linux-kernel - [PATCH 2/2] Documentation/sysctl: Document uclamp sysctl knobs

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Message-Id: <20200511154053.7822-2-qais.yousef@arm.com>
Date:   Mon, 11 May 2020 16:40:53 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Qais Yousef <qais.yousef@....com>,
        Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH 2/2] 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@....com>
CC: Jonathan Corbet <corbet@....net>
CC: Juri Lelli <juri.lelli@...hat.com>
CC: Vincent Guittot <vincent.guittot@...aro.org>
CC: Dietmar Eggemann <dietmar.eggemann@....com>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ben Segall <bsegall@...gle.com>
CC: Mel Gorman <mgorman@...e.de>
CC: Luis Chamberlain <mcgrof@...nel.org>
CC: Kees Cook <keescook@...omium.org>
CC: Iurii Zaikin <yzaikin@...gle.com>
CC: Quentin Perret <qperret@...gle.com>
CC: Valentin Schneider <valentin.schneider@....com>
CC: Patrick Bellasi <patrick.bellasi@...bug.net>
CC: Pavan Kondeti <pkondeti@...eaurora.org>
CC: linux-doc@...r.kernel.org
CC: linux-kernel@...r.kernel.org
CC: linux-fsdevel@...r.kernel.org
---

Changes in v5:
	* Improve the wording about the lazy update and the relationship to
	  sched_util_clamp_min (Patrick)

 Documentation/admin-guide/sysctl/kernel.rst | 57 +++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 0d427fd10941..6cd3cb0e22c0 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -940,6 +940,63 @@ 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.
+
+Any modification is applied lazily to tasks and is effective starting from
+their next wakeup.
 
 seccomp
 =======
-- 
2.17.1