed at boot or runtime. No argument was made yet as to why we should
|
|
provide this, but can be added in the future.
|
|
|
|
For SCHED_FIFO/SCHED_RR tasks:
|
|
|
|
::
|
|
|
|
p_rt->uclamp[UCLAMP_MIN] = 1024
|
|
p_rt->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
That is by default they're boosted to run at the maximum performance point of
|
|
the system which retains the historical behavior of the RT tasks.
|
|
|
|
RT tasks default uclamp_min value can be modified at boot or runtime via
|
|
sysctl. See below section.
|
|
|
|
.. _sched-util-clamp-min-rt-default:
|
|
|
|
3.4.1 sched_util_clamp_min_rt_default
|
|
-------------------------------------
|
|
|
|
Running RT tasks at maximum performance point is expensive on battery powered
|
|
devices and not necessary. To allow system developer to offer good performance
|
|
guarantees for these tasks without pushing it all the way to maximum
|
|
performance point, this sysctl knob allows tuning the best boost value to
|
|
address the system requirement without burning power running at maximum
|
|
performance point all the time.
|
|
|
|
Application developer are encouraged to use the per task util clamp interface
|
|
to ensure they are performance and power aware. Ideally this knob should be set
|
|
to 0 by system designers and leave the task of managing performance
|
|
requirements to the apps.
|
|
|
|
4. How to use util clamp
|
|
========================
|
|
|
|
Util clamp promotes the concept of user space assisted power and performance
|
|
management. At the scheduler level there is no info required to make the best
|
|
decision. However, with util clamp user space can hint to the scheduler to make
|
|
better decision about task placement and frequency selection.
|
|
|
|
Best results are achieved by not making any assumptions about the system the
|
|
application is running on and to use it in conjunction with a feedback loop to
|
|
dynamically monitor and adjust. Ultimately this will allow for a better user
|
|
experience at a better perf/watt.
|
|
|
|
For some systems and use cases, static setup will help to achieve good results.
|
|
Portability will be a problem in this case. How much work one can do at 100,
|
|
200 or 1024 is different for each system. Unless there's a specific target
|
|
system, static setup should be avoided.
|
|
|
|
There are enough possibilities to create a whole framework based on util clamp
|
|
or self contained app that makes use of it directly.
|
|
|
|
4.1. Boost important and DVFS-latency-sensitive tasks
|
|
-----------------------------------------------------
|
|
|
|
A GUI task might not be busy to warrant driving the frequency high when it
|
|
wakes up. However, it requires to finish its work within a specific time window
|
|
to deliver the desired user experience. The right frequency it requires at
|
|
wakeup will be system dependent. On some underpowered systems it will be high,
|
|
on other overpowered ones it will be low or 0.
|
|
|
|
This task can increase its UCLAMP_MIN value every time it misses the deadline
|
|
to ensure on next wake up it runs at a higher performance point. It should try
|
|
to approach the lowest UCLAMP_MIN value that allows to meet its deadline on any
|
|
particular system to achieve the best possible perf/watt for that system.
|
|
|
|
On heterogeneous systems, it might be important for this task to run on
|
|
a faster CPU.
|
|
|
|
**Generally it is advised to perceive the input as performance level or point
|
|
which will imply both task placement and frequency selection**.
|
|
|
|
4.2. Cap background tasks
|
|
-------------------------
|
|
|
|
Like explained for Android case in the introduction. Any app can lower
|
|
UCLAMP_MAX for some background tasks that don't care about performance but
|
|
could end up being busy and consume unnecessary system resources on the system.
|
|
|
|
4.3. Powersave mode
|
|
-------------------
|
|
|
|
sched_util_clamp_max system wide interface can be used to limit all tasks from
|
|
operating at the higher performance points which are usually energy
|
|
inefficient.
|
|
|
|
This is not unique to uclamp as one can achieve the same by reducing max
|
|
frequency of the cpufreq governor. It can be considered a more convenient
|
|
alternative interface.
|
|
|
|
4.4. Per-app performance restriction
|
|
------------------------------------
|
|
|
|
Middleware/Utility can provide the user an option to set UCLAMP_MIN/MAX for an
|
|
app every time it is executed to guarantee a minimum performance point and/or
|
|
limit it from draining system power at the cost of reduced performance for
|
|
these apps.
|
|
|
|
If you want to prevent your laptop from heating up while on the go from
|
|
compiling the kernel and happy to sacrifice performance to save power, but
|
|
still would like to keep your browser performance intact, uclamp makes it
|
|
possible.
|
|
|
|
5. Limitations
|
|
==============
|
|
|
|
.. _uclamp-capping-fail:
|
|
|
|
5.1. Capping frequency with uclamp_max fails under certain conditions
|
|
---------------------------------------------------------------------
|
|
|
|
If task p0 is capped to run at 512:
|
|
|
|
::
|
|
|
|
p0->uclamp[UCLAMP_MAX] = 512
|
|
|
|
and it shares the rq with p1 which is free to run at any performance point:
|
|
|
|
::
|
|
|
|
p1->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
then due to max aggregation the rq will be allowed to reach max performance
|
|
point:
|
|
|
|
::
|
|
|
|
rq->uclamp[UCLAMP_MAX] = max(512, 1024) = 1024
|
|
|
|
Assuming both p0 and p1 have UCLAMP_MIN = 0, then the frequency selection for
|
|
the rq will depend on the actual utilization value of the tasks.
|
|
|
|
If p1 is a small task but p0 is a CPU intensive task, then due to the fact that
|
|
both are running at the same rq, p1 will cause the frequency capping to be left
|
|
from the rq although p1, which is allowed to run at any performance point,
|
|
doesn't actually need to run at that frequency.
|
|
|
|
5.2. UCLAMP_MAX can break PELT (util_avg) signal
|
|
------------------------------------------------
|
|
|
|
PELT assumes that frequency will always increase as the signals grow to ensure
|
|
there's always some idle time on the CPU. But with UCLAMP_MAX, this frequency
|
|
increase will be prevented which can lead to no idle time in some
|
|
circumstances. When there's no idle time, a task will stuck in a busy loop,
|
|
which would result in util_avg being 1024.
|
|
|
|
Combing with issue described below, this can lead to unwanted frequency spikes
|
|
when severely capped tasks share the rq with a small non capped task.
|
|
|
|
As an example if task p, which have:
|
|
|
|
::
|
|
|
|
p0->util_avg = 300
|
|
p0->uclamp[UCLAMP_MAX] = 0
|
|
|
|
wakes up on an idle CPU, then it will run at min frequency (Fmin) this
|
|
CPU is capable of. The max CPU frequency (Fmax) matters here as well,
|
|
since it designates the shortest computational time to finish the task's
|
|
work on this CPU.
|
|
|
|
::
|
|
|
|
rq->uclamp[UCLAMP_MAX] = 0
|
|
|
|
If the ratio of Fmax/Fmin is 3, then maximum value will be:
|
|
|
|
::
|
|
|
|
300 * (Fmax/Fmin) = 900
|
|
|
|
which indicates the CPU will still see idle time since 900 is < 1024. The
|
|
_actual_ util_avg will not be 900 though, but somewhere between 300 and 900. As
|
|
long as there's idle time, p->util_avg updates will be off by a some margin,
|
|
but not proportional to Fmax/Fmin.
|
|
|
|
::
|
|
|
|
p0->util_avg = 300 + small_error
|
|
|
|
Now if the ratio of Fmax/Fmin is 4, the maximum value becomes:
|
|
|
|
::
|
|
|
|
300 * (Fmax/Fmin) = 1200
|
|
|
|
which is higher than 1024 and indicates that the CPU has no idle time. When
|
|
this happens, then the _actual_ util_avg will become:
|
|
|
|
::
|
|
|
|
p0->util_avg = 1024
|
|
|
|
If task p1 wakes up on this CPU, which have:
|
|
|
|
::
|
|
|
|
p1->util_avg = 200
|
|
p1->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
then the effective UCLAMP_MAX for the CPU will be 1024 according to max
|
|
aggregation rule. But since the capped p0 task was running and throttled
|
|
severely, then the rq->util_avg will be:
|
|
|
|
::
|
|
|
|
p0->util_avg = 1024
|
|
p1->util_avg = 200
|
|
|
|
rq->util_avg = 1024
|
|
rq->uclamp[UCLAMP_MAX] = 1024
|
|
|
|
Hence lead to a frequency spike since if p0 wasn't throttled we should get:
|
|
|
|
::
|
|
|
|
p0->util_avg = 300
|
|
p1->util_avg = 200
|
|
|
|
rq->util_avg = 500
|
|
|
|
and run somewhere near mid performance point of that CPU, not the Fmax we get.
|
|
|
|
5.3. Schedutil response time issues
|
|
-----------------------------------
|
|
|
|
schedutil has three limitations:
|
|
|
|
1. Hardware takes non-zero time to respond to any frequency change
|
|
request. On some platforms can be in the order of few ms.
|
|
2. Non fast-switch systems require a worker deadline thread to wake up
|
|
and perform the frequency change, which adds measurable overhead.
|
|
3. schedutil rate_limit_us drops any requests during this rate_limit_us
|
|
window.
|
|
|
|
If a relatively small task is doing critical job and requires a certain
|
|
performance point when it wakes up and starts running, then all these
|
|
limitations will prevent it from getting what it wants in the time scale it
|
|
expects.
|
|
|
|
This limitation is not only impactful when using uclamp, but will be more
|
|
prevalent as we no longer gradually ramp up or down. We could easily be
|
|
jumping between frequencies depending on the order tasks wake up, and their
|
|
respective uclamp values.
|
|
|
|
We regard that as a limitation of the capabilities of the underlying system
|
|
itself.
|
|
|
|
There is room to improve the behavior of schedutil rate_limit_us, but not much
|
|
to be done for 1 or 2. They are considered hard limitations of the system.
|