[patch 16/16] sched: add documentation for bandwidth control

[Paul Turner]

From: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx>

Basic description of usage and effect for CFS Bandwidth Control.

Signed-off-by: Bharata B Rao <bharata@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
---
 Documentation/scheduler/sched-bwc.txt |   98
 ++++++++++++++++++++++++++++++++++
 Documentation/scheduler/sched-bwc.txt |  110 ++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)

Index: tip/Documentation/scheduler/sched-bwc.txt
===================================================================
--- /dev/null
+++ tip/Documentation/scheduler/sched-bwc.txt
@@ -0,0 +1,110 @@
+CFS Bandwidth Control
+=====================
+
+[ This document talks about CPU bandwidth control for CFS groups only.
+  Bandwidth control for RT groups covered in:
+  Documentation/scheduler/sched-rt-group.txt ]
+
+CFS bandwidth control is a group scheduler extension that can be used to
+control the maximum CPU bandwidth obtained by a CPU cgroup.
+
+Bandwidth allowed for a group is specified using quota and period. Within
+a given "period" (microseconds), a group is allowed to consume up to "quota"
+microseconds of CPU time, which is the upper limit or the hard limit. When the
+CPU bandwidth consumption of a group exceeds the hard limit, the tasks in the
+group are throttled and are not allowed to run until the end of the period at
+which time the group's quota is replenished.
+
+Runtime available to the group is tracked globally. At the beginning of
+each period, the group's global runtime pool is replenished with "quota"
+microseconds worth of runtime.  This bandwidth is then transferred to cpu local
+"accounts" on a demand basis.  Thie size of this transfer is described as a
+"slice".
+
+Interface
+---------
+Quota and period can be set via cgroup files.
+
+cpu.cfs_quota_us: the enforcement interval (microseconds)
+cpu.cfs_period_us: the maximum allowed bandwidth (microseconds)
+
+Within a period of cpu.cfs_period_us, the group as a whole will not be allowed
+to consume more than cpu_cfs_quota_us worth of runtime.
+
+The default value of cpu.cfs_period_us is 100ms and the default value
+for cpu.cfs_quota_us is -1.
+
+A group with cpu.cfs_quota_us as -1 indicates that the group has infinite
+bandwidth, which means that it is not bandwidth controlled.
+
+Writing any negative value to cpu.cfs_quota_us will turn the group into
+an infinite bandwidth group. Reading cpu.cfs_quota_us for an unconstrained
+bandwidth group will always return -1.
+
+System wide settings
+--------------------
+The amount of runtime obtained from global pool every time a CPU wants the
+group quota locally is controlled by a sysctl parameter
+sched_cfs_bandwidth_slice_us. The current default is 5ms. This can be changed
+by writing to /proc/sys/kernel/sched_cfs_bandwidth_slice_us.
+
+Statistics
+----------
+cpu.stat file lists three different stats related to bandwidth control's
+activity.
+
+- nr_periods: Number of enforcement intervals that have elapsed.
+- nr_throttled: Number of times the group has been throttled/limited.
+- throttled_time: The total time duration (in nanoseconds) for which entities
+  of the group have been throttled.
+
+These files are read-only.
+
+Hierarchy considerations
+------------------------
+The interface enforces that an individual entity's bandwidth is always
+attainable, that is: max(c_i) <= C. However, over-subscription in the
+aggregate case is explicitly allowed:
+  e.g. \Sum (c_i) may exceed C
+[ Where C is the parent's bandwidth, and c_i its children ]
+
+There are two ways in which a group may become throttled:
+
+a. it fully consumes its own quota within a period
+b. a parent's quota is fully consumed within its period
+
+In case b above, even though the child may have runtime remaining it will not
+be allowed to un until the parent's runtime is refreshed.
+
+Examples
+--------
+1. Limit a group to 1 CPU worth of runtime.
+
+	If period is 250ms and quota is also 250ms, the group will get
+	1 CPU worth of runtime every 250ms.
+
+	# echo 500000 > cpu.cfs_quota_us /* quota = 250ms */
+	# echo 250000 > cpu.cfs_period_us /* period = 250ms */
+
+2. Limit a group to 2 CPUs worth of runtime on a multi-CPU machine.
+
+	With 500ms period and 1000ms quota, the group can get 2 CPUs worth of
+	runtime every 500ms.
+
+	# echo 1000000 > cpu.cfs_quota_us /* quota = 1000ms */
+	# echo 500000 > cpu.cfs_period_us /* period = 500ms */
+
+	The larger period here allows for increased burst capacity.
+
+3. Limit a group to 20% of 1 CPU.
+
+	With 50ms period, 10ms quota will be equivalent to 20% of 1 CPU.
+
+	# echo 10000 > cpu.cfs_quota_us /* quota = 10ms */
+	# echo 50000 > cpu.cfs_period_us /* period = 50ms */
+
+	By using a small period her we are ensuring a consistent latency
+	response at the expense of burst capacity.
+
+
+


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