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
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
<meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<p>December 18, 2016</p>
<p>This article was contributed by Paul E. McKenney</p>
<h3>Introduction</h3>
This document describes RCU's major data structures and their relationship
to each other.
<ol>
<li> <a href="#Data-Structure Relationships">
Data-Structure Relationships</a>
<li> <a href="#The rcu_state Structure">
The <tt>rcu_state</tt> Structure</a>
<li> <a href="#The rcu_node Structure">
The <tt>rcu_node</tt> Structure</a>
<li> <a href="#The rcu_segcblist Structure">
The <tt>rcu_segcblist</tt> Structure</a>
<li> <a href="#The rcu_data Structure">
The <tt>rcu_data</tt> Structure</a>
<li> <a href="#The rcu_head Structure">
The <tt>rcu_head</tt> Structure</a>
<li> <a href="#RCU-Specific Fields in the task_struct Structure">
RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
<li> <a href="#Accessor Functions">
Accessor Functions</a>
</ol>
<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
<p>RCU is for all intents and purposes a large state machine, and its
data structures maintain the state in such a way as to allow RCU readers
to execute extremely quickly, while also processing the RCU grace periods
requested by updaters in an efficient and extremely scalable fashion.
The efficiency and scalability of RCU updaters is provided primarily
by a combining tree, as shown below:
</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
containing a tree of <tt>rcu_node</tt> structures.
Each leaf node of the <tt>rcu_node</tt> tree has up to 16
<tt>rcu_data</tt> structures associated with it, so that there
are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
one for each possible CPU.
This structure is adjusted at boot time, if needed, to handle the
common case where <tt>nr_cpu_ids</tt> is much less than
<tt>NR_CPUs</tt>.
For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
which results in a three-level <tt>rcu_node</tt> tree.
If the actual hardware has only 16 CPUs, RCU will adjust itself
at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
</p><p>The purpose of this combining tree is to allow per-CPU events
such as quiescent states, dyntick-idle transitions,
and CPU hotplug operations to be processed efficiently
and scalably.
Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
and other events are recorded by the leaf-level <tt>rcu_node</tt>
structures.
All of these events are combined at each level of the tree until finally
grace periods are completed at the tree's root <tt>rcu_node</tt>
structure.
A grace period can be completed at the root once every CPU
(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
has passed through a quiescent state.
Once a grace period has completed, record of that fact is propagated
back down the tree.
</p><p>As can be seen from the diagram, on a 64-bit system
a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
of 64 at the root and a fanout of 16 at the leaves.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why isn't the fanout at the leaves also 64?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because there are more types of events that affect the leaf-level
<tt>rcu_node</tt> structures than further up the tree.
Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
64, the contention on these structures' <tt>->structures</tt>
becomes excessive.
Experimentation on a wide variety of systems has shown that a fanout
of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
</font>
<p><font color="ffffff">Of course, further experience with
systems having hundreds or thousands of CPUs may demonstrate
that the fanout for the non-leaf <tt>rcu_node</tt> structures
must also be reduced.
Such reduction can be easily carried out when and if it proves
necessary.
In the meantime, if you are using such a system and running into
contention problems on the non-leaf <tt>rcu_node</tt> structures,
you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
parameter to reduce the non-leaf fanout as needed.
</font>
<p><font color="ffffff">Kernels built for systems with
strong NUMA characteristics might also need to adjust
<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
<tt>rcu_node</tt> structures align with hardware boundaries.
However, there has thus far been no need for this.
</font></td></tr>
<tr><td> </td></tr>
</table>
<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
a 32-bit system), then RCU will automatically add more levels to the
tree.
For example, if you are crazy enough to build a 64-bit system with 65,536
CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
32-bit systems.
On the other hand, you can set both <tt>CONFIG_RCU_FANOUT</tt> and
<tt>CONFIG_RCU_FANOUT_LEAF</tt> to be as small as 2, which would result
in a 16-CPU test using a 4-level tree.
This can be useful for testing large-system capabilities on small test
machines.
</p><p>This multi-level combining tree allows us to get most of the
performance and scalability
benefits of partitioning, even though RCU grace-period detection is
inherently a global operation.
The trick here is that only the last CPU to report a quiescent state
into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
structure at the next level up the tree.
This means that at the leaf-level <tt>rcu_node</tt> structure, only
one access out of sixteen will progress up the tree.
For the internal <tt>rcu_node</tt> structures, the situation is even
more extreme: Only one access out of sixty-four will progress up
the tree.
Because the vast majority of the CPUs do not progress up the tree,
the lock contention remains roughly constant up the tree.
No matter how many CPUs there are in the system, at most 64 quiescent-state
reports per grace period will progress all the way to the root
<tt>rcu_node</tt> structure, thus ensuring that the lock contention
on that root <tt>rcu_node</tt> structure remains acceptably low.
</p><p>In effect, the combining tree acts like a big shock absorber,
keeping lock contention under control at all tree levels regardless
of the level of loading on the system.
</p><p>RCU updaters wait for normal grace periods by registering
RCU callbacks, either directly via <tt>call_rcu()</tt> and
friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
or indirectly via <tt>synchronize_rcu()</tt> and friends.
RCU callbacks are represented by <tt>rcu_head</tt> structures,
which are queued on <tt>rcu_data</tt> structures while they are
waiting for a grace period to elapse, as shown in the following figure:
</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
<tt>PREEMPT_RCU</tt>'s major data structures are related.
Lesser data structures will be introduced with the algorithms that
make use of them.
</p><p>Note that each of the data structures in the above figure has
its own synchronization:
<p><ol>
<li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
and some fields are protected by the corresponding root
<tt>rcu_node</tt> structure's lock.
<li> Each <tt>rcu_node</tt> structure has a spinlock.
<li> The fields in <tt>rcu_data</tt> are private to the corresponding
CPU, although a few can be read and written by other CPUs.
</ol>
<p>It is important to note that different data structures can have
very different ideas about the state of RCU at any given time.
For but one example, awareness of the start or end of a given RCU
grace period propagates slowly through the data structures.
This slow propagation is absolutely necessary for RCU to have good
read-side performance.
If this balkanized implementation seems foreign to you, one useful
trick is to consider each instance of these data structures to be
a different person, each having the usual slightly different
view of reality.
</p><p>The general role of each of these data structures is as
follows:
</p><ol>
<li> <tt>rcu_state</tt>:
This structure forms the interconnection between the
<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
tracks grace periods, serves as short-term repository
for callbacks orphaned by CPU-hotplug events,
maintains <tt>rcu_barrier()</tt> state,
tracks expedited grace-period state,
and maintains state used to force quiescent states when
grace periods extend too long,
<li> <tt>rcu_node</tt>: This structure forms the combining
tree that propagates quiescent-state
information from the leaves to the root, and also propagates
grace-period information from the root to the leaves.
It provides local copies of the grace-period state in order
to allow this information to be accessed in a synchronized
manner without suffering the scalability limitations that
would otherwise be imposed by global locking.
In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
of tasks that have blocked while in their current
RCU read-side critical section.
In <tt>CONFIG_PREEMPT_RCU</tt> with
<tt>CONFIG_RCU_BOOST</tt>, it manages the
per-<tt>rcu_node</tt> priority-boosting
kernel threads (kthreads) and state.
Finally, it records CPU-hotplug state in order to determine
which CPUs should be ignored during a given grace period.
<li> <tt>rcu_data</tt>: This per-CPU structure is the
focus of quiescent-state detection and RCU callback queuing.
It also tracks its relationship to the corresponding leaf
<tt>rcu_node</tt> structure to allow more-efficient
propagation of quiescent states up the <tt>rcu_node</tt>
combining tree.
Like the <tt>rcu_node</tt> structure, it provides a local
copy of the grace-period information to allow for-free
synchronized
access to this information from the corresponding CPU.
Finally, this structure records past dyntick-idle state
for the corresponding CPU and also tracks statistics.
<li> <tt>rcu_head</tt>:
This structure represents RCU callbacks, and is the
only structure allocated and managed by RCU users.
The <tt>rcu_head</tt> structure is normally embedded
within the RCU-protected data structure.
</ol>
<p>If all you wanted from this article was a general notion of how
RCU's data structures are related, you are done.
Otherwise, each of the following sections give more details on
the <tt>rcu_state</tt>, <tt>rcu_node</tt> and <tt>rcu_data</tt> data
structures.
<h3><a name="The rcu_state Structure">
The <tt>rcu_state</tt> Structure</a></h3>
<p>The <tt>rcu_state</tt> structure is the base structure that
represents the state of RCU in the system.
This structure forms the interconnection between the
<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
tracks grace periods, contains the lock used to
synchronize with CPU-hotplug events,
and maintains state used to force quiescent states when
grace periods extend too long,
</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
singly and in groups, in the following sections.
The more specialized fields are covered in the discussion of their
use.
<h5>Relationship to rcu_node and rcu_data Structures</h5>
This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 struct rcu_node node[NUM_RCU_NODES];
2 struct rcu_node *level[NUM_RCU_LVLS + 1];
3 struct rcu_data __percpu *rda;
</pre>
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Wait a minute!
You said that the <tt>rcu_node</tt> structures formed a tree,
but they are declared as a flat array!
What gives?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
The tree is laid out in the array.
The first node In the array is the head, the next set of nodes in the
array are children of the head node, and so on until the last set of
nodes in the array are the leaves.
</font>
<p><font color="ffffff">See the following diagrams to see how
this works.
</font></td></tr>
<tr><td> </td></tr>
</table>
<p>The <tt>rcu_node</tt> tree is embedded into the
<tt>->node[]</tt> array as shown in the following figure:
</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
</p><p>One interesting consequence of this mapping is that a
breadth-first traversal of the tree is implemented as a simple
linear scan of the array, which is in fact what the
<tt>rcu_for_each_node_breadth_first()</tt> macro does.
This macro is used at the beginning and ends of grace periods.
</p><p>Each entry of the <tt>->level</tt> array references
the first <tt>rcu_node</tt> structure on the corresponding level
of the tree, for example, as shown below:
</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
</p><p>The zero<sup>th</sup> element of the array references the root
<tt>rcu_node</tt> structure, the first element references the
first child of the root <tt>rcu_node</tt>, and finally the second
element references the first leaf <tt>rcu_node</tt> structure.
</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
rather than array-shaped, it is easy to draw a planar representation:
</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
</p><p>Finally, the <tt>->rda</tt> field references a per-CPU
pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
</p><p>All of these fields are constant once initialization is complete,
and therefore need no protection.
<h5>Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 unsigned long gp_seq;
</pre>
<p>RCU grace periods are numbered, and
the <tt>->gp_seq</tt> field contains the current grace-period
sequence number.
The bottom two bits are the state of the current grace period,
which can be zero for not yet started or one for in progress.
In other words, if the bottom two bits of <tt>->gp_seq</tt> are
zero, then RCU is idle.
Any other value in the bottom two bits indicates that something is broken.
This field is protected by the root <tt>rcu_node</tt> structure's
<tt>->lock</tt> field.
</p><p>There are <tt>->gp_seq</tt> fields
in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
as well.
The fields in the <tt>rcu_state</tt> structure represent the
most current value, and those of the other structures are compared
in order to detect the beginnings and ends of grace periods in a distributed
fashion.
The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
<h5>Miscellaneous</h5>
<p>This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 unsigned long gp_max;
2 char abbr;
3 char *name;
</pre>
<p>The <tt>->gp_max</tt> field tracks the duration of the longest
grace period in jiffies.
It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>.
<p>The <tt>->name</tt> and <tt>->abbr</tt> fields distinguish
between preemptible RCU (“rcu_preempt” and “p”)
and non-preemptible RCU (“rcu_sched” and “s”).
These fields are used for diagnostic and tracing purposes.
<h3><a name="The rcu_node Structure">
The <tt>rcu_node</tt> Structure</a></h3>
<p>The <tt>rcu_node</tt> structures form the combining
tree that propagates quiescent-state
information from the leaves to the root and also that propagates
grace-period information from the root down to the leaves.
They provides local copies of the grace-period state in order
to allow this information to be accessed in a synchronized
manner without suffering the scalability limitations that
would otherwise be imposed by global locking.
In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
of tasks that have blocked while in their current
RCU read-side critical section.
In <tt>CONFIG_PREEMPT_RCU</tt> with
<tt>CONFIG_RCU_BOOST</tt>, they manage the
per-<tt>rcu_node</tt> priority-boosting
kernel threads (kthreads) and state.
Finally, they record CPU-hotplug state in order to determine
which CPUs should be ignored during a given grace period.
</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
singly and in groups, in the following sections.
<h5>Connection to Combining Tree</h5>
<p>This portion of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 struct rcu_node *parent;
2 u8 level;
3 u8 grpnum;
4 unsigned long grpmask;
5 int grplo;
6 int grphi;
</pre>
<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt>
one level up in the tree, and is <tt>NULL</tt> for the root
<tt>rcu_node</tt>.
The RCU implementation makes heavy use of this field to push quiescent
states up the tree.
The <tt>->level</tt> field gives the level in the tree, with
the root being at level zero, its children at level one, and so on.
The <tt>->grpnum</tt> field gives this node's position within
the children of its parent, so this number can range between 0 and 31
on 32-bit systems and between 0 and 63 on 64-bit systems.
The <tt>->level</tt> and <tt>->grpnum</tt> fields are
used only during initialization and for tracing.
The <tt>->grpmask</tt> field is the bitmask counterpart of
<tt>->grpnum</tt>, and therefore always has exactly one bit set.
This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
structure in its parent's bitmasks, which are described later.
Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields
contain the lowest and highest numbered CPU served by this
<tt>rcu_node</tt> structure, respectively.
</p><p>All of these fields are constant, and thus do not require any
synchronization.
<h5>Synchronization</h5>
<p>This field of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 raw_spinlock_t lock;
</pre>
<p>This field is used to protect the remaining fields in this structure,
unless otherwise stated.
That said, all of the fields in this structure can be accessed without
locking for tracing purposes.
Yes, this can result in confusing traces, but better some tracing confusion
than to be heisenbugged out of existence.
<h5>Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 unsigned long gp_seq;
2 unsigned long gp_seq_needed;
</pre>
<p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are
the counterparts of the field of the same name in the <tt>rcu_state</tt>
structure.
They each may lag up to one step behind their <tt>rcu_state</tt>
counterpart.
If the bottom two bits of a given <tt>rcu_node</tt> structure's
<tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
structure believes that RCU is idle.
</p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt>
structure is updated at the beginning and the end
of each grace period.
<p>The <tt>->gp_seq_needed</tt> fields record the
furthest-in-the-future grace period request seen by the corresponding
<tt>rcu_node</tt> structure. The request is considered fulfilled when
the value of the <tt>->gp_seq</tt> field equals or exceeds that of
the <tt>->gp_seq_needed</tt> field.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Suppose that this <tt>rcu_node</tt> structure doesn't see
a request for a very long time.
Won't wrapping of the <tt>->gp_seq</tt> field cause
problems?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
No, because if the <tt>->gp_seq_needed</tt> field lags behind the
<tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field
will be updated at the end of the grace period.
Modulo-arithmetic comparisons therefore will always get the
correct answer, even with wrapping.
</font></td></tr>
<tr><td> </td></tr>
</table>
<h5>Quiescent-State Tracking</h5>
<p>These fields manage the propagation of quiescent states up the
combining tree.
</p><p>This portion of the <tt>rcu_node</tt> structure has fields
as follows:
<pre>
1 unsigned long qsmask;
2 unsigned long expmask;
3 unsigned long qsmaskinit;
4 unsigned long expmaskinit;
</pre>
<p>The <tt>->qsmask</tt> field tracks which of this
<tt>rcu_node</tt> structure's children still need to report
quiescent states for the current normal grace period.
Such children will have a value of 1 in their corresponding bit.
Note that the leaf <tt>rcu_node</tt> structures should be
thought of as having <tt>rcu_data</tt> structures as their
children.
Similarly, the <tt>->expmask</tt> field tracks which
of this <tt>rcu_node</tt> structure's children still need to report
quiescent states for the current expedited grace period.
An expedited grace period has
the same conceptual properties as a normal grace period, but the
expedited implementation accepts extreme CPU overhead to obtain
much lower grace-period latency, for example, consuming a few
tens of microseconds worth of CPU time to reduce grace-period
duration from milliseconds to tens of microseconds.
The <tt>->qsmaskinit</tt> field tracks which of this
<tt>rcu_node</tt> structure's children cover for at least
one online CPU.
This mask is used to initialize <tt>->qsmask</tt>,
and <tt>->expmaskinit</tt> is used to initialize
<tt>->expmask</tt> and the beginning of the
normal and expedited grace periods, respectively.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why are these bitmasks protected by locking?
Come on, haven't you heard of atomic instructions???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Lockless grace-period computation! Such a tantalizing possibility!
</font>
<p><font color="ffffff">But consider the following sequence of events:
</font>
<ol>
<li> <font color="ffffff">CPU 0 has been in dyntick-idle
mode for quite some time.
When it wakes up, it notices that the current RCU
grace period needs it to report in, so it sets a
flag where the scheduling clock interrupt will find it.
</font><p>
<li> <font color="ffffff">Meanwhile, CPU 1 is running
<tt>force_quiescent_state()</tt>,
and notices that CPU 0 has been in dyntick idle mode,
which qualifies as an extended quiescent state.
</font><p>
<li> <font color="ffffff">CPU 0's scheduling clock
interrupt fires in the
middle of an RCU read-side critical section, and notices
that the RCU core needs something, so commences RCU softirq
processing.
</font>
<p>
<li> <font color="ffffff">CPU 0's softirq handler
executes and is just about ready
to report its quiescent state up the <tt>rcu_node</tt>
tree.
</font><p>
<li> <font color="ffffff">But CPU 1 beats it to the punch,
completing the current
grace period and starting a new one.
</font><p>
<li> <font color="ffffff">CPU 0 now reports its quiescent
state for the wrong
grace period.
That grace period might now end before the RCU read-side
critical section.
If that happens, disaster will ensue.
</font>
</ol>
<p><font color="ffffff">So the locking is absolutely required in
order to coordinate clearing of the bits with updating of the
grace-period sequence number in <tt>->gp_seq</tt>.
</font></td></tr>
<tr><td> </td></tr>
</table>
<h5>Blocked-Task Management</h5>
<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
midst of their RCU read-side critical sections, and these tasks
must be tracked explicitly.
The details of exactly why and how they are tracked will be covered
in a separate article on RCU read-side processing.
For now, it is enough to know that the <tt>rcu_node</tt>
structure tracks them.
<pre>
1 struct list_head blkd_tasks;
2 struct list_head *gp_tasks;
3 struct list_head *exp_tasks;
4 bool wait_blkd_tasks;
</pre>
<p>The <tt>->blkd_tasks</tt> field is a list header for
the list of blocked and preempted tasks.
As tasks undergo context switches within RCU read-side critical
sections, their <tt>task_struct</tt> structures are enqueued
(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt>
field) onto the head of the <tt>->blkd_tasks</tt> list for the
leaf <tt>rcu_node</tt> structure corresponding to the CPU
on which the outgoing context switch executed.
As these tasks later exit their RCU read-side critical sections,
they remove themselves from the list.
This list is therefore in reverse time order, so that if one of the tasks
is blocking the current grace period, all subsequent tasks must
also be blocking that same grace period.
Therefore, a single pointer into this list suffices to track
all tasks blocking a given grace period.
That pointer is stored in <tt>->gp_tasks</tt> for normal
grace periods and in <tt>->exp_tasks</tt> for expedited
grace periods.
These last two fields are <tt>NULL</tt> if either there is
no grace period in flight or if there are no blocked tasks
preventing that grace period from completing.
If either of these two pointers is referencing a task that
removes itself from the <tt>->blkd_tasks</tt> list,
then that task must advance the pointer to the next task on
the list, or set the pointer to <tt>NULL</tt> if there
are no subsequent tasks on the list.
</p><p>For example, suppose that tasks T1, T2, and T3 are
all hard-affinitied to the largest-numbered CPU in the system.
Then if task T1 blocked in an RCU read-side
critical section, then an expedited grace period started,
then task T2 blocked in an RCU read-side critical section,
then a normal grace period started, and finally task 3 blocked
in an RCU read-side critical section, then the state of the
last leaf <tt>rcu_node</tt> structure's blocked-task list
would be as shown below:
</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
</p><p>Task T1 is blocking both grace periods, task T2 is
blocking only the normal grace period, and task T3 is blocking
neither grace period.
Note that these tasks will not remove themselves from this list
immediately upon resuming execution.
They will instead remain on the list until they execute the outermost
<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
section.
<p>
The <tt>->wait_blkd_tasks</tt> field indicates whether or not
the current grace period is waiting on a blocked task.
<h5>Sizing the <tt>rcu_node</tt> Array</h5>
<p>The <tt>rcu_node</tt> array is sized via a series of
C-preprocessor expressions as follows:
<pre>
1 #ifdef CONFIG_RCU_FANOUT
2 #define RCU_FANOUT CONFIG_RCU_FANOUT
3 #else
4 # ifdef CONFIG_64BIT
5 # define RCU_FANOUT 64
6 # else
7 # define RCU_FANOUT 32
8 # endif
9 #endif
10
11 #ifdef CONFIG_RCU_FANOUT_LEAF
12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
13 #else
14 # ifdef CONFIG_64BIT
15 # define RCU_FANOUT_LEAF 64
16 # else
17 # define RCU_FANOUT_LEAF 32
18 # endif
19 #endif
20
21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
25
26 #if NR_CPUS <= RCU_FANOUT_1
27 # define RCU_NUM_LVLS 1
28 # define NUM_RCU_LVL_0 1
29 # define NUM_RCU_NODES NUM_RCU_LVL_0
30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
34 #elif NR_CPUS <= RCU_FANOUT_2
35 # define RCU_NUM_LVLS 2
36 # define NUM_RCU_LVL_0 1
37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
43 #elif NR_CPUS <= RCU_FANOUT_3
44 # define RCU_NUM_LVLS 3
45 # define NUM_RCU_LVL_0 1
46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
53 #elif NR_CPUS <= RCU_FANOUT_4
54 # define RCU_NUM_LVLS 4
55 # define NUM_RCU_LVL_0 1
56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
64 #else
65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
66 #endif
</pre>
<p>The maximum number of levels in the <tt>rcu_node</tt> structure
is currently limited to four, as specified by lines 21-24
and the structure of the subsequent “if” statement.
For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
should be sufficient for the next few years at least.
For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
should see us through the next decade or so.
This four-level tree also allows kernels built with
<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
which might be useful in very large systems having eight CPUs per
socket (but please note that no one has yet shown any measurable
performance degradation due to misaligned socket and <tt>rcu_node</tt>
boundaries).
In addition, building kernels with a full four levels of <tt>rcu_node</tt>
tree permits better testing of RCU's combining-tree code.
</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
it is set based on the word size of the system, which is also
the Kconfig default.
</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
handled by each leaf <tt>rcu_node</tt> structure.
Experience has shown that allowing a given leaf <tt>rcu_node</tt>
structure to handle 64 CPUs, as permitted by the number of bits in
the <tt>->qsmask</tt> field on a 64-bit system, results in
excessive contention for the leaf <tt>rcu_node</tt> structures'
<tt>->lock</tt> fields.
The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
selected is based on the word size of the system, just as for
<tt>CONFIG_RCU_FANOUT</tt>.
Lines 11-19 perform this computation.
</p><p>Lines 21-24 compute the maximum number of CPUs supported by
a single-level (which contains a single <tt>rcu_node</tt> structure),
two-level, three-level, and four-level <tt>rcu_node</tt> tree,
respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
and <tt>RCU_FANOUT_LEAF</tt>.
These numbers of CPUs are retained in the
<tt>RCU_FANOUT_1</tt>,
<tt>RCU_FANOUT_2</tt>,
<tt>RCU_FANOUT_3</tt>, and
<tt>RCU_FANOUT_4</tt>
C-preprocessor variables, respectively.
</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
statement spanning lines 26-66 that computes the number of
<tt>rcu_node</tt> structures required for each level of the tree,
as well as the number of levels required.
The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
C-preprocessor variable by lines 27, 35, 44, and 54.
The number of <tt>rcu_node</tt> structures for the topmost level
of the tree is always exactly one, and this value is unconditionally
placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55.
The rest of the levels (if any) of the <tt>rcu_node</tt> tree
are computed by dividing the maximum number of CPUs by the
fanout supported by the number of levels from the current level down,
rounding up. This computation is performed by lines 37,
46-47, and 56-58.
Lines 31-33, 40-42, 50-52, and 62-63 create initializers
for lockdep lock-class names.
Finally, lines 64-66 produce an error if the maximum number of
CPUs is too large for the specified fanout.
<h3><a name="The rcu_segcblist Structure">
The <tt>rcu_segcblist</tt> Structure</a></h3>
The <tt>rcu_segcblist</tt> structure maintains a segmented list of
callbacks as follows:
<pre>
1 #define RCU_DONE_TAIL 0
2 #define RCU_WAIT_TAIL 1
3 #define RCU_NEXT_READY_TAIL 2
4 #define RCU_NEXT_TAIL 3
5 #define RCU_CBLIST_NSEGS 4
6
7 struct rcu_segcblist {
8 struct rcu_head *head;
9 struct rcu_head **tails[RCU_CBLIST_NSEGS];
10 unsigned long gp_seq[RCU_CBLIST_NSEGS];
11 long len;
12 long len_lazy;
13 };
</pre>
<p>
The segments are as follows:
<ol>
<li> <tt>RCU_DONE_TAIL</tt>: Callbacks whose grace periods have elapsed.
These callbacks are ready to be invoked.
<li> <tt>RCU_WAIT_TAIL</tt>: Callbacks that are waiting for the
current grace period.
Note that different CPUs can have different ideas about which
grace period is current, hence the <tt>->gp_seq</tt> field.
<li> <tt>RCU_NEXT_READY_TAIL</tt>: Callbacks waiting for the next
grace period to start.
<li> <tt>RCU_NEXT_TAIL</tt>: Callbacks that have not yet been
associated with a grace period.
</ol>
<p>
The <tt>->head</tt> pointer references the first callback or
is <tt>NULL</tt> if the list contains no callbacks (which is
<i>not</i> the same as being empty).
Each element of the <tt>->tails[]</tt> array references the
<tt>->next</tt> pointer of the last callback in the corresponding
segment of the list, or the list's <tt>->head</tt> pointer if
that segment and all previous segments are empty.
If the corresponding segment is empty but some previous segment is
not empty, then the array element is identical to its predecessor.
Older callbacks are closer to the head of the list, and new callbacks
are added at the tail.
This relationship between the <tt>->head</tt> pointer, the
<tt>->tails[]</tt> array, and the callbacks is shown in this
diagram:
</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
</p><p>In this figure, the <tt>->head</tt> pointer references the
first
RCU callback in the list.
The <tt>->tails[RCU_DONE_TAIL]</tt> array element references
the <tt>->head</tt> pointer itself, indicating that none
of the callbacks is ready to invoke.
The <tt>->tails[RCU_WAIT_TAIL]</tt> array element references callback
CB 2's <tt>->next</tt> pointer, which indicates that
CB 1 and CB 2 are both waiting on the current grace period,
give or take possible disagreements about exactly which grace period
is the current one.
The <tt>->tails[RCU_NEXT_READY_TAIL]</tt> array element
references the same RCU callback that <tt>->tails[RCU_WAIT_TAIL]</tt>
does, which indicates that there are no callbacks waiting on the next
RCU grace period.
The <tt>->tails[RCU_NEXT_TAIL]</tt> array element references
CB 4's <tt>->next</tt> pointer, indicating that all the
remaining RCU callbacks have not yet been assigned to an RCU grace
period.
Note that the <tt>->tails[RCU_NEXT_TAIL]</tt> array element
always references the last RCU callback's <tt>->next</tt> pointer
unless the callback list is empty, in which case it references
the <tt>->head</tt> pointer.
<p>
There is one additional important special case for the
<tt>->tails[RCU_NEXT_TAIL]</tt> array element: It can be <tt>NULL</tt>
when this list is <i>disabled</i>.
Lists are disabled when the corresponding CPU is offline or when
the corresponding CPU's callbacks are offloaded to a kthread,
both of which are described elsewhere.
</p><p>CPUs advance their callbacks from the
<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
as grace periods advance.
</p><p>The <tt>->gp_seq[]</tt> array records grace-period
numbers corresponding to the list segments.
This is what allows different CPUs to have different ideas as to
which is the current grace period while still avoiding premature
invocation of their callbacks.
In particular, this allows CPUs that go idle for extended periods
to determine which of their callbacks are ready to be invoked after
reawakening.
</p><p>The <tt>->len</tt> counter contains the number of
callbacks in <tt>->head</tt>, and the
<tt>->len_lazy</tt> contains the number of those callbacks that
are known to only free memory, and whose invocation can therefore
be safely deferred.
<p><b>Important note</b>: It is the <tt>->len</tt> field that
determines whether or not there are callbacks associated with
this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>->head</tt>
pointer.
The reason for this is that all the ready-to-invoke callbacks
(that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
all at once at callback-invocation time (<tt>rcu_do_batch</tt>), due
to which <tt>->head</tt> may be set to NULL if there are no not-done
callbacks remaining in the <tt>rcu_segcblist</tt>.
If callback invocation must be postponed, for example, because a
high-priority process just woke up on this CPU, then the remaining
callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment and
<tt>->head</tt> once again points to the start of the segment.
In short, the head field can briefly be <tt>NULL</tt> even though the
CPU has callbacks present the entire time.
Therefore, it is not appropriate to test the <tt>->head</tt> pointer
for <tt>NULL</tt>.
<p>In contrast, the <tt>->len</tt> and <tt>->len_lazy</tt> counts
are adjusted only after the corresponding callbacks have been invoked.
This means that the <tt>->len</tt> count is zero only if
the <tt>rcu_segcblist</tt> structure really is devoid of callbacks.
Of course, off-CPU sampling of the <tt>->len</tt> count requires
careful use of appropriate synchronization, for example, memory barriers.
This synchronization can be a bit subtle, particularly in the case
of <tt>rcu_barrier()</tt>.
<h3><a name="The rcu_data Structure">
The <tt>rcu_data</tt> Structure</a></h3>
<p>The <tt>rcu_data</tt> maintains the per-CPU state for the RCU subsystem.
The fields in this structure may be accessed only from the corresponding
CPU (and from tracing) unless otherwise stated.
This structure is the
focus of quiescent-state detection and RCU callback queuing.
It also tracks its relationship to the corresponding leaf
<tt>rcu_node</tt> structure to allow more-efficient
propagation of quiescent states up the <tt>rcu_node</tt>
combining tree.
Like the <tt>rcu_node</tt> structure, it provides a local
copy of the grace-period information to allow for-free
synchronized
access to this information from the corresponding CPU.
Finally, this structure records past dyntick-idle state
for the corresponding CPU and also tracks statistics.
</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
singly and in groups, in the following sections.
<h5>Connection to Other Data Structures</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 int cpu;
2 struct rcu_node *mynode;
3 unsigned long grpmask;
4 bool beenonline;
</pre>
<p>The <tt>->cpu</tt> field contains the number of the
corresponding CPU and the <tt>->mynode</tt> field references the
corresponding <tt>rcu_node</tt> structure.
The <tt>->mynode</tt> is used to propagate quiescent states
up the combining tree.
These two fields are constant and therefore do not require synchronization.
<p>The <tt>->grpmask</tt> field indicates the bit in
the <tt>->mynode->qsmask</tt> corresponding to this
<tt>rcu_data</tt> structure, and is also used when propagating
quiescent states.
The <tt>->beenonline</tt> flag is set whenever the corresponding
CPU comes online, which means that the debugfs tracing need not dump
out any <tt>rcu_data</tt> structure for which this flag is not set.
<h5>Quiescent-State and Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 unsigned long gp_seq;
2 unsigned long gp_seq_needed;
3 bool cpu_no_qs;
4 bool core_needs_qs;
5 bool gpwrap;
</pre>
<p>The <tt>->gp_seq</tt> field is the counterpart of the field of the same
name in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures. The
<tt>->gp_seq_needed</tt> field is the counterpart of the field of the same
name in the rcu_node</tt> structure.
They may each lag up to one behind their <tt>rcu_node</tt>
counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
will catch up upon exit from dyntick-idle mode).
If the lower two bits of a given <tt>rcu_data</tt> structure's
<tt>->gp_seq</tt> are zero, then this <tt>rcu_data</tt>
structure believes that RCU is idle.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
All this replication of the grace period numbers can only cause
massive confusion.
Why not just keep a global sequence number and be done with it???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because if there was only a single global sequence
numbers, there would need to be a single global lock to allow
safely accessing and updating it.
And if we are not going to have a single global lock, we need
to carefully manage the numbers on a per-node basis.
Recall from the answer to a previous Quick Quiz that the consequences
of applying a previously sampled quiescent state to the wrong
grace period are quite severe.
</font></td></tr>
<tr><td> </td></tr>
</table>
<p>The <tt>->cpu_no_qs</tt> flag indicates that the
CPU has not yet passed through a quiescent state,
while the <tt>->core_needs_qs</tt> flag indicates that the
RCU core needs a quiescent state from the corresponding CPU.
The <tt>->gpwrap</tt> field indicates that the corresponding
CPU has remained idle for so long that the
<tt>gp_seq</tt> counter is in danger of overflow, which
will cause the CPU to disregard the values of its counters on
its next exit from idle.
<h5>RCU Callback Handling</h5>
<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
the same CPU that registered them.
This is strictly a cache-locality optimization: callbacks can and
do get invoked on CPUs other than the one that registered them.
After all, if the CPU that registered a given callback has gone
offline before the callback can be invoked, there really is no other
choice.
</p><p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 struct rcu_segcblist cblist;
2 long qlen_last_fqs_check;
3 unsigned long n_cbs_invoked;
4 unsigned long n_nocbs_invoked;
5 unsigned long n_cbs_orphaned;
6 unsigned long n_cbs_adopted;
7 unsigned long n_force_qs_snap;
8 long blimit;
</pre>
<p>The <tt>->cblist</tt> structure is the segmented callback list
described earlier.
The CPU advances the callbacks in its <tt>rcu_data</tt> structure
whenever it notices that another RCU grace period has completed.
The CPU detects the completion of an RCU grace period by noticing
that the value of its <tt>rcu_data</tt> structure's
<tt>->gp_seq</tt> field differs from that of its leaf
<tt>rcu_node</tt> structure.
Recall that each <tt>rcu_node</tt> structure's
<tt>->gp_seq</tt> field is updated at the beginnings and ends of each
grace period.
<p>
The <tt>->qlen_last_fqs_check</tt> and
<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent
states from <tt>call_rcu()</tt> and friends when callback
lists grow excessively long.
</p><p>The <tt>->n_cbs_invoked</tt>,
<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt>
fields count the number of callbacks invoked,
sent to other CPUs when this CPU goes offline,
and received from other CPUs when those other CPUs go offline.
The <tt>->n_nocbs_invoked</tt> is used when the CPU's callbacks
are offloaded to a kthread.
<p>
Finally, the <tt>->blimit</tt> counter is the maximum number of
RCU callbacks that may be invoked at a given time.
<h5>Dyntick-Idle Handling</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 int dynticks_snap;
2 unsigned long dynticks_fqs;
</pre>
The <tt>->dynticks_snap</tt> field is used to take a snapshot
of the corresponding CPU's dyntick-idle state when forcing
quiescent states, and is therefore accessed from other CPUs.
Finally, the <tt>->dynticks_fqs</tt> field is used to
count the number of times this CPU is determined to be in
dyntick-idle state, and is used for tracing and debugging purposes.
<p>
This portion of the rcu_data structure is declared as follows:
<pre>
1 long dynticks_nesting;
2 long dynticks_nmi_nesting;
3 atomic_t dynticks;
4 bool rcu_need_heavy_qs;
5 bool rcu_urgent_qs;
</pre>
<p>These fields in the rcu_data structure maintain the per-CPU dyntick-idle
state for the corresponding CPU.
The fields may be accessed only from the corresponding CPU (and from tracing)
unless otherwise stated.
<p>The <tt>->dynticks_nesting</tt> field counts the
nesting depth of process execution, so that in normal circumstances
this counter has value zero or one.
NMIs, irqs, and tracers are counted by the <tt>->dynticks_nmi_nesting</tt>
field.
Because NMIs cannot be masked, changes to this variable have to be
undertaken carefully using an algorithm provided by Andy Lutomirski.
The initial transition from idle adds one, and nested transitions
add two, so that a nesting level of five is represented by a
<tt>->dynticks_nmi_nesting</tt> value of nine.
This counter can therefore be thought of as counting the number
of reasons why this CPU cannot be permitted to enter dyntick-idle
mode, aside from process-level transitions.
<p>However, it turns out that when running in non-idle kernel context,
the Linux kernel is fully capable of entering interrupt handlers that
never exit and perhaps also vice versa.
Therefore, whenever the <tt>->dynticks_nesting</tt> field is
incremented up from zero, the <tt>->dynticks_nmi_nesting</tt> field
is set to a large positive number, and whenever the
<tt>->dynticks_nesting</tt> field is decremented down to zero,
the the <tt>->dynticks_nmi_nesting</tt> field is set to zero.
Assuming that the number of misnested interrupts is not sufficient
to overflow the counter, this approach corrects the
<tt>->dynticks_nmi_nesting</tt> field every time the corresponding
CPU enters the idle loop from process context.
</p><p>The <tt>->dynticks</tt> field counts the corresponding
CPU's transitions to and from either dyntick-idle or user mode, so
that this counter has an even value when the CPU is in dyntick-idle
mode or user mode and an odd value otherwise. The transitions to/from
user mode need to be counted for user mode adaptive-ticks support
(see timers/NO_HZ.txt).
</p><p>The <tt>->rcu_need_heavy_qs</tt> field is used
to record the fact that the RCU core code would really like to
see a quiescent state from the corresponding CPU, so much so that
it is willing to call for heavy-weight dyntick-counter operations.
This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
code, which provide a momentary idle sojourn in response.
</p><p>Finally, the <tt>->rcu_urgent_qs</tt> field is used to record
the fact that the RCU core code would really like to see a quiescent state from
the corresponding CPU, with the various other fields indicating just how badly
RCU wants this quiescent state.
This flag is checked by RCU's context-switch path
(<tt>rcu_note_context_switch</tt>) and the cond_resched code.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why not simply combine the <tt>->dynticks_nesting</tt>
and <tt>->dynticks_nmi_nesting</tt> counters into a
single counter that just counts the number of reasons that
the corresponding CPU is non-idle?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because this would fail in the presence of interrupts whose
handlers never return and of handlers that manage to return
from a made-up interrupt.
</font></td></tr>
<tr><td> </td></tr>
</table>
<p>Additional fields are present for some special-purpose
builds, and are discussed separately.
<h3><a name="The rcu_head Structure">
The <tt>rcu_head</tt> Structure</a></h3>
<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
These structures are normally embedded within RCU-protected data
structures whose algorithms use asynchronous grace periods.
In contrast, when using algorithms that block waiting for RCU grace periods,
RCU users need not provide <tt>rcu_head</tt> structures.
</p><p>The <tt>rcu_head</tt> structure has fields as follows:
<pre>
1 struct rcu_head *next;
2 void (*func)(struct rcu_head *head);
</pre>
<p>The <tt>->next</tt> field is used
to link the <tt>rcu_head</tt> structures together in the
lists within the <tt>rcu_data</tt> structures.
The <tt>->func</tt> field is a pointer to the function
to be called when the callback is ready to be invoked, and
this function is passed a pointer to the <tt>rcu_head</tt>
structure.
However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt>
field to record the offset of the <tt>rcu_head</tt>
structure within the enclosing RCU-protected data structure.
</p><p>Both of these fields are used internally by RCU.
From the viewpoint of RCU users, this structure is an
opaque “cookie”.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Given that the callback function <tt>->func</tt>
is passed a pointer to the <tt>rcu_head</tt> structure,
how is that function supposed to find the beginning of the
enclosing RCU-protected data structure?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In actual practice, there is a separate callback function per
type of RCU-protected data structure.
The callback function can therefore use the <tt>container_of()</tt>
macro in the Linux kernel (or other pointer-manipulation facilities
in other software environments) to find the beginning of the
enclosing structure.
</font></td></tr>
<tr><td> </td></tr>
</table>
<h3><a name="RCU-Specific Fields in the task_struct Structure">
RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
additional fields in the <tt>task_struct</tt> structure:
<pre>
1 #ifdef CONFIG_PREEMPT_RCU
2 int rcu_read_lock_nesting;
3 union rcu_special rcu_read_unlock_special;
4 struct list_head rcu_node_entry;
5 struct rcu_node *rcu_blocked_node;
6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
7 #ifdef CONFIG_TASKS_RCU
8 unsigned long rcu_tasks_nvcsw;
9 bool rcu_tasks_holdout;
10 struct list_head rcu_tasks_holdout_list;
11 int rcu_tasks_idle_cpu;
12 #endif /* #ifdef CONFIG_TASKS_RCU */
</pre>
<p>The <tt>->rcu_read_lock_nesting</tt> field records the
nesting level for RCU read-side critical sections, and
the <tt>->rcu_read_unlock_special</tt> field is a bitmask
that records special conditions that require <tt>rcu_read_unlock()</tt>
to do additional work.
The <tt>->rcu_node_entry</tt> field is used to form lists of
tasks that have blocked within preemptible-RCU read-side critical
sections and the <tt>->rcu_blocked_node</tt> field references
the <tt>rcu_node</tt> structure whose list this task is a member of,
or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
read-side critical section.
<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of
voluntary context switches that this task had undergone at the
beginning of the current tasks-RCU grace period,
<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU
grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt>
is a list element enqueuing this task on the holdout list,
and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this
idle task is running, but only if the task is currently running,
that is, if the CPU is currently idle.
<h3><a name="Accessor Functions">
Accessor Functions</a></h3>
<p>The following listing shows the
<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt> and
<tt>rcu_for_each_leaf_node()</tt> function and macros:
<pre>
1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
2 {
3 return &rsp->node[0];
4 }
5
6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
7 for ((rnp) = &(rsp)->node[0]; \
8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
9
10 #define rcu_for_each_leaf_node(rsp, rnp) \
11 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
12 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
</pre>
<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
first element of the specified <tt>rcu_state</tt> structure's
<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt>
structure.
</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
macro takes advantage of the layout of the <tt>rcu_node</tt>
structures in the <tt>rcu_state</tt> structure's
<tt>->node[]</tt> array, performing a breadth-first traversal by
simply traversing the array in order.
Similarly, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
the last part of the array, thus traversing only the leaf
<tt>rcu_node</tt> structures.
<table>
<tr><th> </th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
What does
<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
contains only a single node?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In the single-node case,
<tt>rcu_for_each_leaf_node()</tt> traverses the single node.
</font></td></tr>
<tr><td> </td></tr>
</table>
<h3><a name="Summary">
Summary</a></h3>
So the state of RCU is represented by an <tt>rcu_state</tt> structure,
which contains a combining tree of <tt>rcu_node</tt> and
<tt>rcu_data</tt> structures.
Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
state is tracked by dynticks-related fields in the <tt>rcu_data</tt> structure.
If you made it this far, you are well prepared to read the code
walkthroughs in the other articles in this series.
<h3><a name="Acknowledgments">
Acknowledgments</a></h3>
I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
for helping me get this document into a more human-readable state.
<h3><a name="Legal Statement">
Legal Statement</a></h3>
<p>This work represents the view of the author and does not necessarily
represent the view of IBM.
</p><p>Linux is a registered trademark of Linus Torvalds.
</p><p>Other company, product, and service names may be trademarks or
service marks of others.
</body></html>
|
