[PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Qais Yousef]

The new util clamp feature needs a document explaining what it is and
how to use it. The new document hopefully covers everything one needs to
know about uclamp.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---

Changes in v2:

	* Address various style comments from Bagas

 Documentation/scheduler/index.rst            |   1 +
 Documentation/scheduler/sched-util-clamp.rst | 732 +++++++++++++++++++
 2 files changed, 733 insertions(+)
 create mode 100644 Documentation/scheduler/sched-util-clamp.rst

diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index b430d856056a..f12d0d06de3a 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -15,6 +15,7 @@ Linux Scheduler
     sched-capacity
     sched-energy
     schedutil
+    sched-util-clamp
     sched-nice-design
     sched-rt-group
     sched-stats
diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
new file mode 100644
index 000000000000..da1881e293c3
--- /dev/null
+++ b/Documentation/scheduler/sched-util-clamp.rst
@@ -0,0 +1,732 @@
+====================
+Utilization Clamping
+====================
+
+1.  Introduction
+================
+
+Utilization clamping 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.
+
+It is often referred to as util clamp and uclamp. You'll find all variations
+used interchangeably in this documentation and in the source code.
+
+Uclamp is a hinting mechanism that allows the scheduler to understand the
+performance requirements and restrictions of the tasks. Hence help it make
+a better placement decision. And when schedutil cpufreq governor is used, util
+clamp will influence the frequency selection as well.
+
+Since 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. I.e: 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 performance
+constraints request/hint. It consists of two components:
+
+        * UCLAMP_MIN, which sets a lower bound.
+        * UCLAMP_MAX, which sets an 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 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 100ms
+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 in ensuring 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 constraint 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; 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.
+
+By making these uclamp performance requests, or rather hints, user space can
+ensure system resources are used optimally to deliver the best user experience
+the system is capable of.
+
+Another use case is to help with overcoming the ramp up latency inherit in how
+scheduler utilization signal is calculated.
+
+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.
+
+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 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 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 section 3.4 for default values and 3.4.1 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 rq 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
+impacted 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 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, we increment a counter in the matching bucket. And on
+dequeue we decrement it. This makes keeping track of the effective uclamp value
+at rq level a lot easier.
+
+As we enqueue and dequeue tasks we keep track of the current effective uclamp
+value of the rq. See section 2.1 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. This implies 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 honour uclamp 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.
+
+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: [UCLAMP_MIN, UCLAMP_MAX].
+
+The range of each bucket is 1024/N. For example for the default value of 5 we
+will have 5 buckets, each of which will cover the following range:
+
+.. code-block:: c
+
+        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
+
+.. code-block:: c
+
+        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 as follows:
+
+.. code-block:: c
+
+        // update bucket logic goes here
+        rq->uclamp[UCLAMP_MIN] = max(rq->uclamp[UCLAMP_MIN], p->uclamp[UCLAMP_MIN])
+        // repeat for UCLAMP_MAX
+
+When a task is p dequeued the rq value changes as follows:
+
+.. code-block:: c
+
+        // 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 section 3.4 for 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:
+
+.. code-block:: c
+
+        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
+
+.. code-block:: c
+
+        rq->uclamp[UCLAMP_MIN] = max(300, 500) = 500
+        rq->uclamp[UCLAMP_MAX] = max(900, 500) = 900
+
+As we shall see in section 5.1, this max aggregation is the cause of one of the
+limitations when using util clamp. Particularly 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.
+
+Section 3 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 look
+different; but as soon as the task moves to a different cgroup or a sysadmin
+modifies the system settings, it'll be able to get what it wants if the new
+settings allows it.
+
+In other words, this aggregation will not cause an error when a task changes
+its uclamp values. It just might not be able to achieve it based on those
+factors.
+
+2.4  Range
+----------
+
+Uclamp performance request follow the utilization range: [0:1024] inclusive.
+
+For cgroup interface percentage is used: [0:100] inclusive.
+You can use 'max' instead of 100 like other cgroup 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:
+
+.. code-block:: c
+
+        attr->sched_util_min = 40% * 1024;
+        attr->sched_util_max = 80% * 1024;
+
+Will tell the system that when task @p is running, it should try its best to
+ensure it starts at a performance point no less than 40% of maximum system's
+capability.
+
+And if the task runs for a long enough time so that its actual utilization goes
+above 80%, then it should not cause the system to operate at a performance
+point higher than that.
+
+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 setting the uclamp value to system default.
+
+.. code-block:: c
+
+        attr->sched_util_min = -1  // p0 is reset to system default e.g: 0
+
+not the same as
+
+.. code-block:: c
+
+        attr->sched_util_min = 0   // p0 is set to 0, the fact it is the same
+                                   // as system default is irrelevant
+
+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 section 3-3 in
+  Documentation/admin-guide/cgroup-v2.rst.
+
+  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 section 3-2 in
+  Documentation/admin-guide/cgroup-v2.rst.
+
+  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:
+
+.. code-block:: c
+
+        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
+
+.. code-block:: c
+
+        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
+
+.. code-block:: c
+
+        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 set to 1024, which
+means tasks are allowed to reach an effective UCLAMP_MIN value in the range of
+[0:1024].
+
+By changing it to 512 for example the effective allowed 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 point will still succeed, but effectively
+they won't be achieved until this value is >= 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 set to 1024, which
+means tasks are allowed to reach an effective UCLAMP_MAX value in the range of
+[0:1024].
+
+By changing it to 512 for example the effective allowed range reduces to
+[0:512]. The visible impact of this is that no task can run above 512, which in
+return means 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.
+
+Can be handy to limit performance when running low on battery.
+
+Requests from tasks to go above this point will still succeed, but effectively
+they won't be achieved until this value is >= p->uclamp[UCLAMP_MAX].
+
+The value must be greater than or equal to sched_util_clamp_min.
+
+3.4  Default values
+-------------------
+
+By default all SCHED_NORMAL/SCHED_OTHER tasks are initialized to:
+
+.. code-block:: c
+
+        p_fair->uclamp[UCLAMP_MIN] = 0
+        p_fair->uclamp[UCLAMP_MAX] = 1024
+
+That is no boosting or restriction on any task. These default values can't be
+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:
+
+.. code-block:: c
+
+        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 section 3.4.1.
+
+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 designers to offer good performance
+guarantees for RT 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 designers 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 themselves.
+
+4.  How to use util clamp
+=========================
+
+Util clamp promotes the concept of user space assisted power and performance
+management. At the scheduler level the info required to make the best decision
+are non existent. But 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. After all how much work one can do
+at 100, 200 or 1024 is unknown and a special property of every system. Unless
+there's a specific target system, static setup should be avoided.
+
+All in all 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. But it requires to finish its work within a specific period of time
+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 a 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 bigger 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 restrictions
+-------------------------------------
+
+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 enables that.
+
+5.  Limitations
+===============
+
+5.1  Capping frequency with uclamp_max fails under certain conditions
+---------------------------------------------------------------------
+
+If task p0 is capped to run at 512
+
+.. code-block:: c
+
+        p0->uclamp[UCLAMP_MAX] = 512
+
+is sharing the rq with p1 which is free to run at any performance point
+
+.. code-block:: c
+
+        p1->uclamp[UCLAMP_MAX] = 1024
+
+then due to max aggregation the rq will be allowed to reach max performance
+point
+
+.. code-block:: c
+
+        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, we will prevent
+this frequency increase which can lead to no idle time in some circumstances.
+When there's no idle time, then a task will look like a busy loop, which would
+result in util_avg being 1024.
+
+Combing with issue described in 5.2, this an lead to unwanted frequency spikes
+when severely capped tasks share the rq with a small non capped task.
+
+As an example if task p
+
+.. code-block:: c
+
+        p0->util_avg = 300
+        p0->uclamp[UCLAMP_MAX] = 0
+
+wakes up on an idle CPU, then it will run at min frequency this CPU is capable
+of.
+
+.. code-block:: c
+
+        rq->uclamp[UCLAMP_MAX] = 0
+
+If the ratio of Fmax/Fmin is 3, then
+
+.. code-block:: c
+
+        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. It will be higher than 300, but won't
+approach 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.
+
+.. code-block:: c
+
+        p0->util_avg = 300 + small_error
+
+Now if the ratio of Fmax/Fmin is 4, then
+
+.. code-block:: c
+
+        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 1024.
+
+.. code-block:: c
+
+        p0->util_avg = 1024
+
+If task p1 wakes up on this CPU
+
+.. code-block:: c
+
+        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 1024.
+
+.. code-block:: c
+
+        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
+
+.. code-block:: c
+
+        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.
-- 
2.25.1

Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Jonathan Corbet]

Qais Yousef <qyousef@layalina.io> writes:

> The new util clamp feature needs a document explaining what it is and
> how to use it. The new document hopefully covers everything one needs to
> know about uclamp.
>
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>
> Changes in v2:
>
> 	* Address various style comments from Bagas

Just a handful of nits - this looks like a good addition to our
documentation, thanks.

>  Documentation/scheduler/index.rst            |   1 +
>  Documentation/scheduler/sched-util-clamp.rst | 732 +++++++++++++++++++
>  2 files changed, 733 insertions(+)
>  create mode 100644 Documentation/scheduler/sched-util-clamp.rst
>
> diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
> index b430d856056a..f12d0d06de3a 100644
> --- a/Documentation/scheduler/index.rst
> +++ b/Documentation/scheduler/index.rst
> @@ -15,6 +15,7 @@ Linux Scheduler
>      sched-capacity
>      sched-energy
>      schedutil
> +    sched-util-clamp
>      sched-nice-design
>      sched-rt-group
>      sched-stats
> diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
> new file mode 100644
> index 000000000000..da1881e293c3
> --- /dev/null
> +++ b/Documentation/scheduler/sched-util-clamp.rst
> @@ -0,0 +1,732 @@
> +====================
> +Utilization Clamping
> +====================

RST files, too, should have SPDX tags at the beginning.  The practice
rather lags the rule here, but we're trying...

> +1.  Introduction
> +================
> +
> +Utilization clamping 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.
> +
> +It is often referred to as util clamp and uclamp. You'll find all variations
> +used interchangeably in this documentation and in the source code.
> +
> +Uclamp is a hinting mechanism that allows the scheduler to understand the
> +performance requirements and restrictions of the tasks. Hence help it make

...of the tasks, and thus help it ... 

> +a better placement decision. And when schedutil cpufreq governor is used, util
> +clamp will influence the frequency selection as well.
> +
> +Since scheduler and schedutil are both driven by PELT (util_avg) signals, util

Since *the* scheduler

> +clamp acts on that to achieve its goal by clamping the signal to a certain
> +point; hence the name. I.e: 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 performance
> +constraints request/hint. It consists of two components:
> +
> +        * UCLAMP_MIN, which sets a lower bound.
> +        * UCLAMP_MAX, which sets an 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 FPS. It can dynamically increase the minimum performance point

FPS?

> +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 100ms
> +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 in ensuring 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 constraint how much

to *constrain* how

> +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; 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.
> +
> +By making these uclamp performance requests, or rather hints, user space can
> +ensure system resources are used optimally to deliver the best user experience
> +the system is capable of.
> +
> +Another use case is to help with overcoming the ramp up latency inherit in how
> +scheduler utilization signal is calculated.
> +
> +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.
> +
> +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 thermal subsystem too to ensure the

with *the* thermal subsystem

> +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 SCHED_FIFO/RR case, uclamp gives the option to run RT tasks at any

In *the* SCHED_FIFO...

> +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 section 3.4 for default values and 3.4.1 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 rq 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
> +impacted 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 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, we increment a counter in the matching bucket. And on
> +dequeue we decrement it. This makes keeping track of the effective uclamp value
> +at rq level a lot easier.
> +
> +As we enqueue and dequeue tasks we keep track of the current effective uclamp
> +value of the rq. See section 2.1 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. This implies 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 honour uclamp 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.
> +
> +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: [UCLAMP_MIN, UCLAMP_MAX].
> +
> +The range of each bucket is 1024/N. For example for the default value of 5 we
> +will have 5 buckets, each of which will cover the following range:
> +
> +.. code-block:: c

If you want to minimize markup, you could use basic literal blocks
rather than ..code-block::, which really only has the effect of syntax
coloring in HTML output.  I don't find that worth the extra clutter
myself, but others clearly disagree with me...

> +        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]
> +

I didn't find anything worth quibbling about after this.

Thanks,

jon

Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Bagas Sanjaya]

[-- Attachment #1: Type: text/plain, Size: 889 bytes --]

On Sun, Nov 27, 2022 at 02:26:57PM +0000, Qais Yousef wrote:
> The new util clamp feature needs a document explaining what it is and
> how to use it. The new document hopefully covers everything one needs to
> know about uclamp.
> 

For patch subject, better say "Documentation: sched: Document util clamp
feature".

> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
> 
> Changes in v2:
> 
> 	* Address various style comments from Bagas
> 

I don't see any of my wording suggestions from [1] being applied. Still
a rather confused to read (maybe should be applied on top of jon's
suggestions)?

[1]: https://lore.kernel.org/lkml/Y3II59dyKuvQGIhG@debian.me/
[2]: https://lore.kernel.org/linux-doc/87cz976pwn.fsf@meer.lwn.net/

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Lukasz Luba]

Hi Qais,

I have a few comments below. IMO this documentation
is also for user-space developers who might be missing
some specific 'know how'.

On 11/27/22 14:26, Qais Yousef wrote:
> The new util clamp feature needs a document explaining what it is and
> how to use it. The new document hopefully covers everything one needs to
> know about uclamp.
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
> 
> Changes in v2:
> 
> 	* Address various style comments from Bagas
> 
>   Documentation/scheduler/index.rst            |   1 +
>   Documentation/scheduler/sched-util-clamp.rst | 732 +++++++++++++++++++
>   2 files changed, 733 insertions(+)
>   create mode 100644 Documentation/scheduler/sched-util-clamp.rst
> 
> diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
> index b430d856056a..f12d0d06de3a 100644
> --- a/Documentation/scheduler/index.rst
> +++ b/Documentation/scheduler/index.rst
> @@ -15,6 +15,7 @@ Linux Scheduler
>       sched-capacity
>       sched-energy
>       schedutil
> +    sched-util-clamp
>       sched-nice-design
>       sched-rt-group
>       sched-stats
> diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
> new file mode 100644
> index 000000000000..da1881e293c3
> --- /dev/null
> +++ b/Documentation/scheduler/sched-util-clamp.rst
> @@ -0,0 +1,732 @@
> +====================
> +Utilization Clamping
> +====================
> +
> +1.  Introduction
> +================
> +
> +Utilization clamping 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.
> +
> +It is often referred to as util clamp and uclamp. You'll find all variations
> +used interchangeably in this documentation and in the source code.
> +
> +Uclamp is a hinting mechanism that allows the scheduler to understand the
> +performance requirements and restrictions of the tasks. Hence help it make
> +a better placement decision. And when schedutil cpufreq governor is used, util
> +clamp will influence the frequency selection as well.

s/the frequency/the CPU frequency/

[snip]

> +
> +Another example is in Android where tasks are classified as background,
> +foreground, top-app, etc. Util clamp can be used to constraint 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; the constraint will help bias the background tasks to

I would add some explenation here, since you then reffer to the
'little cores' or 'big cores'.

s/heterogeneous systems/ heterogeneous systems (e.g. Arm big.LITTLE)/

> +stay on the little cores which will ensure that:

s/the little cores/the LITTLE cores (CPUs with capacity < 1024)/

> +
> +        1. The big cores are free to run top-app tasks immediately. top-app

s/the big cores/the big cores (CPUs with capacity = 1024)

> +           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.
> +
> +By making these uclamp performance requests, or rather hints, user space can
> +ensure system resources are used optimally to deliver the best user experience
> +the system is capable of.
> +
> +Another use case is to help with overcoming the ramp up latency inherit in how
> +scheduler utilization signal is calculated.

I would emphasize this section somehow, since this is very important.
Maybe a section header?

'Overcoming the utilization ramp up latency'

> +
> +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.

This example is good. You can add that this issue should go away when
uclamp_min is set for such thread just after it's created by the
developer.

I would add another example, with a latency sensitive task e.g.
garbage collector thread. Such task tries to keep the memory in a
healthy state. This kind of task might benefit from running
on higher performance CPU even if its real utilization might be
small (e.g. 50 util) and considered to fit on the LITTLE CPU.

[snip]

> +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 rq level, which brings us
> +to the main design challenge.

s/rq/CPU runqueue (rq)/

[snip]

> +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:
> +
> +.. code-block:: c
> +
> +        attr->sched_util_min = 40% * 1024;
> +        attr->sched_util_max = 80% * 1024;
> +
> +Will tell the system that when task @p is running, it should try its best to
> +ensure it starts at a performance point no less than 40% of maximum system's
> +capability.

I would emphasize this, since this is very imporant IMO.
The maximum system's capability is ment to be the fastest and most
performant CPU in the SoC (aka the biggest CPU).

[snip]

> +
> +3.3.2  sched_util_clamp_max
> +---------------------------
> +
> +System wide limit of allowed UCLAMP_MAX range. By default set to 1024, which
> +means tasks are allowed to reach an effective UCLAMP_MAX value in the range of
> +[0:1024].
> +
> +By changing it to 512 for example the effective allowed range reduces to
> +[0:512]. The visible impact of this is that no task can run above 512, which in
> +return means 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.
> +
> +Can be handy to limit performance when running low on battery.

Or when the system knows that it can spent more time on computations
because the user is not using the device but updates and re-compilations
are done in background while the screen is idle.


[snip]

> +
> +On heterogeneous systems, it might be important for this task to run on
> +a bigger CPU.
> +
> +Generally it is advised to perceive the input as performance level or point
> +which will imply both task placement and frequency selection.

s/frequency/ CPU frequency/

BTW this sentence is really good. Maybe it could be somewhere in a
header (1. Introduction)?


[snip]

> +
> +Combing with issue described in 5.2, this an lead to unwanted frequency spikes

s/an/can/

> +when severely capped tasks share the rq with a small non capped task.

s/capped/restricted ?

> +
> +As an example if task p
> +
> +.. code-block:: c
> +
> +        p0->util_avg = 300
> +        p0->uclamp[UCLAMP_MAX] = 0
> +
> +wakes up on an idle CPU, then it will run at min frequency this CPU is capable
> +of.

This example description assumes some 'know how' from the reader ;)
Let's try to elaborate a bit this (please correct me if I made a mistake
somewhere here, since this is tricky).

'wakes up on an idle CPU, then it will run at min frequency (Fmin) this
CPU is capable of. The max CPU frequency (Fmax) matter here as well,
since it designates the shortest computational time to finish the task's
work. We are assuming the max capacity of the CPU is 1024.'

> +
> +.. code-block:: c
> +
> +        rq->uclamp[UCLAMP_MAX] = 0
> +
> +If the ratio of Fmax/Fmin is 3, then
> +
> +.. code-block:: c
> +
> +        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. It will be higher than 300, but won't
> +approach 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.
> +
> +.. code-block:: c
> +
> +        p0->util_avg = 300 + small_error
> +
> +Now if the ratio of Fmax/Fmin is 4, then
> +
> +.. code-block:: c
> +
> +        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 1024.

Could we say this differently?
Something with the CPU capacity levels, e.g.

If Fmax/Fmin is 3 and max CPU capacity is 1024, then
min CPU capacity is 1024/3 = 341.

So a task 'p' w/ util=300 would fit and run OK on such
Fmin CPU performance point (w/ capacity 341) (ignoring stollen CPU
cycles due to IRQs, RT/DL classes or thermal).

Although, if the CPU's Fmax/Fmin is 4, then min CPU capacity
is 1024/4 = 256

Thus, in this case the task 'p' w/ util 300 will not have
enough cycles to finish it's periodic work. Therefore, it
will continue the compuation and would be 'seen' as a 'busy loop'.
It's utilization would grow to 1024 unfortunately on such
low CPU's performance point.


> +
> +.. code-block:: c
> +
> +        p0->util_avg = 1024
> +
> +If task p1 wakes up on this CPU
> +
> +.. code-block:: c
> +
> +        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 1024.
> +
> +.. code-block:: c
> +
> +        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
> +
> +.. code-block:: c
> +
> +        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.

Yeah, this is a side effect that we would have to tackle...

> +
> +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.

Good job, this is a very useful doc. I hope user-space folks would
benefit from it as well.

Regards,
Lukasz

Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Qais Yousef]

On 11/28/22 08:48, Jonathan Corbet wrote:
> Qais Yousef <qyousef@layalina.io> writes:
> 
> > The new util clamp feature needs a document explaining what it is and
> > how to use it. The new document hopefully covers everything one needs to
> > know about uclamp.
> >
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> >
> > Changes in v2:
> >
> > 	* Address various style comments from Bagas
> 
> Just a handful of nits - this looks like a good addition to our
> documentation, thanks.

Thanks a lot Jonathan! I've taken all your comments into account. Thanks for
having a look and hopefully it wasn't a bad read!


Cheers

--
Qais Yousef

Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Qais Yousef]

On 11/29/22 10:16, Bagas Sanjaya wrote:
> On Sun, Nov 27, 2022 at 02:26:57PM +0000, Qais Yousef wrote:
> > The new util clamp feature needs a document explaining what it is and
> > how to use it. The new document hopefully covers everything one needs to
> > know about uclamp.
> > 
> 
> For patch subject, better say "Documentation: sched: Document util clamp
> feature".

ACK.

> 
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> > 
> > Changes in v2:
> > 
> > 	* Address various style comments from Bagas
> > 
> 
> I don't see any of my wording suggestions from [1] being applied. Still
> a rather confused to read (maybe should be applied on top of jon's
> suggestions)?

Apologies! It was an oversight from my side when I revisited the thread and
reworked the patch. I got confused somehow.

All applied now almost as is. There was one tweak that used "40% utilization"
that I replaced with "40% performance level" instead.

Thanks a lot!

Cheers

--
Qais Yousef

> 
> [1]: https://lore.kernel.org/lkml/Y3II59dyKuvQGIhG@debian.me/
> [2]: https://lore.kernel.org/linux-doc/87cz976pwn.fsf@meer.lwn.net/

Re: [PATCH v2] Documentation: sched: Add a new sched-util-clamp.rst

[Qais Yousef]

Hi Lukasz

On 11/29/22 10:32, Lukasz Luba wrote:
> Hi Qais,
> 
> I have a few comments below. IMO this documentation
> is also for user-space developers who might be missing
> some specific 'know how'.

Thanks for having a look!

> 
> On 11/27/22 14:26, Qais Yousef wrote:
> > The new util clamp feature needs a document explaining what it is and
> > how to use it. The new document hopefully covers everything one needs to
> > know about uclamp.
> > 
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> > 
> > Changes in v2:
> > 
> > 	* Address various style comments from Bagas
> > 
> >   Documentation/scheduler/index.rst            |   1 +
> >   Documentation/scheduler/sched-util-clamp.rst | 732 +++++++++++++++++++
> >   2 files changed, 733 insertions(+)
> >   create mode 100644 Documentation/scheduler/sched-util-clamp.rst
> > 
> > diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
> > index b430d856056a..f12d0d06de3a 100644
> > --- a/Documentation/scheduler/index.rst
> > +++ b/Documentation/scheduler/index.rst
> > @@ -15,6 +15,7 @@ Linux Scheduler
> >       sched-capacity
> >       sched-energy
> >       schedutil
> > +    sched-util-clamp
> >       sched-nice-design
> >       sched-rt-group
> >       sched-stats
> > diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
> > new file mode 100644
> > index 000000000000..da1881e293c3
> > --- /dev/null
> > +++ b/Documentation/scheduler/sched-util-clamp.rst
> > @@ -0,0 +1,732 @@
> > +====================
> > +Utilization Clamping
> > +====================
> > +
> > +1.  Introduction
> > +================
> > +
> > +Utilization clamping 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.
> > +
> > +It is often referred to as util clamp and uclamp. You'll find all variations
> > +used interchangeably in this documentation and in the source code.
> > +
> > +Uclamp is a hinting mechanism that allows the scheduler to understand the
> > +performance requirements and restrictions of the tasks. Hence help it make
> > +a better placement decision. And when schedutil cpufreq governor is used, util
> > +clamp will influence the frequency selection as well.
> 
> s/the frequency/the CPU frequency/

Done.

> 
> [snip]
> 
> > +
> > +Another example is in Android where tasks are classified as background,
> > +foreground, top-app, etc. Util clamp can be used to constraint 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; the constraint will help bias the background tasks to
> 
> I would add some explenation here, since you then reffer to the
> 'little cores' or 'big cores'.
> 
> s/heterogeneous systems/ heterogeneous systems (e.g. Arm big.LITTLE)/

Done.

> 
> > +stay on the little cores which will ensure that:
> 
> s/the little cores/the LITTLE cores (CPUs with capacity < 1024)/

From Bagas comments previousluy, I think I'll keep 'little' all small letters.

> 
> > +
> > +        1. The big cores are free to run top-app tasks immediately. top-app
> 
> s/the big cores/the big cores (CPUs with capacity = 1024)

I added this note to define little and big cores after the bullet points

.. note::
  little cores: CPUs with capacity < 1024
  big cores: CPUs with capacity = 1024

> 
> > +           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.
> > +
> > +By making these uclamp performance requests, or rather hints, user space can
> > +ensure system resources are used optimally to deliver the best user experience
> > +the system is capable of.
> > +
> > +Another use case is to help with overcoming the ramp up latency inherit in how
> > +scheduler utilization signal is calculated.
> 
> I would emphasize this section somehow, since this is very important.
> Maybe a section header?
> 
> 'Overcoming the utilization ramp up latency'

Hmm. A new section seems misplaced here without having to restructure the doc.

How about if I make the statement starting from 'overcoming the ramp up..' all
in bold? That should make it stand out enough hopefully?

> 
> > +
> > +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.
> 
> This example is good. You can add that this issue should go away when
> uclamp_min is set for such thread just after it's created by the
> developer.

Okay. I added:

  Setting UCLAMP_MIN=1024 will ensure such tasks will always see the highest
  performance level when they start running.

> 
> I would add another example, with a latency sensitive task e.g.
> garbage collector thread. Such task tries to keep the memory in a
> healthy state. This kind of task might benefit from running
> on higher performance CPU even if its real utilization might be
> small (e.g. 50 util) and considered to fit on the LITTLE CPU.

Paul has actually raised an opposite use case for garbage collector. Paul
wanted to use UCLAMP_MAX to cap them.

I'm wary of mentioning latency_sensitive here since we have latency_nice and
other definitions of latency. Would be simpler to keep it as is? I do expand
more in the How to use uclamp section.

> 
> [snip]
> 
> > +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 rq level, which brings us
> > +to the main design challenge.
> 
> s/rq/CPU runqueue (rq)/

Done.

> 
> [snip]
> 
> > +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:
> > +
> > +.. code-block:: c
> > +
> > +        attr->sched_util_min = 40% * 1024;
> > +        attr->sched_util_max = 80% * 1024;
> > +
> > +Will tell the system that when task @p is running, it should try its best to
> > +ensure it starts at a performance point no less than 40% of maximum system's
> > +capability.
> 
> I would emphasize this, since this is very imporant IMO.
> The maximum system's capability is ment to be the fastest and most
> performant CPU in the SoC (aka the biggest CPU).

Making it bold is good enough emphasis?

> 
> [snip]
> 
> > +
> > +3.3.2  sched_util_clamp_max
> > +---------------------------
> > +
> > +System wide limit of allowed UCLAMP_MAX range. By default set to 1024, which
> > +means tasks are allowed to reach an effective UCLAMP_MAX value in the range of
> > +[0:1024].
> > +
> > +By changing it to 512 for example the effective allowed range reduces to
> > +[0:512]. The visible impact of this is that no task can run above 512, which in
> > +return means 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.
> > +
> > +Can be handy to limit performance when running low on battery.
> 
> Or when the system knows that it can spent more time on computations
> because the user is not using the device but updates and re-compilations
> are done in background while the screen is idle.

I appended this:

  when the system wants to limit access to more energy hungry performance
  levels when it's in idle state or screen is off

Good enough?

> 
> 
> [snip]
> 
> > +
> > +On heterogeneous systems, it might be important for this task to run on
> > +a bigger CPU.
> > +
> > +Generally it is advised to perceive the input as performance level or point
> > +which will imply both task placement and frequency selection.
> 
> s/frequency/ CPU frequency/
> 
> BTW this sentence is really good. Maybe it could be somewhere in a
> header (1. Introduction)?

Made it bold, good enough?

I do say in the introduction that the right way to view util clamp as
a mechanism to make request or hint on performance constraints.

> 
> 
> [snip]
> 
> > +
> > +Combing with issue described in 5.2, this an lead to unwanted frequency spikes
> 
> s/an/can/

Done.

> 
> > +when severely capped tasks share the rq with a small non capped task.
> 
> s/capped/restricted ?

Is there something wrong with capping? I'd like to introduce this word as a way
to describe the behavior too.

> 
> > +
> > +As an example if task p
> > +
> > +.. code-block:: c
> > +
> > +        p0->util_avg = 300
> > +        p0->uclamp[UCLAMP_MAX] = 0
> > +
> > +wakes up on an idle CPU, then it will run at min frequency this CPU is capable
> > +of.
> 
> This example description assumes some 'know how' from the reader ;)

It is intended for kernel developers as we describe internal details here. If
a userspace reader reading this, I think what matters is that max aggregation
could mean they don't always get the power savings they expect.

> Let's try to elaborate a bit this (please correct me if I made a mistake
> somewhere here, since this is tricky).
> 
> 'wakes up on an idle CPU, then it will run at min frequency (Fmin) this
> CPU is capable of. The max CPU frequency (Fmax) matter here as well,
> since it designates the shortest computational time to finish the task's
> work. We are assuming the max capacity of the CPU is 1024.'

We are not assuming anything about the capacity of the CPU. This is true for
any CPU. The ratio of Fmax/Fmin will define how long its runtime will be
stretched by.

Incorporated your comments except for the assumption part. Please let me know
if I'm misunderstanding something you're inferring here :)

> 
> > +
> > +.. code-block:: c
> > +
> > +        rq->uclamp[UCLAMP_MAX] = 0
> > +
> > +If the ratio of Fmax/Fmin is 3, then
> > +
> > +.. code-block:: c
> > +
> > +        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. It will be higher than 300, but won't
> > +approach 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.
> > +
> > +.. code-block:: c
> > +
> > +        p0->util_avg = 300 + small_error
> > +
> > +Now if the ratio of Fmax/Fmin is 4, then
> > +
> > +.. code-block:: c
> > +
> > +        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 1024.
> 
> Could we say this differently?

The target audience here is kernel developers. Do you think it's confusing for
them too? I tried to make it less wordy to help them understand the problem.

Happy to reword it for sure, but first I want to make sure you think existing
wording is not good enough for the intended target audience.

> Something with the CPU capacity levels, e.g.
> 
> If Fmax/Fmin is 3 and max CPU capacity is 1024, then
> min CPU capacity is 1024/3 = 341.
> 
> So a task 'p' w/ util=300 would fit and run OK on such
> Fmin CPU performance point (w/ capacity 341) (ignoring stollen CPU
> cycles due to IRQs, RT/DL classes or thermal).
> 
> Although, if the CPU's Fmax/Fmin is 4, then min CPU capacity
> is 1024/4 = 256
> 
> Thus, in this case the task 'p' w/ util 300 will not have
> enough cycles to finish it's periodic work. Therefore, it
> will continue the compuation and would be 'seen' as a 'busy loop'.
> It's utilization would grow to 1024 unfortunately on such
> low CPU's performance point.
> 
> 
> > +
> > +.. code-block:: c
> > +
> > +        p0->util_avg = 1024
> > +
> > +If task p1 wakes up on this CPU
> > +
> > +.. code-block:: c
> > +
> > +        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 1024.
> > +
> > +.. code-block:: c
> > +
> > +        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
> > +
> > +.. code-block:: c
> > +
> > +        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.
> 
> Yeah, this is a side effect that we would have to tackle...

Yep!

> 
> > +
> > +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.
> 
> Good job, this is a very useful doc. I hope user-space folks would
> benefit from it as well.

I hope so too!

Thanks a lot for having a look Lukasz! I should have incorporated most of your
suggestions. Asked for clarifications in few places only.


Cheers

--
Qais Yousef