1=============
2CFS Scheduler
3=============
4
5
61.  OVERVIEW
7============
8
9CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
10scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
11replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
12code.
13
1480% of CFS's design can be summed up in a single sentence: CFS basically models
15an "ideal, precise multi-tasking CPU" on real hardware.
16
17"Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical
18power and which can run each task at precise equal speed, in parallel, each at
191/nr_running speed.  For example: if there are 2 tasks running, then it runs
20each at 50% physical power --- i.e., actually in parallel.
21
22On real hardware, we can run only a single task at once, so we have to
23introduce the concept of "virtual runtime."  The virtual runtime of a task
24specifies when its next timeslice would start execution on the ideal
25multi-tasking CPU described above.  In practice, the virtual runtime of a task
26is its actual runtime normalized to the total number of running tasks.
27
28
29
302.  FEW IMPLEMENTATION DETAILS
31==============================
32
33In CFS the virtual runtime is expressed and tracked via the per-task
34p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately
35timestamp and measure the "expected CPU time" a task should have gotten.
36
37   Small detail: on "ideal" hardware, at any time all tasks would have the same
38   p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
39   would ever get "out of balance" from the "ideal" share of CPU time.
40
41CFS's task picking logic is based on this p->se.vruntime value and it is thus
42very simple: it always tries to run the task with the smallest p->se.vruntime
43value (i.e., the task which executed least so far).  CFS always tries to split
44up CPU time between runnable tasks as close to "ideal multitasking hardware" as
45possible.
46
47Most of the rest of CFS's design just falls out of this really simple concept,
48with a few add-on embellishments like nice levels, multiprocessing and various
49algorithm variants to recognize sleepers.
50
51
52
533.  THE RBTREE
54==============
55
56CFS's design is quite radical: it does not use the old data structures for the
57runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
58task execution, and thus has no "array switch" artifacts (by which both the
59previous vanilla scheduler and RSDL/SD are affected).
60
61CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
62increasing value tracking the smallest vruntime among all tasks in the
63runqueue.  The total amount of work done by the system is tracked using
64min_vruntime; that value is used to place newly activated entities on the left
65side of the tree as much as possible.
66
67The total number of running tasks in the runqueue is accounted through the
68rq->cfs.load value, which is the sum of the weights of the tasks queued on the
69runqueue.
70
71CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
72p->se.vruntime key. CFS picks the "leftmost" task from this tree and sticks to it.
73As the system progresses forwards, the executed tasks are put into the tree
74more and more to the right --- slowly but surely giving a chance for every task
75to become the "leftmost task" and thus get on the CPU within a deterministic
76amount of time.
77
78Summing up, CFS works like this: it runs a task a bit, and when the task
79schedules (or a scheduler tick happens) the task's CPU usage is "accounted
80for": the (small) time it just spent using the physical CPU is added to
81p->se.vruntime.  Once p->se.vruntime gets high enough so that another task
82becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
83small amount of "granularity" distance relative to the leftmost task so that we
84do not over-schedule tasks and trash the cache), then the new leftmost task is
85picked and the current task is preempted.
86
87
88
894.  SOME FEATURES OF CFS
90========================
91
92CFS uses nanosecond granularity accounting and does not rely on any jiffies or
93other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the
94way the previous scheduler had, and has no heuristics whatsoever.  There is
95only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
96
97   /sys/kernel/debug/sched/base_slice_ns
98
99which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
100"server" (i.e., good batching) workloads.  It defaults to a setting suitable
101for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.
102
103Due to its design, the CFS scheduler is not prone to any of the "attacks" that
104exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
105chew.c, ring-test.c, massive_intr.c all work fine and do not impact
106interactivity and produce the expected behavior.
107
108The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
109than the previous vanilla scheduler: both types of workloads are isolated much
110more aggressively.
111
112SMP load-balancing has been reworked/sanitized: the runqueue-walking
113assumptions are gone from the load-balancing code now, and iterators of the
114scheduling modules are used.  The balancing code got quite a bit simpler as a
115result.
116
117
118
1195. Scheduling policies
120======================
121
122CFS implements three scheduling policies:
123
124  - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
125    policy that is used for regular tasks.
126
127  - SCHED_BATCH: Does not preempt nearly as often as regular tasks
128    would, thereby allowing tasks to run longer and make better use of
129    caches but at the cost of interactivity. This is well suited for
130    batch jobs.
131
132  - SCHED_IDLE: This is even weaker than nice 19, but its not a true
133    idle timer scheduler in order to avoid to get into priority
134    inversion problems which would deadlock the machine.
135
136SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
137POSIX.
138
139The command chrt from util-linux-ng 2.13.1.1 can set all of these except
140SCHED_IDLE.
141
142
143
1446.  SCHEDULING CLASSES
145======================
146
147The new CFS scheduler has been designed in such a way to introduce "Scheduling
148Classes," an extensible hierarchy of scheduler modules.  These modules
149encapsulate scheduling policy details and are handled by the scheduler core
150without the core code assuming too much about them.
151
152sched/fair.c implements the CFS scheduler described above.
153
154sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
155the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT
156priority levels, instead of 140 in the previous scheduler) and it needs no
157expired array.
158
159Scheduling classes are implemented through the sched_class structure, which
160contains hooks to functions that must be called whenever an interesting event
161occurs.
162
163This is the (partial) list of the hooks:
164
165 - enqueue_task(...)
166
167   Called when a task enters a runnable state.
168   It puts the scheduling entity (task) into the red-black tree and
169   increments the nr_running variable.
170
171 - dequeue_task(...)
172
173   When a task is no longer runnable, this function is called to keep the
174   corresponding scheduling entity out of the red-black tree.  It decrements
175   the nr_running variable.
176
177 - yield_task(...)
178
179   This function is basically just a dequeue followed by an enqueue, unless the
180   compat_yield sysctl is turned on; in that case, it places the scheduling
181   entity at the right-most end of the red-black tree.
182
183 - check_preempt_curr(...)
184
185   This function checks if a task that entered the runnable state should
186   preempt the currently running task.
187
188 - pick_next_task(...)
189
190   This function chooses the most appropriate task eligible to run next.
191
192 - set_curr_task(...)
193
194   This function is called when a task changes its scheduling class or changes
195   its task group.
196
197 - task_tick(...)
198
199   This function is mostly called from time tick functions; it might lead to
200   process switch.  This drives the running preemption.
201
202
203
204
2057.  GROUP SCHEDULER EXTENSIONS TO CFS
206=====================================
207
208Normally, the scheduler operates on individual tasks and strives to provide
209fair CPU time to each task.  Sometimes, it may be desirable to group tasks and
210provide fair CPU time to each such task group.  For example, it may be
211desirable to first provide fair CPU time to each user on the system and then to
212each task belonging to a user.
213
214CONFIG_CGROUP_SCHED strives to achieve exactly that.  It lets tasks to be
215grouped and divides CPU time fairly among such groups.
216
217CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
218SCHED_RR) tasks.
219
220CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
221SCHED_BATCH) tasks.
222
223   These options need CONFIG_CGROUPS to be defined, and let the administrator
224   create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See
225   Documentation/admin-guide/cgroup-v1/cgroups.rst for more information about this filesystem.
226
227When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
228group created using the pseudo filesystem.  See example steps below to create
229task groups and modify their CPU share using the "cgroups" pseudo filesystem::
230
231	# mount -t tmpfs cgroup_root /sys/fs/cgroup
232	# mkdir /sys/fs/cgroup/cpu
233	# mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
234	# cd /sys/fs/cgroup/cpu
235
236	# mkdir multimedia	# create "multimedia" group of tasks
237	# mkdir browser		# create "browser" group of tasks
238
239	# #Configure the multimedia group to receive twice the CPU bandwidth
240	# #that of browser group
241
242	# echo 2048 > multimedia/cpu.shares
243	# echo 1024 > browser/cpu.shares
244
245	# firefox &	# Launch firefox and move it to "browser" group
246	# echo <firefox_pid> > browser/tasks
247
248	# #Launch gmplayer (or your favourite movie player)
249	# echo <movie_player_pid> > multimedia/tasks
250