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From: Qais Yousef <qais.yousef@arm.com>
To: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Doug Anderson <dianders@chromium.org>,
	Qais Yousef <qais.yousef@arm.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Quentin Perret <qperret@google.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: [PATCH v7 2/3] Documentation/sysctl: Document uclamp sysctl knobs
Date: Thu, 16 Jul 2020 12:03:46 +0100	[thread overview]
Message-ID: <20200716110347.19553-3-qais.yousef@arm.com> (raw)
In-Reply-To: <20200716110347.19553-1-qais.yousef@arm.com>

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 | 54 +++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 83acf5025488..55bf6b4de4ec 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.
 
 seccomp
 =======
-- 
2.17.1


      parent reply	other threads:[~2020-07-16 11:07 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-16 11:03 [PATCH v7 0/3] sched/uclamp: new sysctl for default RT boost value Qais Yousef
2020-07-16 11:03 ` [PATCH v7 1/3] sched/uclamp: Add a new sysctl to control RT default " Qais Yousef
2020-07-24  8:54   ` Peter Zijlstra
2020-07-24  9:16     ` Qais Yousef
2020-07-16 11:03 ` Qais Yousef [this message]

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