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@xxxxxxx>
> Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
> ---
>
> 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