1 .. _sched_design_CFS: 1 .. _sched_design_CFS:
2 2
3 ============= 3 =============
4 CFS Scheduler 4 CFS Scheduler
5 ============= 5 =============
6 6
7 7
8 1. OVERVIEW 8 1. OVERVIEW
9 ============ 9 ============
10 10
11 CFS stands for "Completely Fair Scheduler," an !! 11 CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
12 scheduler implemented by Ingo Molnar and merge !! 12 scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. It is the
13 originally merged, it was the replacement for !! 13 replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
14 scheduler's SCHED_OTHER interactivity code. No !! 14 code.
15 for EEVDF, for which documentation can be foun <<
16 Documentation/scheduler/sched-eevdf.rst. <<
17 15
18 80% of CFS's design can be summed up in a sing 16 80% of CFS's design can be summed up in a single sentence: CFS basically models
19 an "ideal, precise multi-tasking CPU" on real 17 an "ideal, precise multi-tasking CPU" on real hardware.
20 18
21 "Ideal multi-tasking CPU" is a (non-existent 19 "Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100% physical
22 power and which can run each task at precise e 20 power and which can run each task at precise equal speed, in parallel, each at
23 1/nr_running speed. For example: if there are 21 1/nr_running speed. For example: if there are 2 tasks running, then it runs
24 each at 50% physical power --- i.e., actually 22 each at 50% physical power --- i.e., actually in parallel.
25 23
26 On real hardware, we can run only a single tas 24 On real hardware, we can run only a single task at once, so we have to
27 introduce the concept of "virtual runtime." T 25 introduce the concept of "virtual runtime." The virtual runtime of a task
28 specifies when its next timeslice would start 26 specifies when its next timeslice would start execution on the ideal
29 multi-tasking CPU described above. In practic 27 multi-tasking CPU described above. In practice, the virtual runtime of a task
30 is its actual runtime normalized to the total 28 is its actual runtime normalized to the total number of running tasks.
31 29
32 30
33 31
34 2. FEW IMPLEMENTATION DETAILS 32 2. FEW IMPLEMENTATION DETAILS
35 ============================== 33 ==============================
36 34
37 In CFS the virtual runtime is expressed and tr 35 In CFS the virtual runtime is expressed and tracked via the per-task
38 p->se.vruntime (nanosec-unit) value. This way 36 p->se.vruntime (nanosec-unit) value. This way, it's possible to accurately
39 timestamp and measure the "expected CPU time" 37 timestamp and measure the "expected CPU time" a task should have gotten.
40 38
41 Small detail: on "ideal" hardware, at any t 39 Small detail: on "ideal" hardware, at any time all tasks would have the same
42 p->se.vruntime value --- i.e., tasks would 40 p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
43 would ever get "out of balance" from the "i 41 would ever get "out of balance" from the "ideal" share of CPU time.
44 42
45 CFS's task picking logic is based on this p->s 43 CFS's task picking logic is based on this p->se.vruntime value and it is thus
46 very simple: it always tries to run the task w 44 very simple: it always tries to run the task with the smallest p->se.vruntime
47 value (i.e., the task which executed least so 45 value (i.e., the task which executed least so far). CFS always tries to split
48 up CPU time between runnable tasks as close to 46 up CPU time between runnable tasks as close to "ideal multitasking hardware" as
49 possible. 47 possible.
50 48
51 Most of the rest of CFS's design just falls ou 49 Most of the rest of CFS's design just falls out of this really simple concept,
52 with a few add-on embellishments like nice lev 50 with a few add-on embellishments like nice levels, multiprocessing and various
53 algorithm variants to recognize sleepers. 51 algorithm variants to recognize sleepers.
54 52
55 53
56 54
57 3. THE RBTREE 55 3. THE RBTREE
58 ============== 56 ==============
59 57
60 CFS's design is quite radical: it does not use 58 CFS's design is quite radical: it does not use the old data structures for the
61 runqueues, but it uses a time-ordered rbtree t 59 runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
62 task execution, and thus has no "array switch" 60 task execution, and thus has no "array switch" artifacts (by which both the
63 previous vanilla scheduler and RSDL/SD are aff 61 previous vanilla scheduler and RSDL/SD are affected).
64 62
65 CFS also maintains the rq->cfs.min_vruntime va 63 CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
66 increasing value tracking the smallest vruntim 64 increasing value tracking the smallest vruntime among all tasks in the
67 runqueue. The total amount of work done by th 65 runqueue. The total amount of work done by the system is tracked using
68 min_vruntime; that value is used to place newl 66 min_vruntime; that value is used to place newly activated entities on the left
69 side of the tree as much as possible. 67 side of the tree as much as possible.
70 68
71 The total number of running tasks in the runqu 69 The total number of running tasks in the runqueue is accounted through the
72 rq->cfs.load value, which is the sum of the we 70 rq->cfs.load value, which is the sum of the weights of the tasks queued on the
73 runqueue. 71 runqueue.
74 72
75 CFS maintains a time-ordered rbtree, where all 73 CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
76 p->se.vruntime key. CFS picks the "leftmost" t 74 p->se.vruntime key. CFS picks the "leftmost" task from this tree and sticks to it.
77 As the system progresses forwards, the execute 75 As the system progresses forwards, the executed tasks are put into the tree
78 more and more to the right --- slowly but sure 76 more and more to the right --- slowly but surely giving a chance for every task
79 to become the "leftmost task" and thus get on 77 to become the "leftmost task" and thus get on the CPU within a deterministic
80 amount of time. 78 amount of time.
81 79
82 Summing up, CFS works like this: it runs a tas 80 Summing up, CFS works like this: it runs a task a bit, and when the task
83 schedules (or a scheduler tick happens) the ta 81 schedules (or a scheduler tick happens) the task's CPU usage is "accounted
84 for": the (small) time it just spent using the 82 for": the (small) time it just spent using the physical CPU is added to
85 p->se.vruntime. Once p->se.vruntime gets high 83 p->se.vruntime. Once p->se.vruntime gets high enough so that another task
86 becomes the "leftmost task" of the time-ordere 84 becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
87 small amount of "granularity" distance relativ 85 small amount of "granularity" distance relative to the leftmost task so that we
88 do not over-schedule tasks and trash the cache 86 do not over-schedule tasks and trash the cache), then the new leftmost task is
89 picked and the current task is preempted. 87 picked and the current task is preempted.
90 88
91 89
92 90
93 4. SOME FEATURES OF CFS 91 4. SOME FEATURES OF CFS
94 ======================== 92 ========================
95 93
96 CFS uses nanosecond granularity accounting and 94 CFS uses nanosecond granularity accounting and does not rely on any jiffies or
97 other HZ detail. Thus the CFS scheduler has n 95 other HZ detail. Thus the CFS scheduler has no notion of "timeslices" in the
98 way the previous scheduler had, and has no heu 96 way the previous scheduler had, and has no heuristics whatsoever. There is
99 only one central tunable (you have to switch o 97 only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
100 98
101 /sys/kernel/debug/sched/base_slice_ns 99 /sys/kernel/debug/sched/base_slice_ns
102 100
103 which can be used to tune the scheduler from " 101 which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
104 "server" (i.e., good batching) workloads. It 102 "server" (i.e., good batching) workloads. It defaults to a setting suitable
105 for desktop workloads. SCHED_BATCH is handled 103 for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.
106 104
107 In case CONFIG_HZ results in base_slice_ns < T 105 In case CONFIG_HZ results in base_slice_ns < TICK_NSEC, the value of
108 base_slice_ns will have little to no impact on 106 base_slice_ns will have little to no impact on the workloads.
109 107
110 Due to its design, the CFS scheduler is not pr 108 Due to its design, the CFS scheduler is not prone to any of the "attacks" that
111 exist today against the heuristics of the stoc 109 exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
112 chew.c, ring-test.c, massive_intr.c all work f 110 chew.c, ring-test.c, massive_intr.c all work fine and do not impact
113 interactivity and produce the expected behavio 111 interactivity and produce the expected behavior.
114 112
115 The CFS scheduler has a much stronger handling 113 The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
116 than the previous vanilla scheduler: both type 114 than the previous vanilla scheduler: both types of workloads are isolated much
117 more aggressively. 115 more aggressively.
118 116
119 SMP load-balancing has been reworked/sanitized 117 SMP load-balancing has been reworked/sanitized: the runqueue-walking
120 assumptions are gone from the load-balancing c 118 assumptions are gone from the load-balancing code now, and iterators of the
121 scheduling modules are used. The balancing co 119 scheduling modules are used. The balancing code got quite a bit simpler as a
122 result. 120 result.
123 121
124 122
125 123
126 5. Scheduling policies 124 5. Scheduling policies
127 ====================== 125 ======================
128 126
129 CFS implements three scheduling policies: 127 CFS implements three scheduling policies:
130 128
131 - SCHED_NORMAL (traditionally called SCHED_O 129 - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
132 policy that is used for regular tasks. 130 policy that is used for regular tasks.
133 131
134 - SCHED_BATCH: Does not preempt nearly as of 132 - SCHED_BATCH: Does not preempt nearly as often as regular tasks
135 would, thereby allowing tasks to run longe 133 would, thereby allowing tasks to run longer and make better use of
136 caches but at the cost of interactivity. T 134 caches but at the cost of interactivity. This is well suited for
137 batch jobs. 135 batch jobs.
138 136
139 - SCHED_IDLE: This is even weaker than nice 137 - SCHED_IDLE: This is even weaker than nice 19, but its not a true
140 idle timer scheduler in order to avoid to 138 idle timer scheduler in order to avoid to get into priority
141 inversion problems which would deadlock th 139 inversion problems which would deadlock the machine.
142 140
143 SCHED_FIFO/_RR are implemented in sched/rt.c a 141 SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
144 POSIX. 142 POSIX.
145 143
146 The command chrt from util-linux-ng 2.13.1.1 c 144 The command chrt from util-linux-ng 2.13.1.1 can set all of these except
147 SCHED_IDLE. 145 SCHED_IDLE.
148 146
149 147
150 148
151 6. SCHEDULING CLASSES 149 6. SCHEDULING CLASSES
152 ====================== 150 ======================
153 151
154 The new CFS scheduler has been designed in suc 152 The new CFS scheduler has been designed in such a way to introduce "Scheduling
155 Classes," an extensible hierarchy of scheduler 153 Classes," an extensible hierarchy of scheduler modules. These modules
156 encapsulate scheduling policy details and are 154 encapsulate scheduling policy details and are handled by the scheduler core
157 without the core code assuming too much about 155 without the core code assuming too much about them.
158 156
159 sched/fair.c implements the CFS scheduler desc 157 sched/fair.c implements the CFS scheduler described above.
160 158
161 sched/rt.c implements SCHED_FIFO and SCHED_RR 159 sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
162 the previous vanilla scheduler did. It uses 1 160 the previous vanilla scheduler did. It uses 100 runqueues (for all 100 RT
163 priority levels, instead of 140 in the previou 161 priority levels, instead of 140 in the previous scheduler) and it needs no
164 expired array. 162 expired array.
165 163
166 Scheduling classes are implemented through the 164 Scheduling classes are implemented through the sched_class structure, which
167 contains hooks to functions that must be calle 165 contains hooks to functions that must be called whenever an interesting event
168 occurs. 166 occurs.
169 167
170 This is the (partial) list of the hooks: 168 This is the (partial) list of the hooks:
171 169
172 - enqueue_task(...) 170 - enqueue_task(...)
173 171
174 Called when a task enters a runnable state. 172 Called when a task enters a runnable state.
175 It puts the scheduling entity (task) into t 173 It puts the scheduling entity (task) into the red-black tree and
176 increments the nr_running variable. 174 increments the nr_running variable.
177 175
178 - dequeue_task(...) 176 - dequeue_task(...)
179 177
180 When a task is no longer runnable, this fun 178 When a task is no longer runnable, this function is called to keep the
181 corresponding scheduling entity out of the 179 corresponding scheduling entity out of the red-black tree. It decrements
182 the nr_running variable. 180 the nr_running variable.
183 181
184 - yield_task(...) 182 - yield_task(...)
185 183
186 This function is basically just a dequeue f 184 This function is basically just a dequeue followed by an enqueue, unless the
187 compat_yield sysctl is turned on; in that c 185 compat_yield sysctl is turned on; in that case, it places the scheduling
188 entity at the right-most end of the red-bla 186 entity at the right-most end of the red-black tree.
189 187
190 - wakeup_preempt(...) 188 - wakeup_preempt(...)
191 189
192 This function checks if a task that entered 190 This function checks if a task that entered the runnable state should
193 preempt the currently running task. 191 preempt the currently running task.
194 192
195 - pick_next_task(...) 193 - pick_next_task(...)
196 194
197 This function chooses the most appropriate 195 This function chooses the most appropriate task eligible to run next.
198 196
199 - set_next_task(...) 197 - set_next_task(...)
200 198
201 This function is called when a task changes 199 This function is called when a task changes its scheduling class, changes
202 its task group or is scheduled. 200 its task group or is scheduled.
203 201
204 - task_tick(...) 202 - task_tick(...)
205 203
206 This function is mostly called from time ti 204 This function is mostly called from time tick functions; it might lead to
207 process switch. This drives the running pr 205 process switch. This drives the running preemption.
208 206
209 207
210 208
211 209
212 7. GROUP SCHEDULER EXTENSIONS TO CFS 210 7. GROUP SCHEDULER EXTENSIONS TO CFS
213 ===================================== 211 =====================================
214 212
215 Normally, the scheduler operates on individual 213 Normally, the scheduler operates on individual tasks and strives to provide
216 fair CPU time to each task. Sometimes, it may 214 fair CPU time to each task. Sometimes, it may be desirable to group tasks and
217 provide fair CPU time to each such task group. 215 provide fair CPU time to each such task group. For example, it may be
218 desirable to first provide fair CPU time to ea 216 desirable to first provide fair CPU time to each user on the system and then to
219 each task belonging to a user. 217 each task belonging to a user.
220 218
221 CONFIG_CGROUP_SCHED strives to achieve exactly 219 CONFIG_CGROUP_SCHED strives to achieve exactly that. It lets tasks to be
222 grouped and divides CPU time fairly among such 220 grouped and divides CPU time fairly among such groups.
223 221
224 CONFIG_RT_GROUP_SCHED permits to group real-ti 222 CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
225 SCHED_RR) tasks. 223 SCHED_RR) tasks.
226 224
227 CONFIG_FAIR_GROUP_SCHED permits to group CFS ( 225 CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
228 SCHED_BATCH) tasks. 226 SCHED_BATCH) tasks.
229 227
230 These options need CONFIG_CGROUPS to be def 228 These options need CONFIG_CGROUPS to be defined, and let the administrator
231 create arbitrary groups of tasks, using the 229 create arbitrary groups of tasks, using the "cgroup" pseudo filesystem. See
232 Documentation/admin-guide/cgroup-v1/cgroups 230 Documentation/admin-guide/cgroup-v1/cgroups.rst for more information about this filesystem.
233 231
234 When CONFIG_FAIR_GROUP_SCHED is defined, a "cp 232 When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
235 group created using the pseudo filesystem. Se 233 group created using the pseudo filesystem. See example steps below to create
236 task groups and modify their CPU share using t 234 task groups and modify their CPU share using the "cgroups" pseudo filesystem::
237 235
238 # mount -t tmpfs cgroup_root /sys/fs/c 236 # mount -t tmpfs cgroup_root /sys/fs/cgroup
239 # mkdir /sys/fs/cgroup/cpu 237 # mkdir /sys/fs/cgroup/cpu
240 # mount -t cgroup -ocpu none /sys/fs/c 238 # mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
241 # cd /sys/fs/cgroup/cpu 239 # cd /sys/fs/cgroup/cpu
242 240
243 # mkdir multimedia # create "mult 241 # mkdir multimedia # create "multimedia" group of tasks
244 # mkdir browser # create "brow 242 # mkdir browser # create "browser" group of tasks
245 243
246 # #Configure the multimedia group to r 244 # #Configure the multimedia group to receive twice the CPU bandwidth
247 # #that of browser group 245 # #that of browser group
248 246
249 # echo 2048 > multimedia/cpu.shares 247 # echo 2048 > multimedia/cpu.shares
250 # echo 1024 > browser/cpu.shares 248 # echo 1024 > browser/cpu.shares
251 249
252 # firefox & # Launch firefox and m 250 # firefox & # Launch firefox and move it to "browser" group
253 # echo <firefox_pid> > browser/tasks 251 # echo <firefox_pid> > browser/tasks
254 252
255 # #Launch gmplayer (or your favourite 253 # #Launch gmplayer (or your favourite movie player)
256 # echo <movie_player_pid> > multimedia 254 # echo <movie_player_pid> > multimedia/tasks
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