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Subject: [PATCH v3 2/2] Documentation/sysctl: Document uclamp sysctl knobs
Date:   Tue, 28 Apr 2020 17:41:34 +0100
Message-Id: <20200428164134.5588-2-qais.yousef@arm.com>
In-Reply-To: <20200428164134.5588-1-qais.yousef@arm.com>
References: <20200428164134.5588-1-qais.yousef@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
CC: Jonathan Corbet <corbet@lwn.net>
CC: Juri Lelli <juri.lelli@redhat.com>
CC: Vincent Guittot <vincent.guittot@linaro.org>
CC: Dietmar Eggemann <dietmar.eggemann@arm.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ben Segall <bsegall@google.com>
CC: Mel Gorman <mgorman@suse.de>
CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Kees Cook <keescook@chromium.org>
CC: Iurii Zaikin <yzaikin@google.com>
CC: Quentin Perret <qperret@google.com>
CC: Valentin Schneider <valentin.schneider@arm.com>
CC: Patrick Bellasi <patrick.bellasi@matbug.net>
CC: Pavan Kondeti <pkondeti@codeaurora.org>
CC: linux-doc@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
---
 Documentation/admin-guide/sysctl/kernel.rst | 48 +++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 0d427fd10941..e7255f71493c 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -940,6 +940,54 @@ 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 SCHED_CAPACITY_SCALE (1024), which is the maximum possible
+value.
+
+It means that any requested uclamp.min value cannot be greater than
+sched_util_clamp_min, ie: it is restricted to the range
+[0:sched_util_clamp_min].
+
+sched_util_clamp_max:
+=====================
+
+Max allowed *maximum* utilization.
+
+Default value is SCHED_CAPACITY_SCALE (1024), which is the maximum possible
+value.
+
+It means that any requested uclamp.max value cannot be greater than
+sched_util_clamp_max, ie: 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
+SCHED_CAPACITY_SCALE (1024) by default. Which effectively boosts the tasks to
+run at the highest frequency and bias 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 constraint imposed by sched_util_clamp_min
+defined above.
+
+Any modification is applied lazily on the next opportunity the scheduler needs
+to calculate the effective value of uclamp.min of the task.
 
 seccomp
 =======
-- 
2.17.1