1=================================================== 2A Tour Through TREE_RCU's Data Structures [LWN.net] 3=================================================== 4 5December 18, 2016 6 7This article was contributed by Paul E. McKenney 8 9Introduction 10============ 11 12This document describes RCU's major data structures and their relationship 13to each other. 14 15Data-Structure Relationships 16============================ 17 18RCU is for all intents and purposes a large state machine, and its 19data structures maintain the state in such a way as to allow RCU readers 20to execute extremely quickly, while also processing the RCU grace periods 21requested by updaters in an efficient and extremely scalable fashion. 22The efficiency and scalability of RCU updaters is provided primarily 23by a combining tree, as shown below: 24 25.. kernel-figure:: BigTreeClassicRCU.svg 26 27This diagram shows an enclosing ``rcu_state`` structure containing a tree 28of ``rcu_node`` structures. Each leaf node of the ``rcu_node`` tree has up 29to 16 ``rcu_data`` structures associated with it, so that there are 30``NR_CPUS`` number of ``rcu_data`` structures, one for each possible CPU. 31This structure is adjusted at boot time, if needed, to handle the common 32case where ``nr_cpu_ids`` is much less than ``NR_CPUs``. 33For example, a number of Linux distributions set ``NR_CPUs=4096``, 34which results in a three-level ``rcu_node`` tree. 35If the actual hardware has only 16 CPUs, RCU will adjust itself 36at boot time, resulting in an ``rcu_node`` tree with only a single node. 37 38The purpose of this combining tree is to allow per-CPU events 39such as quiescent states, dyntick-idle transitions, 40and CPU hotplug operations to be processed efficiently 41and scalably. 42Quiescent states are recorded by the per-CPU ``rcu_data`` structures, 43and other events are recorded by the leaf-level ``rcu_node`` 44structures. 45All of these events are combined at each level of the tree until finally 46grace periods are completed at the tree's root ``rcu_node`` 47structure. 48A grace period can be completed at the root once every CPU 49(or, in the case of ``CONFIG_PREEMPT_RCU``, task) 50has passed through a quiescent state. 51Once a grace period has completed, record of that fact is propagated 52back down the tree. 53 54As can be seen from the diagram, on a 64-bit system 55a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout 56of 64 at the root and a fanout of 16 at the leaves. 57 58+-----------------------------------------------------------------------+ 59| **Quick Quiz**: | 60+-----------------------------------------------------------------------+ 61| Why isn't the fanout at the leaves also 64? | 62+-----------------------------------------------------------------------+ 63| **Answer**: | 64+-----------------------------------------------------------------------+ 65| Because there are more types of events that affect the leaf-level | 66| ``rcu_node`` structures than further up the tree. Therefore, if the | 67| leaf ``rcu_node`` structures have fanout of 64, the contention on | 68| these structures' ``->structures`` becomes excessive. Experimentation | 69| on a wide variety of systems has shown that a fanout of 16 works well | 70| for the leaves of the ``rcu_node`` tree. | 71| | 72| Of course, further experience with systems having hundreds or | 73| thousands of CPUs may demonstrate that the fanout for the non-leaf | 74| ``rcu_node`` structures must also be reduced. Such reduction can be | 75| easily carried out when and if it proves necessary. In the meantime, | 76| if you are using such a system and running into contention problems | 77| on the non-leaf ``rcu_node`` structures, you may use the | 78| ``CONFIG_RCU_FANOUT`` kernel configuration parameter to reduce the | 79| non-leaf fanout as needed. | 80| | 81| Kernels built for systems with strong NUMA characteristics might | 82| also need to adjust ``CONFIG_RCU_FANOUT`` so that the domains of | 83| the ``rcu_node`` structures align with hardware boundaries. | 84| However, there has thus far been no need for this. | 85+-----------------------------------------------------------------------+ 86 87If your system has more than 1,024 CPUs (or more than 512 CPUs on a 8832-bit system), then RCU will automatically add more levels to the tree. 89For example, if you are crazy enough to build a 64-bit system with 9065,536 CPUs, RCU would configure the ``rcu_node`` tree as follows: 91 92.. kernel-figure:: HugeTreeClassicRCU.svg 93 94RCU currently permits up to a four-level tree, which on a 64-bit system 95accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for 9632-bit systems. On the other hand, you can set both 97``CONFIG_RCU_FANOUT`` and ``CONFIG_RCU_FANOUT_LEAF`` to be as small as 982, which would result in a 16-CPU test using a 4-level tree. This can be 99useful for testing large-system capabilities on small test machines. 100 101This multi-level combining tree allows us to get most of the performance 102and scalability benefits of partitioning, even though RCU grace-period 103detection is inherently a global operation. The trick here is that only 104the last CPU to report a quiescent state into a given ``rcu_node`` 105structure need advance to the ``rcu_node`` structure at the next level 106up the tree. This means that at the leaf-level ``rcu_node`` structure, 107only one access out of sixteen will progress up the tree. For the 108internal ``rcu_node`` structures, the situation is even more extreme: 109Only one access out of sixty-four will progress up the tree. Because the 110vast majority of the CPUs do not progress up the tree, the lock 111contention remains roughly constant up the tree. No matter how many CPUs 112there are in the system, at most 64 quiescent-state reports per grace 113period will progress all the way to the root ``rcu_node`` structure, 114thus ensuring that the lock contention on that root ``rcu_node`` 115structure remains acceptably low. 116 117In effect, the combining tree acts like a big shock absorber, keeping 118lock contention under control at all tree levels regardless of the level 119of loading on the system. 120 121RCU updaters wait for normal grace periods by registering RCU callbacks, 122either directly via ``call_rcu()`` or indirectly via 123``synchronize_rcu()`` and friends. RCU callbacks are represented by 124``rcu_head`` structures, which are queued on ``rcu_data`` structures 125while they are waiting for a grace period to elapse, as shown in the 126following figure: 127 128.. kernel-figure:: BigTreePreemptRCUBHdyntickCB.svg 129 130This figure shows how ``TREE_RCU``'s and ``PREEMPT_RCU``'s major data 131structures are related. Lesser data structures will be introduced with 132the algorithms that make use of them. 133 134Note that each of the data structures in the above figure has its own 135synchronization: 136 137#. Each ``rcu_state`` structures has a lock and a mutex, and some fields 138 are protected by the corresponding root ``rcu_node`` structure's lock. 139#. Each ``rcu_node`` structure has a spinlock. 140#. The fields in ``rcu_data`` are private to the corresponding CPU, 141 although a few can be read and written by other CPUs. 142 143It is important to note that different data structures can have very 144different ideas about the state of RCU at any given time. For but one 145example, awareness of the start or end of a given RCU grace period 146propagates slowly through the data structures. This slow propagation is 147absolutely necessary for RCU to have good read-side performance. If this 148balkanized implementation seems foreign to you, one useful trick is to 149consider each instance of these data structures to be a different 150person, each having the usual slightly different view of reality. 151 152The general role of each of these data structures is as follows: 153 154#. ``rcu_state``: This structure forms the interconnection between the 155 ``rcu_node`` and ``rcu_data`` structures, tracks grace periods, 156 serves as short-term repository for callbacks orphaned by CPU-hotplug 157 events, maintains ``rcu_barrier()`` state, tracks expedited 158 grace-period state, and maintains state used to force quiescent 159 states when grace periods extend too long, 160#. ``rcu_node``: This structure forms the combining tree that propagates 161 quiescent-state information from the leaves to the root, and also 162 propagates grace-period information from the root to the leaves. It 163 provides local copies of the grace-period state in order to allow 164 this information to be accessed in a synchronized manner without 165 suffering the scalability limitations that would otherwise be imposed 166 by global locking. In ``CONFIG_PREEMPT_RCU`` kernels, it manages the 167 lists of tasks that have blocked while in their current RCU read-side 168 critical section. In ``CONFIG_PREEMPT_RCU`` with 169 ``CONFIG_RCU_BOOST``, it manages the per-\ ``rcu_node`` 170 priority-boosting kernel threads (kthreads) and state. Finally, it 171 records CPU-hotplug state in order to determine which CPUs should be 172 ignored during a given grace period. 173#. ``rcu_data``: This per-CPU structure is the focus of quiescent-state 174 detection and RCU callback queuing. It also tracks its relationship 175 to the corresponding leaf ``rcu_node`` structure to allow 176 more-efficient propagation of quiescent states up the ``rcu_node`` 177 combining tree. Like the ``rcu_node`` structure, it provides a local 178 copy of the grace-period information to allow for-free synchronized 179 access to this information from the corresponding CPU. Finally, this 180 structure records past dyntick-idle state for the corresponding CPU 181 and also tracks statistics. 182#. ``rcu_head``: This structure represents RCU callbacks, and is the 183 only structure allocated and managed by RCU users. The ``rcu_head`` 184 structure is normally embedded within the RCU-protected data 185 structure. 186 187If all you wanted from this article was a general notion of how RCU's 188data structures are related, you are done. Otherwise, each of the 189following sections give more details on the ``rcu_state``, ``rcu_node`` 190and ``rcu_data`` data structures. 191 192The ``rcu_state`` Structure 193~~~~~~~~~~~~~~~~~~~~~~~~~~~ 194 195The ``rcu_state`` structure is the base structure that represents the 196state of RCU in the system. This structure forms the interconnection 197between the ``rcu_node`` and ``rcu_data`` structures, tracks grace 198periods, contains the lock used to synchronize with CPU-hotplug events, 199and maintains state used to force quiescent states when grace periods 200extend too long, 201 202A few of the ``rcu_state`` structure's fields are discussed, singly and 203in groups, in the following sections. The more specialized fields are 204covered in the discussion of their use. 205 206Relationship to rcu_node and rcu_data Structures 207'''''''''''''''''''''''''''''''''''''''''''''''' 208 209This portion of the ``rcu_state`` structure is declared as follows: 210 211:: 212 213 1 struct rcu_node node[NUM_RCU_NODES]; 214 2 struct rcu_node *level[NUM_RCU_LVLS + 1]; 215 3 struct rcu_data __percpu *rda; 216 217+-----------------------------------------------------------------------+ 218| **Quick Quiz**: | 219+-----------------------------------------------------------------------+ 220| Wait a minute! You said that the ``rcu_node`` structures formed a | 221| tree, but they are declared as a flat array! What gives? | 222+-----------------------------------------------------------------------+ 223| **Answer**: | 224+-----------------------------------------------------------------------+ 225| The tree is laid out in the array. The first node In the array is the | 226| head, the next set of nodes in the array are children of the head | 227| node, and so on until the last set of nodes in the array are the | 228| leaves. | 229| See the following diagrams to see how this works. | 230+-----------------------------------------------------------------------+ 231 232The ``rcu_node`` tree is embedded into the ``->node[]`` array as shown 233in the following figure: 234 235.. kernel-figure:: TreeMapping.svg 236 237One interesting consequence of this mapping is that a breadth-first 238traversal of the tree is implemented as a simple linear scan of the 239array, which is in fact what the ``rcu_for_each_node_breadth_first()`` 240macro does. This macro is used at the beginning and ends of grace 241periods. 242 243Each entry of the ``->level`` array references the first ``rcu_node`` 244structure on the corresponding level of the tree, for example, as shown 245below: 246 247.. kernel-figure:: TreeMappingLevel.svg 248 249The zero\ :sup:`th` element of the array references the root 250``rcu_node`` structure, the first element references the first child of 251the root ``rcu_node``, and finally the second element references the 252first leaf ``rcu_node`` structure. 253 254For whatever it is worth, if you draw the tree to be tree-shaped rather 255than array-shaped, it is easy to draw a planar representation: 256 257.. kernel-figure:: TreeLevel.svg 258 259Finally, the ``->rda`` field references a per-CPU pointer to the 260corresponding CPU's ``rcu_data`` structure. 261 262All of these fields are constant once initialization is complete, and 263therefore need no protection. 264 265Grace-Period Tracking 266''''''''''''''''''''' 267 268This portion of the ``rcu_state`` structure is declared as follows: 269 270:: 271 272 1 unsigned long gp_seq; 273 274RCU grace periods are numbered, and the ``->gp_seq`` field contains the 275current grace-period sequence number. The bottom two bits are the state 276of the current grace period, which can be zero for not yet started or 277one for in progress. In other words, if the bottom two bits of 278``->gp_seq`` are zero, then RCU is idle. Any other value in the bottom 279two bits indicates that something is broken. This field is protected by 280the root ``rcu_node`` structure's ``->lock`` field. 281 282There are ``->gp_seq`` fields in the ``rcu_node`` and ``rcu_data`` 283structures as well. The fields in the ``rcu_state`` structure represent 284the most current value, and those of the other structures are compared 285in order to detect the beginnings and ends of grace periods in a 286distributed fashion. The values flow from ``rcu_state`` to ``rcu_node`` 287(down the tree from the root to the leaves) to ``rcu_data``. 288 289Miscellaneous 290''''''''''''' 291 292This portion of the ``rcu_state`` structure is declared as follows: 293 294:: 295 296 1 unsigned long gp_max; 297 2 char abbr; 298 3 char *name; 299 300The ``->gp_max`` field tracks the duration of the longest grace period 301in jiffies. It is protected by the root ``rcu_node``'s ``->lock``. 302 303The ``->name`` and ``->abbr`` fields distinguish between preemptible RCU 304(“rcu_preempt” and “p”) and non-preemptible RCU (“rcu_sched” and “s”). 305These fields are used for diagnostic and tracing purposes. 306 307The ``rcu_node`` Structure 308~~~~~~~~~~~~~~~~~~~~~~~~~~ 309 310The ``rcu_node`` structures form the combining tree that propagates 311quiescent-state information from the leaves to the root and also that 312propagates grace-period information from the root down to the leaves. 313They provides local copies of the grace-period state in order to allow 314this information to be accessed in a synchronized manner without 315suffering the scalability limitations that would otherwise be imposed by 316global locking. In ``CONFIG_PREEMPT_RCU`` kernels, they manage the lists 317of tasks that have blocked while in their current RCU read-side critical 318section. In ``CONFIG_PREEMPT_RCU`` with ``CONFIG_RCU_BOOST``, they 319manage the per-\ ``rcu_node`` priority-boosting kernel threads 320(kthreads) and state. Finally, they record CPU-hotplug state in order to 321determine which CPUs should be ignored during a given grace period. 322 323The ``rcu_node`` structure's fields are discussed, singly and in groups, 324in the following sections. 325 326Connection to Combining Tree 327'''''''''''''''''''''''''''' 328 329This portion of the ``rcu_node`` structure is declared as follows: 330 331:: 332 333 1 struct rcu_node *parent; 334 2 u8 level; 335 3 u8 grpnum; 336 4 unsigned long grpmask; 337 5 int grplo; 338 6 int grphi; 339 340The ``->parent`` pointer references the ``rcu_node`` one level up in the 341tree, and is ``NULL`` for the root ``rcu_node``. The RCU implementation 342makes heavy use of this field to push quiescent states up the tree. The 343``->level`` field gives the level in the tree, with the root being at 344level zero, its children at level one, and so on. The ``->grpnum`` field 345gives this node's position within the children of its parent, so this 346number can range between 0 and 31 on 32-bit systems and between 0 and 63 347on 64-bit systems. The ``->level`` and ``->grpnum`` fields are used only 348during initialization and for tracing. The ``->grpmask`` field is the 349bitmask counterpart of ``->grpnum``, and therefore always has exactly 350one bit set. This mask is used to clear the bit corresponding to this 351``rcu_node`` structure in its parent's bitmasks, which are described 352later. Finally, the ``->grplo`` and ``->grphi`` fields contain the 353lowest and highest numbered CPU served by this ``rcu_node`` structure, 354respectively. 355 356All of these fields are constant, and thus do not require any 357synchronization. 358 359Synchronization 360''''''''''''''' 361 362This field of the ``rcu_node`` structure is declared as follows: 363 364:: 365 366 1 raw_spinlock_t lock; 367 368This field is used to protect the remaining fields in this structure, 369unless otherwise stated. That said, all of the fields in this structure 370can be accessed without locking for tracing purposes. Yes, this can 371result in confusing traces, but better some tracing confusion than to be 372heisenbugged out of existence. 373 374.. _grace-period-tracking-1: 375 376Grace-Period Tracking 377''''''''''''''''''''' 378 379This portion of the ``rcu_node`` structure is declared as follows: 380 381:: 382 383 1 unsigned long gp_seq; 384 2 unsigned long gp_seq_needed; 385 386The ``rcu_node`` structures' ``->gp_seq`` fields are the counterparts of 387the field of the same name in the ``rcu_state`` structure. They each may 388lag up to one step behind their ``rcu_state`` counterpart. If the bottom 389two bits of a given ``rcu_node`` structure's ``->gp_seq`` field is zero, 390then this ``rcu_node`` structure believes that RCU is idle. 391 392The ``>gp_seq`` field of each ``rcu_node`` structure is updated at the 393beginning and the end of each grace period. 394 395The ``->gp_seq_needed`` fields record the furthest-in-the-future grace 396period request seen by the corresponding ``rcu_node`` structure. The 397request is considered fulfilled when the value of the ``->gp_seq`` field 398equals or exceeds that of the ``->gp_seq_needed`` field. 399 400+-----------------------------------------------------------------------+ 401| **Quick Quiz**: | 402+-----------------------------------------------------------------------+ 403| Suppose that this ``rcu_node`` structure doesn't see a request for a | 404| very long time. Won't wrapping of the ``->gp_seq`` field cause | 405| problems? | 406+-----------------------------------------------------------------------+ 407| **Answer**: | 408+-----------------------------------------------------------------------+ 409| No, because if the ``->gp_seq_needed`` field lags behind the | 410| ``->gp_seq`` field, the ``->gp_seq_needed`` field will be updated at | 411| the end of the grace period. Modulo-arithmetic comparisons therefore | 412| will always get the correct answer, even with wrapping. | 413+-----------------------------------------------------------------------+ 414 415Quiescent-State Tracking 416'''''''''''''''''''''''' 417 418These fields manage the propagation of quiescent states up the combining 419tree. 420 421This portion of the ``rcu_node`` structure has fields as follows: 422 423:: 424 425 1 unsigned long qsmask; 426 2 unsigned long expmask; 427 3 unsigned long qsmaskinit; 428 4 unsigned long expmaskinit; 429 430The ``->qsmask`` field tracks which of this ``rcu_node`` structure's 431children still need to report quiescent states for the current normal 432grace period. Such children will have a value of 1 in their 433corresponding bit. Note that the leaf ``rcu_node`` structures should be 434thought of as having ``rcu_data`` structures as their children. 435Similarly, the ``->expmask`` field tracks which of this ``rcu_node`` 436structure's children still need to report quiescent states for the 437current expedited grace period. An expedited grace period has the same 438conceptual properties as a normal grace period, but the expedited 439implementation accepts extreme CPU overhead to obtain much lower 440grace-period latency, for example, consuming a few tens of microseconds 441worth of CPU time to reduce grace-period duration from milliseconds to 442tens of microseconds. The ``->qsmaskinit`` field tracks which of this 443``rcu_node`` structure's children cover for at least one online CPU. 444This mask is used to initialize ``->qsmask``, and ``->expmaskinit`` is 445used to initialize ``->expmask`` and the beginning of the normal and 446expedited grace periods, respectively. 447 448+-----------------------------------------------------------------------+ 449| **Quick Quiz**: | 450+-----------------------------------------------------------------------+ 451| Why are these bitmasks protected by locking? Come on, haven't you | 452| heard of atomic instructions??? | 453+-----------------------------------------------------------------------+ 454| **Answer**: | 455+-----------------------------------------------------------------------+ 456| Lockless grace-period computation! Such a tantalizing possibility! | 457| But consider the following sequence of events: | 458| | 459| #. CPU 0 has been in dyntick-idle mode for quite some time. When it | 460| wakes up, it notices that the current RCU grace period needs it to | 461| report in, so it sets a flag where the scheduling clock interrupt | 462| will find it. | 463| #. Meanwhile, CPU 1 is running ``force_quiescent_state()``, and | 464| notices that CPU 0 has been in dyntick idle mode, which qualifies | 465| as an extended quiescent state. | 466| #. CPU 0's scheduling clock interrupt fires in the middle of an RCU | 467| read-side critical section, and notices that the RCU core needs | 468| something, so commences RCU softirq processing. | 469| #. CPU 0's softirq handler executes and is just about ready to report | 470| its quiescent state up the ``rcu_node`` tree. | 471| #. But CPU 1 beats it to the punch, completing the current grace | 472| period and starting a new one. | 473| #. CPU 0 now reports its quiescent state for the wrong grace period. | 474| That grace period might now end before the RCU read-side critical | 475| section. If that happens, disaster will ensue. | 476| | 477| So the locking is absolutely required in order to coordinate clearing | 478| of the bits with updating of the grace-period sequence number in | 479| ``->gp_seq``. | 480+-----------------------------------------------------------------------+ 481 482Blocked-Task Management 483''''''''''''''''''''''' 484 485``PREEMPT_RCU`` allows tasks to be preempted in the midst of their RCU 486read-side critical sections, and these tasks must be tracked explicitly. 487The details of exactly why and how they are tracked will be covered in a 488separate article on RCU read-side processing. For now, it is enough to 489know that the ``rcu_node`` structure tracks them. 490 491:: 492 493 1 struct list_head blkd_tasks; 494 2 struct list_head *gp_tasks; 495 3 struct list_head *exp_tasks; 496 4 bool wait_blkd_tasks; 497 498The ``->blkd_tasks`` field is a list header for the list of blocked and 499preempted tasks. As tasks undergo context switches within RCU read-side 500critical sections, their ``task_struct`` structures are enqueued (via 501the ``task_struct``'s ``->rcu_node_entry`` field) onto the head of the 502``->blkd_tasks`` list for the leaf ``rcu_node`` structure corresponding 503to the CPU on which the outgoing context switch executed. As these tasks 504later exit their RCU read-side critical sections, they remove themselves 505from the list. This list is therefore in reverse time order, so that if 506one of the tasks is blocking the current grace period, all subsequent 507tasks must also be blocking that same grace period. Therefore, a single 508pointer into this list suffices to track all tasks blocking a given 509grace period. That pointer is stored in ``->gp_tasks`` for normal grace 510periods and in ``->exp_tasks`` for expedited grace periods. These last 511two fields are ``NULL`` if either there is no grace period in flight or 512if there are no blocked tasks preventing that grace period from 513completing. If either of these two pointers is referencing a task that 514removes itself from the ``->blkd_tasks`` list, then that task must 515advance the pointer to the next task on the list, or set the pointer to 516``NULL`` if there are no subsequent tasks on the list. 517 518For example, suppose that tasks T1, T2, and T3 are all hard-affinitied 519to the largest-numbered CPU in the system. Then if task T1 blocked in an 520RCU read-side critical section, then an expedited grace period started, 521then task T2 blocked in an RCU read-side critical section, then a normal 522grace period started, and finally task 3 blocked in an RCU read-side 523critical section, then the state of the last leaf ``rcu_node`` 524structure's blocked-task list would be as shown below: 525 526.. kernel-figure:: blkd_task.svg 527 528Task T1 is blocking both grace periods, task T2 is blocking only the 529normal grace period, and task T3 is blocking neither grace period. Note 530that these tasks will not remove themselves from this list immediately 531upon resuming execution. They will instead remain on the list until they 532execute the outermost ``rcu_read_unlock()`` that ends their RCU 533read-side critical section. 534 535The ``->wait_blkd_tasks`` field indicates whether or not the current 536grace period is waiting on a blocked task. 537 538Sizing the ``rcu_node`` Array 539''''''''''''''''''''''''''''' 540 541The ``rcu_node`` array is sized via a series of C-preprocessor 542expressions as follows: 543 544:: 545 546 1 #ifdef CONFIG_RCU_FANOUT 547 2 #define RCU_FANOUT CONFIG_RCU_FANOUT 548 3 #else 549 4 # ifdef CONFIG_64BIT 550 5 # define RCU_FANOUT 64 551 6 # else 552 7 # define RCU_FANOUT 32 553 8 # endif 554 9 #endif 555 10 556 11 #ifdef CONFIG_RCU_FANOUT_LEAF 557 12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF 558 13 #else 559 14 # ifdef CONFIG_64BIT 560 15 # define RCU_FANOUT_LEAF 64 561 16 # else 562 17 # define RCU_FANOUT_LEAF 32 563 18 # endif 564 19 #endif 565 20 566 21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF) 567 22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT) 568 23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT) 569 24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT) 570 25 571 26 #if NR_CPUS <= RCU_FANOUT_1 572 27 # define RCU_NUM_LVLS 1 573 28 # define NUM_RCU_LVL_0 1 574 29 # define NUM_RCU_NODES NUM_RCU_LVL_0 575 30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 } 576 31 # define RCU_NODE_NAME_INIT { "rcu_node_0" } 577 32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" } 578 33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" } 579 34 #elif NR_CPUS <= RCU_FANOUT_2 580 35 # define RCU_NUM_LVLS 2 581 36 # define NUM_RCU_LVL_0 1 582 37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) 583 38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1) 584 39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 } 585 40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" } 586 41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" } 587 42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" } 588 43 #elif NR_CPUS <= RCU_FANOUT_3 589 44 # define RCU_NUM_LVLS 3 590 45 # define NUM_RCU_LVL_0 1 591 46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) 592 47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) 593 48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2) 594 49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 } 595 50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" } 596 51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" } 597 52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" } 598 53 #elif NR_CPUS <= RCU_FANOUT_4 599 54 # define RCU_NUM_LVLS 4 600 55 # define NUM_RCU_LVL_0 1 601 56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3) 602 57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) 603 58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) 604 59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3) 605 60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 } 606 61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" } 607 62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" } 608 63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" } 609 64 #else 610 65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS" 611 66 #endif 612 613The maximum number of levels in the ``rcu_node`` structure is currently 614limited to four, as specified by lines 21-24 and the structure of the 615subsequent “if” statement. For 32-bit systems, this allows 61616*32*32*32=524,288 CPUs, which should be sufficient for the next few 617years at least. For 64-bit systems, 16*64*64*64=4,194,304 CPUs is 618allowed, which should see us through the next decade or so. This 619four-level tree also allows kernels built with ``CONFIG_RCU_FANOUT=8`` 620to support up to 4096 CPUs, which might be useful in very large systems 621having eight CPUs per socket (but please note that no one has yet shown 622any measurable performance degradation due to misaligned socket and 623``rcu_node`` boundaries). In addition, building kernels with a full four 624levels of ``rcu_node`` tree permits better testing of RCU's 625combining-tree code. 626 627The ``RCU_FANOUT`` symbol controls how many children are permitted at 628each non-leaf level of the ``rcu_node`` tree. If the 629``CONFIG_RCU_FANOUT`` Kconfig option is not specified, it is set based 630on the word size of the system, which is also the Kconfig default. 631 632The ``RCU_FANOUT_LEAF`` symbol controls how many CPUs are handled by 633each leaf ``rcu_node`` structure. Experience has shown that allowing a 634given leaf ``rcu_node`` structure to handle 64 CPUs, as permitted by the 635number of bits in the ``->qsmask`` field on a 64-bit system, results in 636excessive contention for the leaf ``rcu_node`` structures' ``->lock`` 637fields. The number of CPUs per leaf ``rcu_node`` structure is therefore 638limited to 16 given the default value of ``CONFIG_RCU_FANOUT_LEAF``. If 639``CONFIG_RCU_FANOUT_LEAF`` is unspecified, the value selected is based 640on the word size of the system, just as for ``CONFIG_RCU_FANOUT``. 641Lines 11-19 perform this computation. 642 643Lines 21-24 compute the maximum number of CPUs supported by a 644single-level (which contains a single ``rcu_node`` structure), 645two-level, three-level, and four-level ``rcu_node`` tree, respectively, 646given the fanout specified by ``RCU_FANOUT`` and ``RCU_FANOUT_LEAF``. 647These numbers of CPUs are retained in the ``RCU_FANOUT_1``, 648``RCU_FANOUT_2``, ``RCU_FANOUT_3``, and ``RCU_FANOUT_4`` C-preprocessor 649variables, respectively. 650 651These variables are used to control the C-preprocessor ``#if`` statement 652spanning lines 26-66 that computes the number of ``rcu_node`` structures 653required for each level of the tree, as well as the number of levels 654required. The number of levels is placed in the ``NUM_RCU_LVLS`` 655C-preprocessor variable by lines 27, 35, 44, and 54. The number of 656``rcu_node`` structures for the topmost level of the tree is always 657exactly one, and this value is unconditionally placed into 658``NUM_RCU_LVL_0`` by lines 28, 36, 45, and 55. The rest of the levels 659(if any) of the ``rcu_node`` tree are computed by dividing the maximum 660number of CPUs by the fanout supported by the number of levels from the 661current level down, rounding up. This computation is performed by 662lines 37, 46-47, and 56-58. Lines 31-33, 40-42, 50-52, and 62-63 create 663initializers for lockdep lock-class names. Finally, lines 64-66 produce 664an error if the maximum number of CPUs is too large for the specified 665fanout. 666 667The ``rcu_segcblist`` Structure 668~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 669 670The ``rcu_segcblist`` structure maintains a segmented list of callbacks 671as follows: 672 673:: 674 675 1 #define RCU_DONE_TAIL 0 676 2 #define RCU_WAIT_TAIL 1 677 3 #define RCU_NEXT_READY_TAIL 2 678 4 #define RCU_NEXT_TAIL 3 679 5 #define RCU_CBLIST_NSEGS 4 680 6 681 7 struct rcu_segcblist { 682 8 struct rcu_head *head; 683 9 struct rcu_head **tails[RCU_CBLIST_NSEGS]; 684 10 unsigned long gp_seq[RCU_CBLIST_NSEGS]; 685 11 long len; 686 12 long len_lazy; 687 13 }; 688 689The segments are as follows: 690 691#. ``RCU_DONE_TAIL``: Callbacks whose grace periods have elapsed. These 692 callbacks are ready to be invoked. 693#. ``RCU_WAIT_TAIL``: Callbacks that are waiting for the current grace 694 period. Note that different CPUs can have different ideas about which 695 grace period is current, hence the ``->gp_seq`` field. 696#. ``RCU_NEXT_READY_TAIL``: Callbacks waiting for the next grace period 697 to start. 698#. ``RCU_NEXT_TAIL``: Callbacks that have not yet been associated with a 699 grace period. 700 701The ``->head`` pointer references the first callback or is ``NULL`` if 702the list contains no callbacks (which is *not* the same as being empty). 703Each element of the ``->tails[]`` array references the ``->next`` 704pointer of the last callback in the corresponding segment of the list, 705or the list's ``->head`` pointer if that segment and all previous 706segments are empty. If the corresponding segment is empty but some 707previous segment is not empty, then the array element is identical to 708its predecessor. Older callbacks are closer to the head of the list, and 709new callbacks are added at the tail. This relationship between the 710``->head`` pointer, the ``->tails[]`` array, and the callbacks is shown 711in this diagram: 712 713.. kernel-figure:: nxtlist.svg 714 715In this figure, the ``->head`` pointer references the first RCU callback 716in the list. The ``->tails[RCU_DONE_TAIL]`` array element references the 717``->head`` pointer itself, indicating that none of the callbacks is 718ready to invoke. The ``->tails[RCU_WAIT_TAIL]`` array element references 719callback CB 2's ``->next`` pointer, which indicates that CB 1 and CB 2 720are both waiting on the current grace period, give or take possible 721disagreements about exactly which grace period is the current one. The 722``->tails[RCU_NEXT_READY_TAIL]`` array element references the same RCU 723callback that ``->tails[RCU_WAIT_TAIL]`` does, which indicates that 724there are no callbacks waiting on the next RCU grace period. The 725``->tails[RCU_NEXT_TAIL]`` array element references CB 4's ``->next`` 726pointer, indicating that all the remaining RCU callbacks have not yet 727been assigned to an RCU grace period. Note that the 728``->tails[RCU_NEXT_TAIL]`` array element always references the last RCU 729callback's ``->next`` pointer unless the callback list is empty, in 730which case it references the ``->head`` pointer. 731 732There is one additional important special case for the 733``->tails[RCU_NEXT_TAIL]`` array element: It can be ``NULL`` when this 734list is *disabled*. Lists are disabled when the corresponding CPU is 735offline or when the corresponding CPU's callbacks are offloaded to a 736kthread, both of which are described elsewhere. 737 738CPUs advance their callbacks from the ``RCU_NEXT_TAIL`` to the 739``RCU_NEXT_READY_TAIL`` to the ``RCU_WAIT_TAIL`` to the 740``RCU_DONE_TAIL`` list segments as grace periods advance. 741 742The ``->gp_seq[]`` array records grace-period numbers corresponding to 743the list segments. This is what allows different CPUs to have different 744ideas as to which is the current grace period while still avoiding 745premature invocation of their callbacks. In particular, this allows CPUs 746that go idle for extended periods to determine which of their callbacks 747are ready to be invoked after reawakening. 748 749The ``->len`` counter contains the number of callbacks in ``->head``, 750and the ``->len_lazy`` contains the number of those callbacks that are 751known to only free memory, and whose invocation can therefore be safely 752deferred. 753 754.. important:: 755 756 It is the ``->len`` field that determines whether or 757 not there are callbacks associated with this ``rcu_segcblist`` 758 structure, *not* the ``->head`` pointer. The reason for this is that all 759 the ready-to-invoke callbacks (that is, those in the ``RCU_DONE_TAIL`` 760 segment) are extracted all at once at callback-invocation time 761 (``rcu_do_batch``), due to which ``->head`` may be set to NULL if there 762 are no not-done callbacks remaining in the ``rcu_segcblist``. If 763 callback invocation must be postponed, for example, because a 764 high-priority process just woke up on this CPU, then the remaining 765 callbacks are placed back on the ``RCU_DONE_TAIL`` segment and 766 ``->head`` once again points to the start of the segment. In short, the 767 head field can briefly be ``NULL`` even though the CPU has callbacks 768 present the entire time. Therefore, it is not appropriate to test the 769 ``->head`` pointer for ``NULL``. 770 771In contrast, the ``->len`` and ``->len_lazy`` counts are adjusted only 772after the corresponding callbacks have been invoked. This means that the 773``->len`` count is zero only if the ``rcu_segcblist`` structure really 774is devoid of callbacks. Of course, off-CPU sampling of the ``->len`` 775count requires careful use of appropriate synchronization, for example, 776memory barriers. This synchronization can be a bit subtle, particularly 777in the case of ``rcu_barrier()``. 778 779The ``rcu_data`` Structure 780~~~~~~~~~~~~~~~~~~~~~~~~~~ 781 782The ``rcu_data`` maintains the per-CPU state for the RCU subsystem. The 783fields in this structure may be accessed only from the corresponding CPU 784(and from tracing) unless otherwise stated. This structure is the focus 785of quiescent-state detection and RCU callback queuing. It also tracks 786its relationship to the corresponding leaf ``rcu_node`` structure to 787allow more-efficient propagation of quiescent states up the ``rcu_node`` 788combining tree. Like the ``rcu_node`` structure, it provides a local 789copy of the grace-period information to allow for-free synchronized 790access to this information from the corresponding CPU. Finally, this 791structure records past dyntick-idle state for the corresponding CPU and 792also tracks statistics. 793 794The ``rcu_data`` structure's fields are discussed, singly and in groups, 795in the following sections. 796 797Connection to Other Data Structures 798''''''''''''''''''''''''''''''''''' 799 800This portion of the ``rcu_data`` structure is declared as follows: 801 802:: 803 804 1 int cpu; 805 2 struct rcu_node *mynode; 806 3 unsigned long grpmask; 807 4 bool beenonline; 808 809The ``->cpu`` field contains the number of the corresponding CPU and the 810``->mynode`` field references the corresponding ``rcu_node`` structure. 811The ``->mynode`` is used to propagate quiescent states up the combining 812tree. These two fields are constant and therefore do not require 813synchronization. 814 815The ``->grpmask`` field indicates the bit in the ``->mynode->qsmask`` 816corresponding to this ``rcu_data`` structure, and is also used when 817propagating quiescent states. The ``->beenonline`` flag is set whenever 818the corresponding CPU comes online, which means that the debugfs tracing 819need not dump out any ``rcu_data`` structure for which this flag is not 820set. 821 822Quiescent-State and Grace-Period Tracking 823''''''''''''''''''''''''''''''''''''''''' 824 825This portion of the ``rcu_data`` structure is declared as follows: 826 827:: 828 829 1 unsigned long gp_seq; 830 2 unsigned long gp_seq_needed; 831 3 bool cpu_no_qs; 832 4 bool core_needs_qs; 833 5 bool gpwrap; 834 835The ``->gp_seq`` field is the counterpart of the field of the same name 836in the ``rcu_state`` and ``rcu_node`` structures. The 837``->gp_seq_needed`` field is the counterpart of the field of the same 838name in the rcu_node structure. They may each lag up to one behind their 839``rcu_node`` counterparts, but in ``CONFIG_NO_HZ_IDLE`` and 840``CONFIG_NO_HZ_FULL`` kernels can lag arbitrarily far behind for CPUs in 841dyntick-idle mode (but these counters will catch up upon exit from 842dyntick-idle mode). If the lower two bits of a given ``rcu_data`` 843structure's ``->gp_seq`` are zero, then this ``rcu_data`` structure 844believes that RCU is idle. 845 846+-----------------------------------------------------------------------+ 847| **Quick Quiz**: | 848+-----------------------------------------------------------------------+ 849| All this replication of the grace period numbers can only cause | 850| massive confusion. Why not just keep a global sequence number and be | 851| done with it??? | 852+-----------------------------------------------------------------------+ 853| **Answer**: | 854+-----------------------------------------------------------------------+ 855| Because if there was only a single global sequence numbers, there | 856| would need to be a single global lock to allow safely accessing and | 857| updating it. And if we are not going to have a single global lock, we | 858| need to carefully manage the numbers on a per-node basis. Recall from | 859| the answer to a previous Quick Quiz that the consequences of applying | 860| a previously sampled quiescent state to the wrong grace period are | 861| quite severe. | 862+-----------------------------------------------------------------------+ 863 864The ``->cpu_no_qs`` flag indicates that the CPU has not yet passed 865through a quiescent state, while the ``->core_needs_qs`` flag indicates 866that the RCU core needs a quiescent state from the corresponding CPU. 867The ``->gpwrap`` field indicates that the corresponding CPU has remained 868idle for so long that the ``gp_seq`` counter is in danger of overflow, 869which will cause the CPU to disregard the values of its counters on its 870next exit from idle. 871 872RCU Callback Handling 873''''''''''''''''''''' 874 875In the absence of CPU-hotplug events, RCU callbacks are invoked by the 876same CPU that registered them. This is strictly a cache-locality 877optimization: callbacks can and do get invoked on CPUs other than the 878one that registered them. After all, if the CPU that registered a given 879callback has gone offline before the callback can be invoked, there 880really is no other choice. 881 882This portion of the ``rcu_data`` structure is declared as follows: 883 884:: 885 886 1 struct rcu_segcblist cblist; 887 2 long qlen_last_fqs_check; 888 3 unsigned long n_cbs_invoked; 889 4 unsigned long n_nocbs_invoked; 890 5 unsigned long n_cbs_orphaned; 891 6 unsigned long n_cbs_adopted; 892 7 unsigned long n_force_qs_snap; 893 8 long blimit; 894 895The ``->cblist`` structure is the segmented callback list described 896earlier. The CPU advances the callbacks in its ``rcu_data`` structure 897whenever it notices that another RCU grace period has completed. The CPU 898detects the completion of an RCU grace period by noticing that the value 899of its ``rcu_data`` structure's ``->gp_seq`` field differs from that of 900its leaf ``rcu_node`` structure. Recall that each ``rcu_node`` 901structure's ``->gp_seq`` field is updated at the beginnings and ends of 902each grace period. 903 904The ``->qlen_last_fqs_check`` and ``->n_force_qs_snap`` coordinate the 905forcing of quiescent states from ``call_rcu()`` and friends when 906callback lists grow excessively long. 907 908The ``->n_cbs_invoked``, ``->n_cbs_orphaned``, and ``->n_cbs_adopted`` 909fields count the number of callbacks invoked, sent to other CPUs when 910this CPU goes offline, and received from other CPUs when those other 911CPUs go offline. The ``->n_nocbs_invoked`` is used when the CPU's 912callbacks are offloaded to a kthread. 913 914Finally, the ``->blimit`` counter is the maximum number of RCU callbacks 915that may be invoked at a given time. 916 917Dyntick-Idle Handling 918''''''''''''''''''''' 919 920This portion of the ``rcu_data`` structure is declared as follows: 921 922:: 923 924 1 int dynticks_snap; 925 2 unsigned long dynticks_fqs; 926 927The ``->dynticks_snap`` field is used to take a snapshot of the 928corresponding CPU's dyntick-idle state when forcing quiescent states, 929and is therefore accessed from other CPUs. Finally, the 930``->dynticks_fqs`` field is used to count the number of times this CPU 931is determined to be in dyntick-idle state, and is used for tracing and 932debugging purposes. 933 934This portion of the rcu_data structure is declared as follows: 935 936:: 937 938 1 long dynticks_nesting; 939 2 long dynticks_nmi_nesting; 940 3 atomic_t dynticks; 941 4 bool rcu_need_heavy_qs; 942 5 bool rcu_urgent_qs; 943 944These fields in the rcu_data structure maintain the per-CPU dyntick-idle 945state for the corresponding CPU. The fields may be accessed only from 946the corresponding CPU (and from tracing) unless otherwise stated. 947 948The ``->dynticks_nesting`` field counts the nesting depth of process 949execution, so that in normal circumstances this counter has value zero 950or one. NMIs, irqs, and tracers are counted by the 951``->dynticks_nmi_nesting`` field. Because NMIs cannot be masked, changes 952to this variable have to be undertaken carefully using an algorithm 953provided by Andy Lutomirski. The initial transition from idle adds one, 954and nested transitions add two, so that a nesting level of five is 955represented by a ``->dynticks_nmi_nesting`` value of nine. This counter 956can therefore be thought of as counting the number of reasons why this 957CPU cannot be permitted to enter dyntick-idle mode, aside from 958process-level transitions. 959 960However, it turns out that when running in non-idle kernel context, the 961Linux kernel is fully capable of entering interrupt handlers that never 962exit and perhaps also vice versa. Therefore, whenever the 963``->dynticks_nesting`` field is incremented up from zero, the 964``->dynticks_nmi_nesting`` field is set to a large positive number, and 965whenever the ``->dynticks_nesting`` field is decremented down to zero, 966the the ``->dynticks_nmi_nesting`` field is set to zero. Assuming that 967the number of misnested interrupts is not sufficient to overflow the 968counter, this approach corrects the ``->dynticks_nmi_nesting`` field 969every time the corresponding CPU enters the idle loop from process 970context. 971 972The ``->dynticks`` field counts the corresponding CPU's transitions to 973and from either dyntick-idle or user mode, so that this counter has an 974even value when the CPU is in dyntick-idle mode or user mode and an odd 975value otherwise. The transitions to/from user mode need to be counted 976for user mode adaptive-ticks support (see timers/NO_HZ.txt). 977 978The ``->rcu_need_heavy_qs`` field is used to record the fact that the 979RCU core code would really like to see a quiescent state from the 980corresponding CPU, so much so that it is willing to call for 981heavy-weight dyntick-counter operations. This flag is checked by RCU's 982context-switch and ``cond_resched()`` code, which provide a momentary 983idle sojourn in response. 984 985Finally, the ``->rcu_urgent_qs`` field is used to record the fact that 986the RCU core code would really like to see a quiescent state from the 987corresponding CPU, with the various other fields indicating just how 988badly RCU wants this quiescent state. This flag is checked by RCU's 989context-switch path (``rcu_note_context_switch``) and the cond_resched 990code. 991 992+-----------------------------------------------------------------------+ 993| **Quick Quiz**: | 994+-----------------------------------------------------------------------+ 995| Why not simply combine the ``->dynticks_nesting`` and | 996| ``->dynticks_nmi_nesting`` counters into a single counter that just | 997| counts the number of reasons that the corresponding CPU is non-idle? | 998+-----------------------------------------------------------------------+ 999| **Answer**: | 1000+-----------------------------------------------------------------------+ 1001| Because this would fail in the presence of interrupts whose handlers | 1002| never return and of handlers that manage to return from a made-up | 1003| interrupt. | 1004+-----------------------------------------------------------------------+ 1005 1006Additional fields are present for some special-purpose builds, and are 1007discussed separately. 1008 1009The ``rcu_head`` Structure 1010~~~~~~~~~~~~~~~~~~~~~~~~~~ 1011 1012Each ``rcu_head`` structure represents an RCU callback. These structures 1013are normally embedded within RCU-protected data structures whose 1014algorithms use asynchronous grace periods. In contrast, when using 1015algorithms that block waiting for RCU grace periods, RCU users need not 1016provide ``rcu_head`` structures. 1017 1018The ``rcu_head`` structure has fields as follows: 1019 1020:: 1021 1022 1 struct rcu_head *next; 1023 2 void (*func)(struct rcu_head *head); 1024 1025The ``->next`` field is used to link the ``rcu_head`` structures 1026together in the lists within the ``rcu_data`` structures. The ``->func`` 1027field is a pointer to the function to be called when the callback is 1028ready to be invoked, and this function is passed a pointer to the 1029``rcu_head`` structure. However, ``kfree_rcu()`` uses the ``->func`` 1030field to record the offset of the ``rcu_head`` structure within the 1031enclosing RCU-protected data structure. 1032 1033Both of these fields are used internally by RCU. From the viewpoint of 1034RCU users, this structure is an opaque “cookie”. 1035 1036+-----------------------------------------------------------------------+ 1037| **Quick Quiz**: | 1038+-----------------------------------------------------------------------+ 1039| Given that the callback function ``->func`` is passed a pointer to | 1040| the ``rcu_head`` structure, how is that function supposed to find the | 1041| beginning of the enclosing RCU-protected data structure? | 1042+-----------------------------------------------------------------------+ 1043| **Answer**: | 1044+-----------------------------------------------------------------------+ 1045| In actual practice, there is a separate callback function per type of | 1046| RCU-protected data structure. The callback function can therefore use | 1047| the ``container_of()`` macro in the Linux kernel (or other | 1048| pointer-manipulation facilities in other software environments) to | 1049| find the beginning of the enclosing structure. | 1050+-----------------------------------------------------------------------+ 1051 1052RCU-Specific Fields in the ``task_struct`` Structure 1053~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1054 1055The ``CONFIG_PREEMPT_RCU`` implementation uses some additional fields in 1056the ``task_struct`` structure: 1057 1058:: 1059 1060 1 #ifdef CONFIG_PREEMPT_RCU 1061 2 int rcu_read_lock_nesting; 1062 3 union rcu_special rcu_read_unlock_special; 1063 4 struct list_head rcu_node_entry; 1064 5 struct rcu_node *rcu_blocked_node; 1065 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */ 1066 7 #ifdef CONFIG_TASKS_RCU 1067 8 unsigned long rcu_tasks_nvcsw; 1068 9 bool rcu_tasks_holdout; 1069 10 struct list_head rcu_tasks_holdout_list; 1070 11 int rcu_tasks_idle_cpu; 1071 12 #endif /* #ifdef CONFIG_TASKS_RCU */ 1072 1073The ``->rcu_read_lock_nesting`` field records the nesting level for RCU 1074read-side critical sections, and the ``->rcu_read_unlock_special`` field 1075is a bitmask that records special conditions that require 1076``rcu_read_unlock()`` to do additional work. The ``->rcu_node_entry`` 1077field is used to form lists of tasks that have blocked within 1078preemptible-RCU read-side critical sections and the 1079``->rcu_blocked_node`` field references the ``rcu_node`` structure whose 1080list this task is a member of, or ``NULL`` if it is not blocked within a 1081preemptible-RCU read-side critical section. 1082 1083The ``->rcu_tasks_nvcsw`` field tracks the number of voluntary context 1084switches that this task had undergone at the beginning of the current 1085tasks-RCU grace period, ``->rcu_tasks_holdout`` is set if the current 1086tasks-RCU grace period is waiting on this task, 1087``->rcu_tasks_holdout_list`` is a list element enqueuing this task on 1088the holdout list, and ``->rcu_tasks_idle_cpu`` tracks which CPU this 1089idle task is running, but only if the task is currently running, that 1090is, if the CPU is currently idle. 1091 1092Accessor Functions 1093~~~~~~~~~~~~~~~~~~ 1094 1095The following listing shows the ``rcu_get_root()``, 1096``rcu_for_each_node_breadth_first`` and ``rcu_for_each_leaf_node()`` 1097function and macros: 1098 1099:: 1100 1101 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp) 1102 2 { 1103 3 return &rsp->node[0]; 1104 4 } 1105 5 1106 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \ 1107 7 for ((rnp) = &(rsp)->node[0]; \ 1108 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) 1109 9 1110 10 #define rcu_for_each_leaf_node(rsp, rnp) \ 1111 11 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \ 1112 12 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) 1113 1114The ``rcu_get_root()`` simply returns a pointer to the first element of 1115the specified ``rcu_state`` structure's ``->node[]`` array, which is the 1116root ``rcu_node`` structure. 1117 1118As noted earlier, the ``rcu_for_each_node_breadth_first()`` macro takes 1119advantage of the layout of the ``rcu_node`` structures in the 1120``rcu_state`` structure's ``->node[]`` array, performing a breadth-first 1121traversal by simply traversing the array in order. Similarly, the 1122``rcu_for_each_leaf_node()`` macro traverses only the last part of the 1123array, thus traversing only the leaf ``rcu_node`` structures. 1124 1125+-----------------------------------------------------------------------+ 1126| **Quick Quiz**: | 1127+-----------------------------------------------------------------------+ 1128| What does ``rcu_for_each_leaf_node()`` do if the ``rcu_node`` tree | 1129| contains only a single node? | 1130+-----------------------------------------------------------------------+ 1131| **Answer**: | 1132+-----------------------------------------------------------------------+ 1133| In the single-node case, ``rcu_for_each_leaf_node()`` traverses the | 1134| single node. | 1135+-----------------------------------------------------------------------+ 1136 1137Summary 1138~~~~~~~ 1139 1140So the state of RCU is represented by an ``rcu_state`` structure, which 1141contains a combining tree of ``rcu_node`` and ``rcu_data`` structures. 1142Finally, in ``CONFIG_NO_HZ_IDLE`` kernels, each CPU's dyntick-idle state 1143is tracked by dynticks-related fields in the ``rcu_data`` structure. If 1144you made it this far, you are well prepared to read the code 1145walkthroughs in the other articles in this series. 1146 1147Acknowledgments 1148~~~~~~~~~~~~~~~ 1149 1150I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul 1151Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn for 1152helping me get this document into a more human-readable state. 1153 1154Legal Statement 1155~~~~~~~~~~~~~~~ 1156 1157This work represents the view of the author and does not necessarily 1158represent the view of IBM. 1159 1160Linux is a registered trademark of Linus Torvalds. 1161 1162Other company, product, and service names may be trademarks or service 1163marks of others. 1164
