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    Subject[PATCH v3 00/14] Add utilization clamping support
    Date
    This is a respin of:

    https://lore.kernel.org/lkml/20180716082906.6061-1-patrick.bellasi@arm.com

    Which has been rebased on today's tip/sched/core:

    commit 1b6266ebe3da ("watchdog: Reduce message verbosity")

    and addresses all the comments from Tejun and Suren, thanks for your feedback!

    Further comments and feedbacks are more than welcome!

    Cheers Patrick


    Main changes
    ============

    .:: Properly implemented the cgroup delegation model
    ----------------------------------------------------

    As Tejun pointed out in:

    https://lore.kernel.org/lkml/20180409222417.GK3126663@devbig577.frc2.facebook.com

    the cgroup delegation model requires that a parent group can always restrict
    the resources of a child group. To properly support this behavior we need to
    add a concept of "effective" clamp values beside the one of "requested" clamp
    values.

    This means that a child group can always ask for certain clamp values but the
    effective clamps it get depends on the parent group configuration. More
    specifically, the effective clamps for a group are defined as the most
    restrictive value (i.e. the minimum) between a task group clamp value and the
    effective value of its parent task group.

    This new feature is introduced by this new patch in this series:

    [PATCH v3 09/14] sched/core: uclamp: propagate parent clamps


    .:: Added support for system defaults
    -------------------------------------

    The introduction of the previous patch implies that the root task group must be
    configured with a default min utilization clamping which corresponds to the
    maximum value, i.e. root_task_group::uclamp[UTIL_MIN].value = 100%.
    Otherwise, subgroups will always have an effective 0% minimum clamp.

    To fix this misbehavior, as well as to overcome the (cgroup imposed) limitation
    of non configurable attributes of the root task group, in this new patch:

    [PATCH v3 12/14] sched/core: uclamp: add system default clamps

    we add sysfs support for system wide defaults.

    This should satisfy another comment by Tejun and it also provides a convenient
    system wide configuration API, which is available independently from cgroup.


    .:: Improved syscall API semantics
    ----------------------------------

    As pointed out and suggested by Suren, the __sched_setscheduler() syscall
    semantics has been improved to support:
    - single attribute configuration
    by using a new set of dedicated sched_attr::sched_flags we can now specify
    which clamp values we want to configure
    - atomic setting or failure in case of two attributes being configured at the
    same time.

    These changes affect mainly:

    [PATCH v3 01/14] sched/core: uclamp: extend sched_setattr to support utilization clamping
    [PATCH v3 02/14] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups


    .:: Other notes
    ---------------

    The rest of the series is similar to v2 and split into these sections:

    - Patches [01-04]: Per task (primary) API
    - Patches [05-06]: Schedutil integration
    - Patches [08-13]: Per task group (secondary) API
    - Patches [07,14]: Additional improvements


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

    The Linux scheduler is able to drive frequency selection, when the schedutil
    cpufreq's governor is in use, based on task utilization aggregated at CPU
    level. The CPU utilization is then used to select the frequency which best
    fits the task's generated workload. 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.

    While this simple mechanism is good enough for DL tasks, for RT and FAIR tasks
    we can aim at some better frequency driving which can 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 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 5 and 6 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. A similar solution (SchedTune) is already used on Android kernels, which
    targets the biasing of both 'frequency selection' and 'task placement'.
    This series provides the foundation bits to add similar features in mainline
    and its first simple client with the schedutil integration.


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

    Changes in v3:
    Message-ID: <CAJuCfpF6=L=0LrmNnJrTNPazT4dWKqNv+thhN0dwpKCgUzs9sg@mail.gmail.com>
    - 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@mail.gmail.com>
    - few typos fixed
    Message-ID: <20180409222417.GK3126663@devbig577.frc2.facebook.com>
    - 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@hirez.programming.kicks-ass.net>
    - 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@hirez.programming.kicks-ass.net>
    - 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@hirez.programming.kicks-ass.net>
    - 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@devbig577.frc2.facebook.com>
    - 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@devbig577.frc2.facebook.com>
    - 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 (14):
    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/cpufreq: uclamp: add utilization clamping for FAIR tasks
    sched/cpufreq: uclamp: add utilization clamping for RT tasks
    sched/core: uclamp: enforce last task UCLAMP_MAX
    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

    Documentation/admin-guide/cgroup-v2.rst | 46 +
    include/linux/sched.h | 58 ++
    include/linux/sched/sysctl.h | 11 +
    include/uapi/linux/sched.h | 8 +-
    include/uapi/linux/sched/types.h | 66 +-
    init/Kconfig | 61 ++
    kernel/sched/core.c | 1186 +++++++++++++++++++++++
    kernel/sched/cpufreq_schedutil.c | 38 +-
    kernel/sched/fair.c | 4 +
    kernel/sched/features.h | 10 +
    kernel/sched/rt.c | 4 +
    kernel/sched/sched.h | 194 ++++
    kernel/sysctl.c | 16 +
    13 files changed, 1688 insertions(+), 14 deletions(-)

    --
    2.18.0

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