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Documentation/sysctl: Document uclamp sysctl knobs

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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
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Qais Yousef 2020-07-16 12:03:46 +01:00 committed by Peter Zijlstra
parent 13685c4a08
commit 1f73d1abe5
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Documentation/admin-guide/sysctl/kernel.rst
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@ -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.
seccomp
=======