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1 =============
2 CFS Scheduler
3 =============
4
5
6 1. OVERVIEW
7
8 CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
9 scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. It is the
10 replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
11 code.
12
13 80% of CFS's design can be summed up in a single sentence: CFS basically models
14 an "ideal, precise multi-tasking CPU" on real hardware.
15
16 "Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100% physical
17 power and which can run each task at precise equal speed, in parallel, each at
18 1/nr_running speed. For example: if there are 2 tasks running, then it runs
19 each at 50% physical power --- i.e., actually in parallel.
20
21 On real hardware, we can run only a single task at once, so we have to
22 introduce the concept of "virtual runtime." The virtual runtime of a task
23 specifies when its next timeslice would start execution on the ideal
24 multi-tasking CPU described above. In practice, the virtual runtime of a task
25 is its actual runtime normalized to the total number of running tasks.
26
27
28
29 2. FEW IMPLEMENTATION DETAILS
30
31 In CFS the virtual runtime is expressed and tracked via the per-task
32 p->se.vruntime (nanosec-unit) value. This way, it's possible to accurately
33 timestamp and measure the "expected CPU time" a task should have gotten.
34
35 [ small detail: on "ideal" hardware, at any time all tasks would have the same
36 p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
37 would ever get "out of balance" from the "ideal" share of CPU time. ]
38
39 CFS's task picking logic is based on this p->se.vruntime value and it is thus
40 very simple: it always tries to run the task with the smallest p->se.vruntime
41 value (i.e., the task which executed least so far). CFS always tries to split
42 up CPU time between runnable tasks as close to "ideal multitasking hardware" as
43 possible.
44
45 Most of the rest of CFS's design just falls out of this really simple concept,
46 with a few add-on embellishments like nice levels, multiprocessing and various
47 algorithm variants to recognize sleepers.
48
49
50
51 3. THE RBTREE
52
53 CFS's design is quite radical: it does not use the old data structures for the
54 runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
55 task execution, and thus has no "array switch" artifacts (by which both the
56 previous vanilla scheduler and RSDL/SD are affected).
57
58 CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
59 increasing value tracking the smallest vruntime among all tasks in the
60 runqueue. The total amount of work done by the system is tracked using
61 min_vruntime; that value is used to place newly activated entities on the left
62 side of the tree as much as possible.
63
64 The total number of running tasks in the runqueue is accounted through the
65 rq->cfs.load value, which is the sum of the weights of the tasks queued on the
66 runqueue.
67
68 CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
69 p->se.vruntime key. CFS picks the "leftmost" task from this tree and sticks to it.
70 As the system progresses forwards, the executed tasks are put into the tree
71 more and more to the right --- slowly but surely giving a chance for every task
72 to become the "leftmost task" and thus get on the CPU within a deterministic
73 amount of time.
74
75 Summing up, CFS works like this: it runs a task a bit, and when the task
76 schedules (or a scheduler tick happens) the task's CPU usage is "accounted
77 for": the (small) time it just spent using the physical CPU is added to
78 p->se.vruntime. Once p->se.vruntime gets high enough so that another task
79 becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
80 small amount of "granularity" distance relative to the leftmost task so that we
81 do not over-schedule tasks and trash the cache), then the new leftmost task is
82 picked and the current task is preempted.
83
84
85
86 4. SOME FEATURES OF CFS
87
88 CFS uses nanosecond granularity accounting and does not rely on any jiffies or
89 other HZ detail. Thus the CFS scheduler has no notion of "timeslices" in the
90 way the previous scheduler had, and has no heuristics whatsoever. There is
91 only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
92
93 /proc/sys/kernel/sched_min_granularity_ns
94
95 which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
96 "server" (i.e., good batching) workloads. It defaults to a setting suitable
97 for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.
98
99 Due to its design, the CFS scheduler is not prone to any of the "attacks" that
100 exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
101 chew.c, ring-test.c, massive_intr.c all work fine and do not impact
102 interactivity and produce the expected behavior.
103
104 The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
105 than the previous vanilla scheduler: both types of workloads are isolated much
106 more aggressively.
107
108 SMP load-balancing has been reworked/sanitized: the runqueue-walking
109 assumptions are gone from the load-balancing code now, and iterators of the
110 scheduling modules are used. The balancing code got quite a bit simpler as a
111 result.
112
113
114
115 5. Scheduling policies
116
117 CFS implements three scheduling policies:
118
119 - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
120 policy that is used for regular tasks.
121
122 - SCHED_BATCH: Does not preempt nearly as often as regular tasks
123 would, thereby allowing tasks to run longer and make better use of
124 caches but at the cost of interactivity. This is well suited for
125 batch jobs.
126
127 - SCHED_IDLE: This is even weaker than nice 19, but its not a true
128 idle timer scheduler in order to avoid to get into priority
129 inversion problems which would deadlock the machine.
130
131 SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
132 POSIX.
133
134 The command chrt from util-linux-ng 2.13.1.1 can set all of these except
135 SCHED_IDLE.
136
137
138
139 6. SCHEDULING CLASSES
140
141 The new CFS scheduler has been designed in such a way to introduce "Scheduling
142 Classes," an extensible hierarchy of scheduler modules. These modules
143 encapsulate scheduling policy details and are handled by the scheduler core
144 without the core code assuming too much about them.
145
146 sched/fair.c implements the CFS scheduler described above.
147
148 sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
149 the previous vanilla scheduler did. It uses 100 runqueues (for all 100 RT
150 priority levels, instead of 140 in the previous scheduler) and it needs no
151 expired array.
152
153 Scheduling classes are implemented through the sched_class structure, which
154 contains hooks to functions that must be called whenever an interesting event
155 occurs.
156
157 This is the (partial) list of the hooks:
158
159 - enqueue_task(...)
160
161 Called when a task enters a runnable state.
162 It puts the scheduling entity (task) into the red-black tree and
163 increments the nr_running variable.
164
165 - dequeue_task(...)
166
167 When a task is no longer runnable, this function is called to keep the
168 corresponding scheduling entity out of the red-black tree. It decrements
169 the nr_running variable.
170
171 - yield_task(...)
172
173 This function is basically just a dequeue followed by an enqueue, unless the
174 compat_yield sysctl is turned on; in that case, it places the scheduling
175 entity at the right-most end of the red-black tree.
176
177 - check_preempt_curr(...)
178
179 This function checks if a task that entered the runnable state should
180 preempt the currently running task.
181
182 - pick_next_task(...)
183
184 This function chooses the most appropriate task eligible to run next.
185
186 - set_curr_task(...)
187
188 This function is called when a task changes its scheduling class or changes
189 its task group.
190
191 - task_tick(...)
192
193 This function is mostly called from time tick functions; it might lead to
194 process switch. This drives the running preemption.
195
196
197
198
199 7. GROUP SCHEDULER EXTENSIONS TO CFS
200
201 Normally, the scheduler operates on individual tasks and strives to provide
202 fair CPU time to each task. Sometimes, it may be desirable to group tasks and
203 provide fair CPU time to each such task group. For example, it may be
204 desirable to first provide fair CPU time to each user on the system and then to
205 each task belonging to a user.
206
207 CONFIG_CGROUP_SCHED strives to achieve exactly that. It lets tasks to be
208 grouped and divides CPU time fairly among such groups.
209
210 CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
211 SCHED_RR) tasks.
212
213 CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
214 SCHED_BATCH) tasks.
215
216 These options need CONFIG_CGROUPS to be defined, and let the administrator
217 create arbitrary groups of tasks, using the "cgroup" pseudo filesystem. See
218 Documentation/cgroup-v1/cgroups.txt for more information about this filesystem.
219
220 When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
221 group created using the pseudo filesystem. See example steps below to create
222 task groups and modify their CPU share using the "cgroups" pseudo filesystem.
223
224 # mount -t tmpfs cgroup_root /sys/fs/cgroup
225 # mkdir /sys/fs/cgroup/cpu
226 # mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
227 # cd /sys/fs/cgroup/cpu
228
229 # mkdir multimedia # create "multimedia" group of tasks
230 # mkdir browser # create "browser" group of tasks
231
232 # #Configure the multimedia group to receive twice the CPU bandwidth
233 # #that of browser group
234
235 # echo 2048 > multimedia/cpu.shares
236 # echo 1024 > browser/cpu.shares
237
238 # firefox & # Launch firefox and move it to "browser" group
239 # echo <firefox_pid> > browser/tasks
240
241 # #Launch gmplayer (or your favourite movie player)
242 # echo <movie_player_pid> > multimedia/tasks