1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
|
Control Group v2
October, 2015 Tejun Heo <tj@kernel.org>
This is the authoritative documentation on the design, interface and
conventions of cgroup v2. It describes all userland-visible aspects
of cgroup including core and specific controller behaviors. All
future changes must be reflected in this document. Documentation for
v1 is available under Documentation/cgroup-legacy/.
CONTENTS
1. Introduction
1-1. Terminology
1-2. What is cgroup?
2. Basic Operations
2-1. Mounting
2-2. Organizing Processes
2-3. [Un]populated Notification
2-4. Controlling Controllers
2-4-1. Enabling and Disabling
2-4-2. Top-down Constraint
2-4-3. No Internal Process Constraint
2-5. Delegation
2-5-1. Model of Delegation
2-5-2. Delegation Containment
2-6. Guidelines
2-6-1. Organize Once and Control
2-6-2. Avoid Name Collisions
3. Resource Distribution Models
3-1. Weights
3-2. Limits
3-3. Protections
3-4. Allocations
4. Interface Files
4-1. Format
4-2. Conventions
4-3. Core Interface Files
5. Controllers
5-1. CPU
5-1-1. CPU Interface Files
5-2. Memory
5-2-1. Memory Interface Files
5-2-2. Usage Guidelines
5-2-3. Memory Ownership
5-3. IO
5-3-1. IO Interface Files
5-3-2. Writeback
P. Information on Kernel Programming
P-1. Filesystem Support for Writeback
D. Deprecated v1 Core Features
R. Issues with v1 and Rationales for v2
R-1. Multiple Hierarchies
R-2. Thread Granularity
R-3. Competition Between Inner Nodes and Threads
R-4. Other Interface Issues
R-5. Controller Issues and Remedies
R-5-1. Memory
1. Introduction
1-1. Terminology
"cgroup" stands for "control group" and is never capitalized. The
singular form is used to designate the whole feature and also as a
qualifier as in "cgroup controllers". When explicitly referring to
multiple individual control groups, the plural form "cgroups" is used.
1-2. What is cgroup?
cgroup is a mechanism to organize processes hierarchically and
distribute system resources along the hierarchy in a controlled and
configurable manner.
cgroup is largely composed of two parts - the core and controllers.
cgroup core is primarily responsible for hierarchically organizing
processes. A cgroup controller is usually responsible for
distributing a specific type of system resource along the hierarchy
although there are utility controllers which serve purposes other than
resource distribution.
cgroups form a tree structure and every process in the system belongs
to one and only one cgroup. All threads of a process belong to the
same cgroup. On creation, all processes are put in the cgroup that
the parent process belongs to at the time. A process can be migrated
to another cgroup. Migration of a process doesn't affect already
existing descendant processes.
Following certain structural constraints, controllers may be enabled or
disabled selectively on a cgroup. All controller behaviors are
hierarchical - if a controller is enabled on a cgroup, it affects all
processes which belong to the cgroups consisting the inclusive
sub-hierarchy of the cgroup. When a controller is enabled on a nested
cgroup, it always restricts the resource distribution further. The
restrictions set closer to the root in the hierarchy can not be
overridden from further away.
2. Basic Operations
2-1. Mounting
Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
hierarchy can be mounted with the following mount command.
# mount -t cgroup2 none $MOUNT_POINT
cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
controllers which support v2 and are not bound to a v1 hierarchy are
automatically bound to the v2 hierarchy and show up at the root.
Controllers which are not in active use in the v2 hierarchy can be
bound to other hierarchies. This allows mixing v2 hierarchy with the
legacy v1 multiple hierarchies in a fully backward compatible way.
A controller can be moved across hierarchies only after the controller
is no longer referenced in its current hierarchy. Because per-cgroup
controller states are destroyed asynchronously and controllers may
have lingering references, a controller may not show up immediately on
the v2 hierarchy after the final umount of the previous hierarchy.
Similarly, a controller should be fully disabled to be moved out of
the unified hierarchy and it may take some time for the disabled
controller to become available for other hierarchies; furthermore, due
to inter-controller dependencies, other controllers may need to be
disabled too.
While useful for development and manual configurations, moving
controllers dynamically between the v2 and other hierarchies is
strongly discouraged for production use. It is recommended to decide
the hierarchies and controller associations before starting using the
controllers after system boot.
2-2. Organizing Processes
Initially, only the root cgroup exists to which all processes belong.
A child cgroup can be created by creating a sub-directory.
# mkdir $CGROUP_NAME
A given cgroup may have multiple child cgroups forming a tree
structure. Each cgroup has a read-writable interface file
"cgroup.procs". When read, it lists the PIDs of all processes which
belong to the cgroup one-per-line. The PIDs are not ordered and the
same PID may show up more than once if the process got moved to
another cgroup and then back or the PID got recycled while reading.
A process can be migrated into a cgroup by writing its PID to the
target cgroup's "cgroup.procs" file. Only one process can be migrated
on a single write(2) call. If a process is composed of multiple
threads, writing the PID of any thread migrates all threads of the
process.
When a process forks a child process, the new process is born into the
cgroup that the forking process belongs to at the time of the
operation. After exit, a process stays associated with the cgroup
that it belonged to at the time of exit until it's reaped; however, a
zombie process does not appear in "cgroup.procs" and thus can't be
moved to another cgroup.
A cgroup which doesn't have any children or live processes can be
destroyed by removing the directory. Note that a cgroup which doesn't
have any children and is associated only with zombie processes is
considered empty and can be removed.
# rmdir $CGROUP_NAME
"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
cgroup is in use in the system, this file may contain multiple lines,
one for each hierarchy. The entry for cgroup v2 is always in the
format "0::$PATH".
# cat /proc/842/cgroup
...
0::/test-cgroup/test-cgroup-nested
If the process becomes a zombie and the cgroup it was associated with
is removed subsequently, " (deleted)" is appended to the path.
# cat /proc/842/cgroup
...
0::/test-cgroup/test-cgroup-nested (deleted)
2-3. [Un]populated Notification
Each non-root cgroup has a "cgroup.events" file which contains
"populated" field indicating whether the cgroup's sub-hierarchy has
live processes in it. Its value is 0 if there is no live process in
the cgroup and its descendants; otherwise, 1. poll and [id]notify
events are triggered when the value changes. This can be used, for
example, to start a clean-up operation after all processes of a given
sub-hierarchy have exited. The populated state updates and
notifications are recursive. Consider the following sub-hierarchy
where the numbers in the parentheses represent the numbers of processes
in each cgroup.
A(4) - B(0) - C(1)
\ D(0)
A, B and C's "populated" fields would be 1 while D's 0. After the one
process in C exits, B and C's "populated" fields would flip to "0" and
file modified events will be generated on the "cgroup.events" files of
both cgroups.
2-4. Controlling Controllers
2-4-1. Enabling and Disabling
Each cgroup has a "cgroup.controllers" file which lists all
controllers available for the cgroup to enable.
# cat cgroup.controllers
cpu io memory
No controller is enabled by default. Controllers can be enabled and
disabled by writing to the "cgroup.subtree_control" file.
# echo "+cpu +memory -io" > cgroup.subtree_control
Only controllers which are listed in "cgroup.controllers" can be
enabled. When multiple operations are specified as above, either they
all succeed or fail. If multiple operations on the same controller
are specified, the last one is effective.
Enabling a controller in a cgroup indicates that the distribution of
the target resource across its immediate children will be controlled.
Consider the following sub-hierarchy. The enabled controllers are
listed in parentheses.
A(cpu,memory) - B(memory) - C()
\ D()
As A has "cpu" and "memory" enabled, A will control the distribution
of CPU cycles and memory to its children, in this case, B. As B has
"memory" enabled but not "CPU", C and D will compete freely on CPU
cycles but their division of memory available to B will be controlled.
As a controller regulates the distribution of the target resource to
the cgroup's children, enabling it creates the controller's interface
files in the child cgroups. In the above example, enabling "cpu" on B
would create the "cpu." prefixed controller interface files in C and
D. Likewise, disabling "memory" from B would remove the "memory."
prefixed controller interface files from C and D. This means that the
controller interface files - anything which doesn't start with
"cgroup." are owned by the parent rather than the cgroup itself.
2-4-2. Top-down Constraint
Resources are distributed top-down and a cgroup can further distribute
a resource only if the resource has been distributed to it from the
parent. This means that all non-root "cgroup.subtree_control" files
can only contain controllers which are enabled in the parent's
"cgroup.subtree_control" file. A controller can be enabled only if
the parent has the controller enabled and a controller can't be
disabled if one or more children have it enabled.
2-4-3. No Internal Process Constraint
Non-root cgroups can only distribute resources to their children when
they don't have any processes of their own. In other words, only
cgroups which don't contain any processes can have controllers enabled
in their "cgroup.subtree_control" files.
This guarantees that, when a controller is looking at the part of the
hierarchy which has it enabled, processes are always only on the
leaves. This rules out situations where child cgroups compete against
internal processes of the parent.
The root cgroup is exempt from this restriction. Root contains
processes and anonymous resource consumption which can't be associated
with any other cgroups and requires special treatment from most
controllers. How resource consumption in the root cgroup is governed
is up to each controller.
Note that the restriction doesn't get in the way if there is no
enabled controller in the cgroup's "cgroup.subtree_control". This is
important as otherwise it wouldn't be possible to create children of a
populated cgroup. To control resource distribution of a cgroup, the
cgroup must create children and transfer all its processes to the
children before enabling controllers in its "cgroup.subtree_control"
file.
2-5. Delegation
2-5-1. Model of Delegation
A cgroup can be delegated to a less privileged user by granting write
access of the directory and its "cgroup.procs" file to the user. Note
that resource control interface files in a given directory control the
distribution of the parent's resources and thus must not be delegated
along with the directory.
Once delegated, the user can build sub-hierarchy under the directory,
organize processes as it sees fit and further distribute the resources
it received from the parent. The limits a
|