[PATCH v4 00/16] Add utilization clamping support

[Patrick Bellasi]

This is a respin of:

   https://lore.kernel.org/lkml/20180806163946.28380-1-patrick.bellasi@xxxxxxx/

Which has been rebased on v4.19-rc1.

Thanks for all the valuable comments collected so far!
Further comments and feedbacks are more than welcome!

Cheers Patrick


Main changes in v4
==================

.:: Fix issues due to limited number of clamp groups
----------------------------------------------------

As Juri pointed out in:

   https://lore.kernel.org/lkml/20180807123550.GA3062@xxxxxxxxxxxxxxxxxxxxx/

we had an issue related to the limited number of supported clamp groups, which
could have affected both normal users as well as cgroups users.

This problem has been fixed by a couple of new patches:

   [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default
   [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support

which allows to ensure that only privileged tasks can create clamp groups
and/or the clamps groups are transformed into buckets to always ensure a
mapping for each possible userspace request.


.:: Better integrate RT tasks
-----------------------------

Quentin pointed out a change in behavior for RT task in:

   https://lore.kernel.org/lkml/20180809155551.bp46sixk4u3ilcnh@queper01-lin/

which has been fixed by improving this patch:

   [PATCH v4 16/16] sched/cpufreq: uclamp: add utilization clamping for RT tasks

This patch has also been moved to the end of the series since the solution is
partially based on some bits already added by other patches of this series.


.:: Improved refcounting code
-----------------------------

Pavan reported some code paths not covered by refcounting:

   https://lore.kernel.org/lkml/20180816091348.GD2661@xxxxxxxxxxxxxx/

Which translated into a complete review and improvement of the slow-path code
where we refcount clamp groups availability.

We now properly refcount clamp groups usage for all the main entities, i.e.
init_task, root_task_group and sysfs defaults.
The same refcounting code has been properly integrated with fork/exit as well
as cgroups creation/release code paths.


.:: Series Organization
-----------------------

The series is organized into these main sections:

- Patches [01-05]: Per task (primary) API
- Patches [06]   : Schedutil integration for CFS tasks
- Patches [07-13]: Per task group (secondary) API
- Patches [14-15]: Fix issues related to limited clamp groups
- Patches [16]   : Schedutil integration for RT tasks


Newcomer's Short Abstract (Updated)
===================================

The Linux scheduler tracks a "utilization" signal for each scheduling entity
(SE), e.g. tasks, to know how much CPU time they use. This signal allows the
scheduler to know how "big" a task is and, in principle, it can support
advanced task placement strategies by selecting the best CPU to run a task.
Some of these strategies are represented by the Energy Aware scheduler
extension [1].

When the schedutil cpufreq's governor is in use, the utilization signal allows
also the Linux scheduler to drive frequency selection. The CPU utilization
signal, which represents the aggregated utilization of tasks scheduled on that
CPU, is used to select the frequency which best fits the task's generated
workload.

However, the current translation of utilization values into a frequency
selection is pretty simple: we just go to max for RT tasks or to the minimum
frequency which can accommodate the utilization of DL+FAIR tasks. Regarding
task placement instead utilization is of limited usage since its value alone is
not enough to properly describe what's the expected power/performance behaviors
of each task.

In general, for both RT and FAIR tasks we can aim at better tasks placement and
frequency selection policies if take into consideration hints coming from
user-space.

Utilization clamping is a mechanism which allows to "clamp" (i.e. filter) the
utilization generated by RT and FAIR tasks within a range defined from
user-space. The clamped utilization value can then be used, for example, to
enforce a minimum and/or maximum frequency depending on which tasks are
currently active on a CPU.

The main use-cases for utilization clamping are:

 - boosting: better interactive response for small tasks which
   are affecting the user experience.

   Consider for example the case of a small control thread for an external
   accelerator (e.g. GPU, DSP, other devices). In this case, from its
   utilization the scheduler does not have a complete view of what are the task
   requirements and, if it's a small utilization task, schedutil will keep
   selecting a more energy efficient CPU, with smaller capacity and lower
   frequency, thus affecting the overall time required to complete the task
   activations.

 - capping: increase energy efficiency for background tasks not directly
   affecting the user experience.

   Since running on a lower capacity CPU at a lower frequency is in general
   more energy efficient, when the completion time is not a main goal, then
   capping the utilization considered for certain (maybe big) tasks can have
   positive effects, both on energy consumption and thermal stress.
   Moreover, this last support allows also to make RT tasks more energy
   friendly on mobile systems, where running them on high capacity CPUs and at
   the maximum frequency is not strictly required.

>From these two use-cases, it's worth to notice that frequency selection
biasing, introduced by patches 6 and 16 of this series, is just one possible
usage of utilization clamping. Another compelling extension of utilization
clamping is in helping the scheduler on tasks placement decisions.

Utilization is a task specific property which is used by the scheduler to know
how much CPU bandwidth a task requires (under certain conditions).
Thus, the utilization clamp values, defined either per-task or via the CPU
controller, can be used to represent tasks to the scheduler as being bigger
(or smaller) than what they really are.

Utilization clamping thus ultimately enable interesting additional
optimizations, especially on asymmetric capacity systems like Arm
big.LITTLE and DynamIQ CPUs, where:

 - boosting: small tasks are preferably scheduled on higher-capacity CPUs
   where, despite being less energy efficient, they can complete faster

 - capping: big/background tasks are preferably scheduled on low-capacity CPUs
   where, being more energy efficient, they can still run but save power and
   thermal headroom for more important tasks.

This additional usage of the utilization clamping is not presented in this
series but it's an integral part of the Energy Aware Scheduler (EAS) feature
set, where [1] is one of its main components. A solution similar to utilization
clamping, namely SchedTune, is already used on Android kernels to biasing of
both 'frequency selection' and 'task placement'.
This series provides the foundation bits to add a similar features to mainline
by focusing just on schedutil integration.

[1] https://lore.kernel.org/lkml/20180820094420.26590-1-quentin.perret@xxxxxxx/


Detailed Changelog
==================

Changes in v4:
 Message-ID: <20180809152313.lewfhufidhxb2qrk@darkstar>
 - implements the idea discussed in this thread
 Message-ID: <87897157-0b49-a0be-f66c-81cc2942b4dd@xxxxxxxxxxxxx>
 - remove not required default setting
 - fixed some tabs/spaces
 Message-ID: <20180807095905.GB2288@xxxxxxxxxxxxxxxxxxxxx>
 - replace/rephrase "bandwidth" references to use "capacity"
 - better stress that this do not enforce any bandwidth requirement
   but "just" give hints to the scheduler
 - fixed some typos
 Message-ID: <20180814112509.GB2661@xxxxxxxxxxxxxx>
 - add uclamp_exit_task() to release clamp refcount from do_exit()
 Message-ID: <20180816133249.GA2964@e110439-lin>
 - keep the WARN but beautify a bit that code
 - keep the WARN in uclamp_cpu_put_id() but beautify a bit that code
 - add another WARN on the unexpected condition of releasing a refcount
   from a CPU which has a lower clamp value active
 Message-ID: <20180413082648.GP4043@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - move uclamp_enabled at the top of sched_class to keep it on the same
   cache line of other main wakeup time callbacks
 Message-ID: <20180816132249.GA2960@e110439-lin>
 - inline uclamp_task_active() code into uclamp_task_update_active()
 - get rid of the now unused uclamp_task_active()
 Message-ID: <20180816172016.GG2960@e110439-lin>
 - ensure to always reset clamp holding on wakeup from IDLE
 Message-ID: <CAKfTPtC2adLupg7wy1JU9zxKx1466Sza6fSCcr92wcawm1OYkg@xxxxxxxxxxxxxx>
 - use *rq instead of cpu for both uclamp_util() and uclamp_value()
 Message-ID: <20180816135300.GC2960@e110439-lin>
 - remove uclamp_value() which is never used outside CONFIG_UCLAMP_TASK
 Message-ID: <20180816140731.GD2960@e110439-lin>
 - add ".effective" attributes to the default hierarchy
 - reuse already existing:
     task_struct::uclamp::effective::group_id
   instead of adding:
     task_struct::uclamp_group_id
   to back annotate the effective clamp group in which a task has been
   refcounted
 Message-ID: <20180820122728.GM2960@e110439-lin>
 - fix unwanted reset of clamp values on refcount success
 Other:
 - by default all tasks have a UCLAMP_NOT_VALID task specific clamp
 - always use:
      p->uclamp[clamp_id].effective.value
   to track the actual clamp value the task has been refcounted into.
   This matches with the usage of
      p->uclamp[clamp_id].effective.group_id
 - allow to call uclamp_group_get() without a task pointer, which is
   used to refcount the initial clamp group for all the global objects
   (init_task, root_task_group and system_defaults)
 - ensure (and check) that all tasks have a valid group_id at
   uclamp_cpu_get_id()
 - rework uclamp_cpu layout to better fit into just 2x64B cache lines
 - fix some s/SCHED_DEBUG/CONFIG_SCHED_DEBUG/
 - init uclamp for the init_task and refcount its clamp groups
 - add uclamp specific fork time code into uclamp_fork
 - add support for SCHED_FLAG_RESET_ON_FORK
   default clamps are now set for init_task and inherited/reset at
   fork time (when then flag is set for the parent)
 - enable uclamp only for FAIR tasks, RT class will be enabled only
   by a following patch which also integrate the class to schedutil
 - define uclamp_maps ____cacheline_aligned_in_smp
 - in uclamp_group_get() ensure to include uclamp_group_available() and
   uclamp_group_init() into the atomic section defined by:
      uc_map[next_group_id].se_lock
 - do not use mutex_lock(&uclamp_mutex) in uclamp_exit_task
   which is also not needed since refcounting is already guarded by
   the uc_map[group_id].se_lock spinlock
 - consolidate init_uclamp_sched_group() into init_uclamp()
 - refcount root_task_group's clamp groups from init_uclamp()
 - small documentation fixes
 - rebased on v4.19-rc1

Changes in v3:
 Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@xxxxxxxxxxxxxx>
 - removed UCLAMP_NONE not used by this patch
 - remove not necessary checks in uclamp_group_find()
 - add WARN on unlikely un-referenced decrement in uclamp_group_put()
 - add WARN on unlikely un-referenced decrement in uclamp_cpu_put_id()
 - make __setscheduler_uclamp() able to set just one clamp value
 - make __setscheduler_uclamp() failing if both clamps are required but
   there is no clamp groups available for one of them
 - remove uclamp_group_find() from uclamp_group_get() which now takes a
   group_id as a parameter
 - add explicit calls to uclamp_group_find()
   which is now not more part of uclamp_group_get()
 - fixed a not required override
 - fixed some typos in comments and changelog
 Message-ID: <CAJuCfpGaKvxKcO=RLcmveHRB9qbMrvFs2yFVrk=k-v_m7JkxwQ@xxxxxxxxxxxxxx>
 - few typos fixed
 Message-ID: <20180409222417.GK3126663@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - use "." notation for attributes naming
   i.e. s/util_{min,max}/util.{min,max}/
 - added new patches: 09 and 12
 Other changes:
 - rebased on tip/sched/core

Changes in v2:
 Message-ID: <20180413093822.GM4129@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - refactored struct rq::uclamp_cpu to be more cache efficient
   no more holes, re-arranged vectors to match cache lines with expected
   data locality
 Message-ID: <20180413094615.GT4043@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - use *rq as parameter whenever already available
 - add scheduling class's uclamp_enabled marker
 - get rid of the "confusing" single callback uclamp_task_update()
   and use uclamp_cpu_{get,put}() directly from {en,de}queue_task()
 - fix/remove "bad" comments
 Message-ID: <20180413113337.GU14248@e110439-lin>
 - remove inline from init_uclamp, flag it __init
 Message-ID: <20180413111900.GF4082@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - get rid of the group_id back annotation
   which is not requires at this stage where we have only per-task
   clamping support. It will be introduce later when cgroup support is
   added.
 Message-ID: <20180409222417.GK3126663@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - make attributes available only on non-root nodes
   a system wide API seems of not immediate interest and thus it's not
   supported anymore
 - remove implicit parent-child constraints and dependencies
 Message-ID: <20180410200514.GA793541@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
 - add some cgroup-v2 documentation for the new attributes
 - (hopefully) better explain intended use-cases
   the changelog above has been extended to better justify the naming
   proposed by the new attributes
 Other changes:
 - improved documentation to make more explicit some concepts
 - set UCLAMP_GROUPS_COUNT=2 by default
   which allows to fit all the hot-path CPU clamps data into a single cache
   line while still supporting up to 2 different {min,max}_utiql clamps.
 - use -ERANGE as range violation error
 - add attributes to the default hierarchy as well as the legacy one
 - implement a "nice" semantics where cgroup clamp values are always
   used to restrict task specific clamp values,
   i.e. tasks running on a TG are only allowed to demote themself.
 - patches re-ordering in top-down way
 - rebased on v4.18-rc4

Patrick Bellasi (16):
  sched/core: uclamp: extend sched_setattr to support utilization
    clamping
  sched/core: uclamp: map TASK's clamp values into CPU's clamp groups
  sched/core: uclamp: add CPU's clamp groups accounting
  sched/core: uclamp: update CPU's refcount on clamp changes
  sched/core: uclamp: enforce last task UCLAMP_MAX
  sched/cpufreq: uclamp: add utilization clamping for FAIR tasks
  sched/core: uclamp: extend cpu's cgroup controller
  sched/core: uclamp: propagate parent clamps
  sched/core: uclamp: map TG's clamp values into CPU's clamp groups
  sched/core: uclamp: use TG's clamps to restrict Task's clamps
  sched/core: uclamp: add system default clamps
  sched/core: uclamp: update CPU's refcount on TG's clamp changes
  sched/core: uclamp: use percentage clamp values
  sched/core: uclamp: request CAP_SYS_ADMIN by default
  sched/core: uclamp: add clamp group discretization support
  sched/cpufreq: uclamp: add utilization clamping for RT tasks

 Documentation/admin-guide/cgroup-v2.rst       |   46 +
 .../admin-guide/kernel-parameters.txt         |    3 +
 include/linux/sched.h                         |   65 +
 include/linux/sched/sysctl.h                  |   11 +
 include/linux/sched/task.h                    |    6 +
 include/uapi/linux/sched.h                    |    8 +-
 include/uapi/linux/sched/types.h              |   68 +-
 init/Kconfig                                  |   63 +
 init/init_task.c                              |    1 +
 kernel/exit.c                                 |    1 +
 kernel/sched/core.c                           | 1368 ++++++++++++++++-
 kernel/sched/cpufreq_schedutil.c              |   31 +-
 kernel/sched/fair.c                           |    4 +
 kernel/sched/features.h                       |   10 +
 kernel/sched/rt.c                             |    4 +
 kernel/sched/sched.h                          |  177 ++-
 kernel/sysctl.c                               |   16 +
 17 files changed, 1863 insertions(+), 19 deletions(-)

-- 
2.18.0