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    Subject[tip: sched/core] Documentation/sysctl: Document uclamp sysctl knobs
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: 1f73d1abe5836bd8ffe747ff5cb7561b17ce5bc6
    Gitweb: https://git.kernel.org/tip/1f73d1abe5836bd8ffe747ff5cb7561b17ce5bc6
    Author: Qais Yousef <qais.yousef@arm.com>
    AuthorDate: Thu, 16 Jul 2020 12:03:46 +01:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Wed, 29 Jul 2020 13:51:48 +02:00

    Documentation/sysctl: Document uclamp sysctl knobs

    Uclamp exposes 3 sysctl knobs:

    * sched_util_clamp_min
    * sched_util_clamp_max
    * sched_util_clamp_min_rt_default

    Document them in sysctl/kernel.rst.

    Signed-off-by: Qais Yousef <qais.yousef@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20200716110347.19553-3-qais.yousef@arm.com
    ---
    Documentation/admin-guide/sysctl/kernel.rst | 54 ++++++++++++++++++++-
    1 file changed, 54 insertions(+)

    diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
    index 83acf50..55bf6b4 100644
    --- a/Documentation/admin-guide/sysctl/kernel.rst
    +++ b/Documentation/admin-guide/sysctl/kernel.rst
    @@ -1062,6 +1062,60 @@ Enables/disables scheduler statistics. Enabling this feature
    incurs a small amount of overhead in the scheduler but is
    useful for debugging and performance tuning.

    +sched_util_clamp_min:
    +=====================
    +
    +Max allowed *minimum* utilization.
    +
    +Default value is 1024, which is the maximum possible value.
    +
    +It means that any requested uclamp.min value cannot be greater than
    +sched_util_clamp_min, i.e., it is restricted to the range
    +[0:sched_util_clamp_min].
    +
    +sched_util_clamp_max:
    +=====================
    +
    +Max allowed *maximum* utilization.
    +
    +Default value is 1024, which is the maximum possible value.
    +
    +It means that any requested uclamp.max value cannot be greater than
    +sched_util_clamp_max, i.e., it is restricted to the range
    +[0:sched_util_clamp_max].
    +
    +sched_util_clamp_min_rt_default:
    +================================
    +
    +By default Linux is tuned for performance. Which means that RT tasks always run
    +at the highest frequency and most capable (highest capacity) CPU (in
    +heterogeneous systems).
    +
    +Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
    +1024 by default, which effectively boosts the tasks to run at the highest
    +frequency and biases them to run on the biggest CPU.
    +
    +This knob allows admins to change the default behavior when uclamp is being
    +used. In battery powered devices particularly, running at the maximum
    +capacity and frequency will increase energy consumption and shorten the battery
    +life.
    +
    +This knob is only effective for RT tasks which the user hasn't modified their
    +requested uclamp.min value via sched_setattr() syscall.
    +
    +This knob will not escape the range constraint imposed by sched_util_clamp_min
    +defined above.
    +
    +For example if
    +
    + sched_util_clamp_min_rt_default = 800
    + sched_util_clamp_min = 600
    +
    +Then the boost will be clamped to 600 because 800 is outside of the permissible
    +range of [0:600]. This could happen for instance if a powersave mode will
    +restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
    +this restriction is lifted, the requested sched_util_clamp_min_rt_default
    +will take effect.

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