Message-Id: <1390214440-2711-1-git-send-email-juri.lelli@gmail.com>
Date:	Mon, 20 Jan 2014 11:40:40 +0100
From:	Juri Lelli <juri.lelli@...il.com>
To:	peterz@...radead.org, tglx@...utronix.de
Cc:	mingo@...hat.com, rostedt@...dmis.org, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	juri.lelli@...il.com, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com, rob@...dley.net
Subject: [PATCH] sched/deadline: Add sched_dl documentation

From: Dario Faggioli <raistlin@...ux.it>

Add in Documentation/scheduler/ some hints about the design
choices, the usage and the future possible developments of the
sched_dl scheduling class and of the SCHED_DEADLINE policy.

Cc: bruce.ashfield@...driver.com
Cc: claudio@...dence.eu.com
Cc: darren@...art.com
Cc: dhaval.giani@...il.com
Cc: fchecconi@...il.com
Cc: fweisbec@...il.com
Cc: harald.gustafsson@...csson.com
Cc: hgu1972@...il.com
Cc: insop.song@...il.com
Cc: jkacur@...hat.com
Cc: johan.eker@...csson.com
Cc: liming.wang@...driver.com
Cc: luca.abeni@...tn.it
Cc: michael@...rulasolutions.com
Cc: mingo@...hat.com
Cc: nicola.manica@...i.unitn.it
Cc: oleg@...hat.com
Cc: paulmck@...ux.vnet.ibm.com
Cc: p.faure@...tech.ch
Cc: rob@...dley.net
Cc: rostedt@...dmis.org
Cc: tglx@...utronix.de
Cc: tommaso.cucinotta@...up.it
Cc: vincent.guittot@...aro.org
Signed-off-by: Dario Faggioli <raistlin@...ux.it>
Signed-off-by: Juri Lelli <juri.lelli@...il.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 Documentation/scheduler/00-INDEX           |    2 +
 Documentation/scheduler/sched-deadline.txt |  189 ++++++++++++++++++++++++++++
 kernel/sched/deadline.c                    |    3 +-
 3 files changed, 193 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/scheduler/sched-deadline.txt

diff --git a/Documentation/scheduler/00-INDEX b/Documentation/scheduler/00-INDEX
index d2651c4..46702e4 100644
--- a/Documentation/scheduler/00-INDEX
+++ b/Documentation/scheduler/00-INDEX
@@ -10,5 +10,7 @@ sched-nice-design.txt
 	- How and why the scheduler's nice levels are implemented.
 sched-rt-group.txt
 	- real-time group scheduling.
+sched-deadline.txt
+	- deadline scheduling.
 sched-stats.txt
 	- information on schedstats (Linux Scheduler Statistics).
diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
new file mode 100644
index 0000000..8980de1
--- /dev/null
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -0,0 +1,189 @@
+			  Deadline Task Scheduling
+			  ------------------------
+
+CONTENTS
+========
+
+ 0. WARNING
+ 1. Overview
+ 2. Task scheduling
+ 2. The Interface
+ 3. Bandwidth management
+   3.1 System-wide settings
+   3.2 Task interface
+   3.4 Default behavior
+ 4. Tasks CPU affinity
+   4.1 SCHED_DEADLINE and cpusets HOWTO
+ 5. Future plans
+
+
+0. WARNING
+==========
+
+ Fiddling with these settings can result in an unpredictable or even unstable
+ system behavior. As for -rt (group) scheduling, it is assumed that root users
+ know what they're doing.
+
+
+1. Overview
+===========
+
+ The SCHED_DEADLINE policy contained inside the sched_dl scheduling class is
+ basically an implementation of the Earliest Deadline First (EDF) scheduling
+ algorithm, augmented with a mechanism (called Constant Bandwidth Server, CBS)
+ that makes it possible to isolate the behavior of tasks between each other.
+
+
+2. Task scheduling
+==================
+
+ The typical -deadline task is composed of a computation phase (instance)
+ which is activated on a periodic or sporadic fashion. The expected (maximum)
+ duration of such computation is called the task's runtime; the time interval
+ by which each instance needs to be completed is called the task's relative
+ deadline. The task's absolute deadline is dynamically calculated as the
+ time instant a task (or, more properly) activates plus the relative
+ deadline.
+
+ The EDF[1] algorithm selects the task with the smallest absolute deadline as
+ the one to be executed first, while the CBS[2,3] ensures that each task runs
+ for at most its runtime every period, avoiding any interference between
+ different tasks (bandwidth isolation).
+ Thanks to this feature, also tasks that do not strictly comply with the
+ computational model described above can effectively use the new policy.
+ IOW, there are no limitations on what kind of task can exploit this new
+ scheduling discipline, even if it must be said that it is particularly
+ suited for periodic or sporadic tasks that need guarantees on their
+ timing behavior, e.g., multimedia, streaming, control applications, etc.
+
+ References:
+  1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram-
+      ming in a hard-real-time environment. Journal of the Association for
+      Computing Machinery, 20(1), 1973.
+  2 - L. Abeni , G. Buttazzo. Integrating Multimedia Applications in Hard
+      Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems
+      Symposium, 1998. http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf
+  3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab
+      Technical Report. http://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps
+
+3. Bandwidth management
+=======================
+
+ In order for the -deadline scheduling to be effective and useful, it is
+ important to have some method to keep the allocation of the available CPU
+ bandwidth to the tasks under control.
+ This is usually called "admission control" and if it is not performed at all,
+ no guarantee can be given on the actual scheduling of the -deadline tasks.
+
+ Since when RT-throttling has been introduced each task group has a bandwidth
+ associated, calculated as a certain amount of runtime over a period.
+ Moreover, to make it possible to manipulate such bandwidth, readable/writable
+ controls have been added to both procfs (for system wide settings) and cgroupfs
+ (for per-group settings).
+ Therefore, the same interface is being used for controlling the bandwidth
+ distrubution to -deadline tasks.
+
+ However, more discussion is needed in order to figure out how we want to manage
+ SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
+ uses (for now) a less sophisticated, but actually very sensible, mechanism to
+ ensure that a certain utilization cap is not overcome per each root_domain.
+
+ Another main difference between deadline bandwidth management and RT-throttling
+ is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
+ and thus we don't need an higher level throttling mechanism to enforce the
+ desired bandwidth.
+
+3.1 System wide settings
+------------------------
+
+ The system wide settings are configured under the /proc virtual file system.
+
+ For now the -rt knobs are used for dl admission control and the -deadline
+ runtime is accounted against the -rt runtime. We realise that this isn't
+ entirely desirable; however, it is better to have a small interface for now,
+ and be able to change it easily later. The ideal situation (see 5.) is to run
+ -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct
+ subset of dl_bw.
+
+ This means that, for a root_domain comprising M CPUs, -deadline tasks
+ can be created while the sum of their bandwidths stays below:
+
+   M * (sched_rt_runtime_us / sched_rt_period_us)
+
+ It is also possible to disable this bandwidth management logic, and
+ be thus free of oversubscribing the system up to any arbitrary level.
+ This is done by writing -1 in /proc/sys/kernel/sched_rt_runtime_us.
+
+
+3.2 Task interface
+------------------
+
+ Specifying a periodic/sporadic task that executes for a given amount of
+ runtime at each instance, and that is scheduled according to the urgency of
+ its own timing constraints needs, in general, a way of declaring:
+  - a (maximum/typical) instance execution time,
+  - a minimum interval between consecutive instances,
+  - a time constraint by which each instance must be completed.
+
+ Therefore:
+  * a new struct sched_attr, containing all the necessary fields is
+    provided;
+  * the new scheduling related syscalls that manipulate it, i.e.,
+    sched_setattr() and sched_getattr() are implemented.
+
+
+3.3 Default behavior
+---------------------
+
+ The default value for SCHED_DEADLINE bandwidth is to have rt_runtime equal to
+ 95000. With rt_period equal to 1000000, by default, it means that -deadline
+ tasks can use at most 95%, multiplied by the number of CPUs that compose the
+ root_domain, for each root_domain.
+
+ A -deadline task cannot fork.
+
+4. Tasks CPU affinity
+=====================
+
+ -deadline tasks cannot have an affinity mask smaller that the entire
+ root_domain they are created on. However, affinities can be specified
+ through the cpuset facility (Documentation/cgroups/cpusets.txt).
+
+4.1 SCHED_DEADLINE and cpusets HOWTO
+------------------------------------
+
+ An example of a simple configuration (pin a -deadline task to CPU0)
+ follows (rt-app is used to create a -deadline task).
+
+ mkdir /dev/cpuset
+ mount -t cgroup -o cpuset cpuset /dev/cpuset
+ cd /dev/cpuset
+ mkdir cpu0
+ echo 0 > cpu0/cpuset.cpus
+ echo 0 > cpu0/cpuset.mems
+ echo 1 > cpuset.cpu_exclusive
+ echo 0 > cpuset.sched_load_balance
+ echo 1 > cpu0/cpuset.cpu_exclusive
+ echo 1 > cpu0/cpuset.mem_exclusive
+ echo $$ > cpu0/tasks
+ rt-app -t 100000:10000:d:0 -D5 (it is now actually superfluous to specify
+ task affinity)
+
+5. Future plans
+===============
+
+ Still missing:
+
+  - refinements to deadline inheritance, especially regarding the possibility
+    of retaining bandwidth isolation among non-interacting tasks. This is
+    being studied from both theoretical and practical points of view, and
+    hopefully we should be able to produce some demonstrative code soon;
+  - (c)group based bandwidth management, and maybe scheduling;
+  - access control for non-root users (and related security concerns to
+    address), which is the best way to allow unprivileged use of the mechanisms
+    and how to prevent non-root users "cheat" the system?
+
+ As already discussed, we are planning also to merge this work with the EDF
+ throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
+ the preliminary phases of the merge and we really seek feedback that would
+ help us decide on the direction it should take.
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0de2482..0dd5e09 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -351,7 +351,8 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se,
  * disrupting the schedulability of the system. Otherwise, we should
  * refill the runtime and set the deadline a period in the future,
  * because keeping the current (absolute) deadline of the task would
- * result in breaking guarantees promised to other tasks.
+ * result in breaking guarantees promised to other tasks (refer to
+ * Documentation/scheduler/sched-deadline.txt for more informations).
  *
  * This function returns true if:
  *
-- 
1.7.9.5

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