1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
2 "http://www.w3.org/TR/html4/loose.dtd">
4 <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
5 <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
7 <p>December 18, 2016</p>
8 <p>This article was contributed by Paul E. McKenney</p>
12 This document describes RCU's major data structures and their relationship
16 <li> <a href="#Data-Structure Relationships">
17 Data-Structure Relationships</a>
18 <li> <a href="#The rcu_state Structure">
19 The <tt>rcu_state</tt> Structure</a>
20 <li> <a href="#The rcu_node Structure">
21 The <tt>rcu_node</tt> Structure</a>
22 <li> <a href="#The rcu_segcblist Structure">
23 The <tt>rcu_segcblist</tt> Structure</a>
24 <li> <a href="#The rcu_data Structure">
25 The <tt>rcu_data</tt> Structure</a>
26 <li> <a href="#The rcu_head Structure">
27 The <tt>rcu_head</tt> Structure</a>
28 <li> <a href="#RCU-Specific Fields in the task_struct Structure">
29 RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
30 <li> <a href="#Accessor Functions">
31 Accessor Functions</a>
34 <h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
36 <p>RCU is for all intents and purposes a large state machine, and its
37 data structures maintain the state in such a way as to allow RCU readers
38 to execute extremely quickly, while also processing the RCU grace periods
39 requested by updaters in an efficient and extremely scalable fashion.
40 The efficiency and scalability of RCU updaters is provided primarily
41 by a combining tree, as shown below:
43 </p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
45 </p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
46 containing a tree of <tt>rcu_node</tt> structures.
47 Each leaf node of the <tt>rcu_node</tt> tree has up to 16
48 <tt>rcu_data</tt> structures associated with it, so that there
49 are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
50 one for each possible CPU.
51 This structure is adjusted at boot time, if needed, to handle the
52 common case where <tt>nr_cpu_ids</tt> is much less than
54 For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
55 which results in a three-level <tt>rcu_node</tt> tree.
56 If the actual hardware has only 16 CPUs, RCU will adjust itself
57 at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
59 </p><p>The purpose of this combining tree is to allow per-CPU events
60 such as quiescent states, dyntick-idle transitions,
61 and CPU hotplug operations to be processed efficiently
63 Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
64 and other events are recorded by the leaf-level <tt>rcu_node</tt>
66 All of these events are combined at each level of the tree until finally
67 grace periods are completed at the tree's root <tt>rcu_node</tt>
69 A grace period can be completed at the root once every CPU
70 (or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
71 has passed through a quiescent state.
72 Once a grace period has completed, record of that fact is propagated
75 </p><p>As can be seen from the diagram, on a 64-bit system
76 a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
77 of 64 at the root and a fanout of 16 at the leaves.
80 <tr><th> </th></tr>
81 <tr><th align="left">Quick Quiz:</th></tr>
83 Why isn't the fanout at the leaves also 64?
85 <tr><th align="left">Answer:</th></tr>
86 <tr><td bgcolor="#ffffff"><font color="ffffff">
87 Because there are more types of events that affect the leaf-level
88 <tt>rcu_node</tt> structures than further up the tree.
89 Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
90 64, the contention on these structures' <tt>->structures</tt>
92 Experimentation on a wide variety of systems has shown that a fanout
93 of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
96 <p><font color="ffffff">Of course, further experience with
97 systems having hundreds or thousands of CPUs may demonstrate
98 that the fanout for the non-leaf <tt>rcu_node</tt> structures
100 Such reduction can be easily carried out when and if it proves
102 In the meantime, if you are using such a system and running into
103 contention problems on the non-leaf <tt>rcu_node</tt> structures,
104 you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
105 parameter to reduce the non-leaf fanout as needed.
108 <p><font color="ffffff">Kernels built for systems with
109 strong NUMA characteristics might also need to adjust
110 <tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
111 <tt>rcu_node</tt> structures align with hardware boundaries.
112 However, there has thus far been no need for this.
114 <tr><td> </td></tr>
117 <p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
118 a 32-bit system), then RCU will automatically add more levels to the
120 For example, if you are crazy enough to build a 64-bit system with 65,536
121 CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
123 </p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
125 </p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
126 accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
128 On the other hand, you can set both <tt>CONFIG_RCU_FANOUT</tt> and
129 <tt>CONFIG_RCU_FANOUT_LEAF</tt> to be as small as 2, which would result
130 in a 16-CPU test using a 4-level tree.
131 This can be useful for testing large-system capabilities on small test
134 </p><p>This multi-level combining tree allows us to get most of the
135 performance and scalability
136 benefits of partitioning, even though RCU grace-period detection is
137 inherently a global operation.
138 The trick here is that only the last CPU to report a quiescent state
139 into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
140 structure at the next level up the tree.
141 This means that at the leaf-level <tt>rcu_node</tt> structure, only
142 one access out of sixteen will progress up the tree.
143 For the internal <tt>rcu_node</tt> structures, the situation is even
144 more extreme: Only one access out of sixty-four will progress up
146 Because the vast majority of the CPUs do not progress up the tree,
147 the lock contention remains roughly constant up the tree.
148 No matter how many CPUs there are in the system, at most 64 quiescent-state
149 reports per grace period will progress all the way to the root
150 <tt>rcu_node</tt> structure, thus ensuring that the lock contention
151 on that root <tt>rcu_node</tt> structure remains acceptably low.
153 </p><p>In effect, the combining tree acts like a big shock absorber,
154 keeping lock contention under control at all tree levels regardless
155 of the level of loading on the system.
157 </p><p>RCU updaters wait for normal grace periods by registering
158 RCU callbacks, either directly via <tt>call_rcu()</tt>
159 or indirectly via <tt>synchronize_rcu()</tt> and friends.
160 RCU callbacks are represented by <tt>rcu_head</tt> structures,
161 which are queued on <tt>rcu_data</tt> structures while they are
162 waiting for a grace period to elapse, as shown in the following figure:
164 </p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
166 </p><p>This figure shows how <tt>TREE_RCU</tt>'s and
167 <tt>PREEMPT_RCU</tt>'s major data structures are related.
168 Lesser data structures will be introduced with the algorithms that
171 </p><p>Note that each of the data structures in the above figure has
172 its own synchronization:
175 <li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
176 and some fields are protected by the corresponding root
177 <tt>rcu_node</tt> structure's lock.
178 <li> Each <tt>rcu_node</tt> structure has a spinlock.
179 <li> The fields in <tt>rcu_data</tt> are private to the corresponding
180 CPU, although a few can be read and written by other CPUs.
183 <p>It is important to note that different data structures can have
184 very different ideas about the state of RCU at any given time.
185 For but one example, awareness of the start or end of a given RCU
186 grace period propagates slowly through the data structures.
187 This slow propagation is absolutely necessary for RCU to have good
188 read-side performance.
189 If this balkanized implementation seems foreign to you, one useful
190 trick is to consider each instance of these data structures to be
191 a different person, each having the usual slightly different
194 </p><p>The general role of each of these data structures is as
198 <li> <tt>rcu_state</tt>:
199 This structure forms the interconnection between the
200 <tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
201 tracks grace periods, serves as short-term repository
202 for callbacks orphaned by CPU-hotplug events,
203 maintains <tt>rcu_barrier()</tt> state,
204 tracks expedited grace-period state,
205 and maintains state used to force quiescent states when
206 grace periods extend too long,
207 <li> <tt>rcu_node</tt>: This structure forms the combining
208 tree that propagates quiescent-state
209 information from the leaves to the root, and also propagates
210 grace-period information from the root to the leaves.
211 It provides local copies of the grace-period state in order
212 to allow this information to be accessed in a synchronized
213 manner without suffering the scalability limitations that
214 would otherwise be imposed by global locking.
215 In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
216 of tasks that have blocked while in their current
217 RCU read-side critical section.
218 In <tt>CONFIG_PREEMPT_RCU</tt> with
219 <tt>CONFIG_RCU_BOOST</tt>, it manages the
220 per-<tt>rcu_node</tt> priority-boosting
221 kernel threads (kthreads) and state.
222 Finally, it records CPU-hotplug state in order to determine
223 which CPUs should be ignored during a given grace period.
224 <li> <tt>rcu_data</tt>: This per-CPU structure is the
225 focus of quiescent-state detection and RCU callback queuing.
226 It also tracks its relationship to the corresponding leaf
227 <tt>rcu_node</tt> structure to allow more-efficient
228 propagation of quiescent states up the <tt>rcu_node</tt>
230 Like the <tt>rcu_node</tt> structure, it provides a local
231 copy of the grace-period information to allow for-free
233 access to this information from the corresponding CPU.
234 Finally, this structure records past dyntick-idle state
235 for the corresponding CPU and also tracks statistics.
236 <li> <tt>rcu_head</tt>:
237 This structure represents RCU callbacks, and is the
238 only structure allocated and managed by RCU users.
239 The <tt>rcu_head</tt> structure is normally embedded
240 within the RCU-protected data structure.
243 <p>If all you wanted from this article was a general notion of how
244 RCU's data structures are related, you are done.
245 Otherwise, each of the following sections give more details on
246 the <tt>rcu_state</tt>, <tt>rcu_node</tt> and <tt>rcu_data</tt> data
249 <h3><a name="The rcu_state Structure">
250 The <tt>rcu_state</tt> Structure</a></h3>
252 <p>The <tt>rcu_state</tt> structure is the base structure that
253 represents the state of RCU in the system.
254 This structure forms the interconnection between the
255 <tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
256 tracks grace periods, contains the lock used to
257 synchronize with CPU-hotplug events,
258 and maintains state used to force quiescent states when
259 grace periods extend too long,
261 </p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
262 singly and in groups, in the following sections.
263 The more specialized fields are covered in the discussion of their
266 <h5>Relationship to rcu_node and rcu_data Structures</h5>
268 This portion of the <tt>rcu_state</tt> structure is declared
272 1 struct rcu_node node[NUM_RCU_NODES];
273 2 struct rcu_node *level[NUM_RCU_LVLS + 1];
274 3 struct rcu_data __percpu *rda;
278 <tr><th> </th></tr>
279 <tr><th align="left">Quick Quiz:</th></tr>
282 You said that the <tt>rcu_node</tt> structures formed a tree,
283 but they are declared as a flat array!
286 <tr><th align="left">Answer:</th></tr>
287 <tr><td bgcolor="#ffffff"><font color="ffffff">
288 The tree is laid out in the array.
289 The first node In the array is the head, the next set of nodes in the
290 array are children of the head node, and so on until the last set of
291 nodes in the array are the leaves.
294 <p><font color="ffffff">See the following diagrams to see how
297 <tr><td> </td></tr>
300 <p>The <tt>rcu_node</tt> tree is embedded into the
301 <tt>->node[]</tt> array as shown in the following figure:
303 </p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
305 </p><p>One interesting consequence of this mapping is that a
306 breadth-first traversal of the tree is implemented as a simple
307 linear scan of the array, which is in fact what the
308 <tt>rcu_for_each_node_breadth_first()</tt> macro does.
309 This macro is used at the beginning and ends of grace periods.
311 </p><p>Each entry of the <tt>->level</tt> array references
312 the first <tt>rcu_node</tt> structure on the corresponding level
313 of the tree, for example, as shown below:
315 </p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
317 </p><p>The zero<sup>th</sup> element of the array references the root
318 <tt>rcu_node</tt> structure, the first element references the
319 first child of the root <tt>rcu_node</tt>, and finally the second
320 element references the first leaf <tt>rcu_node</tt> structure.
322 </p><p>For whatever it is worth, if you draw the tree to be tree-shaped
323 rather than array-shaped, it is easy to draw a planar representation:
325 </p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
327 </p><p>Finally, the <tt>->rda</tt> field references a per-CPU
328 pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
330 </p><p>All of these fields are constant once initialization is complete,
331 and therefore need no protection.
333 <h5>Grace-Period Tracking</h5>
335 <p>This portion of the <tt>rcu_state</tt> structure is declared
339 1 unsigned long gp_seq;
342 <p>RCU grace periods are numbered, and
343 the <tt>->gp_seq</tt> field contains the current grace-period
345 The bottom two bits are the state of the current grace period,
346 which can be zero for not yet started or one for in progress.
347 In other words, if the bottom two bits of <tt>->gp_seq</tt> are
348 zero, then RCU is idle.
349 Any other value in the bottom two bits indicates that something is broken.
350 This field is protected by the root <tt>rcu_node</tt> structure's
351 <tt>->lock</tt> field.
353 </p><p>There are <tt>->gp_seq</tt> fields
354 in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
356 The fields in the <tt>rcu_state</tt> structure represent the
357 most current value, and those of the other structures are compared
358 in order to detect the beginnings and ends of grace periods in a distributed
360 The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
361 (down the tree from the root to the leaves) to <tt>rcu_data</tt>.
363 <h5>Miscellaneous</h5>
365 <p>This portion of the <tt>rcu_state</tt> structure is declared
369 1 unsigned long gp_max;
374 <p>The <tt>->gp_max</tt> field tracks the duration of the longest
375 grace period in jiffies.
376 It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>.
378 <p>The <tt>->name</tt> and <tt>->abbr</tt> fields distinguish
379 between preemptible RCU (“rcu_preempt” and “p”)
380 and non-preemptible RCU (“rcu_sched” and “s”).
381 These fields are used for diagnostic and tracing purposes.
383 <h3><a name="The rcu_node Structure">
384 The <tt>rcu_node</tt> Structure</a></h3>
386 <p>The <tt>rcu_node</tt> structures form the combining
387 tree that propagates quiescent-state
388 information from the leaves to the root and also that propagates
389 grace-period information from the root down to the leaves.
390 They provides local copies of the grace-period state in order
391 to allow this information to be accessed in a synchronized
392 manner without suffering the scalability limitations that
393 would otherwise be imposed by global locking.
394 In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
395 of tasks that have blocked while in their current
396 RCU read-side critical section.
397 In <tt>CONFIG_PREEMPT_RCU</tt> with
398 <tt>CONFIG_RCU_BOOST</tt>, they manage the
399 per-<tt>rcu_node</tt> priority-boosting
400 kernel threads (kthreads) and state.
401 Finally, they record CPU-hotplug state in order to determine
402 which CPUs should be ignored during a given grace period.
404 </p><p>The <tt>rcu_node</tt> structure's fields are discussed,
405 singly and in groups, in the following sections.
407 <h5>Connection to Combining Tree</h5>
409 <p>This portion of the <tt>rcu_node</tt> structure is declared
413 1 struct rcu_node *parent;
416 4 unsigned long grpmask;
421 <p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt>
422 one level up in the tree, and is <tt>NULL</tt> for the root
424 The RCU implementation makes heavy use of this field to push quiescent
426 The <tt>->level</tt> field gives the level in the tree, with
427 the root being at level zero, its children at level one, and so on.
428 The <tt>->grpnum</tt> field gives this node's position within
429 the children of its parent, so this number can range between 0 and 31
430 on 32-bit systems and between 0 and 63 on 64-bit systems.
431 The <tt>->level</tt> and <tt>->grpnum</tt> fields are
432 used only during initialization and for tracing.
433 The <tt>->grpmask</tt> field is the bitmask counterpart of
434 <tt>->grpnum</tt>, and therefore always has exactly one bit set.
435 This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
436 structure in its parent's bitmasks, which are described later.
437 Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields
438 contain the lowest and highest numbered CPU served by this
439 <tt>rcu_node</tt> structure, respectively.
441 </p><p>All of these fields are constant, and thus do not require any
444 <h5>Synchronization</h5>
446 <p>This field of the <tt>rcu_node</tt> structure is declared
450 1 raw_spinlock_t lock;
453 <p>This field is used to protect the remaining fields in this structure,
454 unless otherwise stated.
455 That said, all of the fields in this structure can be accessed without
456 locking for tracing purposes.
457 Yes, this can result in confusing traces, but better some tracing confusion
458 than to be heisenbugged out of existence.
460 <h5>Grace-Period Tracking</h5>
462 <p>This portion of the <tt>rcu_node</tt> structure is declared
466 1 unsigned long gp_seq;
467 2 unsigned long gp_seq_needed;
470 <p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are
471 the counterparts of the field of the same name in the <tt>rcu_state</tt>
473 They each may lag up to one step behind their <tt>rcu_state</tt>
475 If the bottom two bits of a given <tt>rcu_node</tt> structure's
476 <tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
477 structure believes that RCU is idle.
478 </p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt>
479 structure is updated at the beginning and the end
480 of each grace period.
482 <p>The <tt>->gp_seq_needed</tt> fields record the
483 furthest-in-the-future grace period request seen by the corresponding
484 <tt>rcu_node</tt> structure. The request is considered fulfilled when
485 the value of the <tt>->gp_seq</tt> field equals or exceeds that of
486 the <tt>->gp_seq_needed</tt> field.
489 <tr><th> </th></tr>
490 <tr><th align="left">Quick Quiz:</th></tr>
492 Suppose that this <tt>rcu_node</tt> structure doesn't see
493 a request for a very long time.
494 Won't wrapping of the <tt>->gp_seq</tt> field cause
497 <tr><th align="left">Answer:</th></tr>
498 <tr><td bgcolor="#ffffff"><font color="ffffff">
499 No, because if the <tt>->gp_seq_needed</tt> field lags behind the
500 <tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field
501 will be updated at the end of the grace period.
502 Modulo-arithmetic comparisons therefore will always get the
503 correct answer, even with wrapping.
505 <tr><td> </td></tr>
508 <h5>Quiescent-State Tracking</h5>
510 <p>These fields manage the propagation of quiescent states up the
513 </p><p>This portion of the <tt>rcu_node</tt> structure has fields
517 1 unsigned long qsmask;
518 2 unsigned long expmask;
519 3 unsigned long qsmaskinit;
520 4 unsigned long expmaskinit;
523 <p>The <tt>->qsmask</tt> field tracks which of this
524 <tt>rcu_node</tt> structure's children still need to report
525 quiescent states for the current normal grace period.
526 Such children will have a value of 1 in their corresponding bit.
527 Note that the leaf <tt>rcu_node</tt> structures should be
528 thought of as having <tt>rcu_data</tt> structures as their
530 Similarly, the <tt>->expmask</tt> field tracks which
531 of this <tt>rcu_node</tt> structure's children still need to report
532 quiescent states for the current expedited grace period.
533 An expedited grace period has
534 the same conceptual properties as a normal grace period, but the
535 expedited implementation accepts extreme CPU overhead to obtain
536 much lower grace-period latency, for example, consuming a few
537 tens of microseconds worth of CPU time to reduce grace-period
538 duration from milliseconds to tens of microseconds.
539 The <tt>->qsmaskinit</tt> field tracks which of this
540 <tt>rcu_node</tt> structure's children cover for at least
542 This mask is used to initialize <tt>->qsmask</tt>,
543 and <tt>->expmaskinit</tt> is used to initialize
544 <tt>->expmask</tt> and the beginning of the
545 normal and expedited grace periods, respectively.
548 <tr><th> </th></tr>
549 <tr><th align="left">Quick Quiz:</th></tr>
551 Why are these bitmasks protected by locking?
552 Come on, haven't you heard of atomic instructions???
554 <tr><th align="left">Answer:</th></tr>
555 <tr><td bgcolor="#ffffff"><font color="ffffff">
556 Lockless grace-period computation! Such a tantalizing possibility!
559 <p><font color="ffffff">But consider the following sequence of events:
563 <li> <font color="ffffff">CPU 0 has been in dyntick-idle
564 mode for quite some time.
565 When it wakes up, it notices that the current RCU
566 grace period needs it to report in, so it sets a
567 flag where the scheduling clock interrupt will find it.
569 <li> <font color="ffffff">Meanwhile, CPU 1 is running
570 <tt>force_quiescent_state()</tt>,
571 and notices that CPU 0 has been in dyntick idle mode,
572 which qualifies as an extended quiescent state.
574 <li> <font color="ffffff">CPU 0's scheduling clock
575 interrupt fires in the
576 middle of an RCU read-side critical section, and notices
577 that the RCU core needs something, so commences RCU softirq
581 <li> <font color="ffffff">CPU 0's softirq handler
582 executes and is just about ready
583 to report its quiescent state up the <tt>rcu_node</tt>
586 <li> <font color="ffffff">But CPU 1 beats it to the punch,
587 completing the current
588 grace period and starting a new one.
590 <li> <font color="ffffff">CPU 0 now reports its quiescent
593 That grace period might now end before the RCU read-side
595 If that happens, disaster will ensue.
599 <p><font color="ffffff">So the locking is absolutely required in
600 order to coordinate clearing of the bits with updating of the
601 grace-period sequence number in <tt>->gp_seq</tt>.
603 <tr><td> </td></tr>
606 <h5>Blocked-Task Management</h5>
608 <p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
609 midst of their RCU read-side critical sections, and these tasks
610 must be tracked explicitly.
611 The details of exactly why and how they are tracked will be covered
612 in a separate article on RCU read-side processing.
613 For now, it is enough to know that the <tt>rcu_node</tt>
614 structure tracks them.
617 1 struct list_head blkd_tasks;
618 2 struct list_head *gp_tasks;
619 3 struct list_head *exp_tasks;
620 4 bool wait_blkd_tasks;
623 <p>The <tt>->blkd_tasks</tt> field is a list header for
624 the list of blocked and preempted tasks.
625 As tasks undergo context switches within RCU read-side critical
626 sections, their <tt>task_struct</tt> structures are enqueued
627 (via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt>
628 field) onto the head of the <tt>->blkd_tasks</tt> list for the
629 leaf <tt>rcu_node</tt> structure corresponding to the CPU
630 on which the outgoing context switch executed.
631 As these tasks later exit their RCU read-side critical sections,
632 they remove themselves from the list.
633 This list is therefore in reverse time order, so that if one of the tasks
634 is blocking the current grace period, all subsequent tasks must
635 also be blocking that same grace period.
636 Therefore, a single pointer into this list suffices to track
637 all tasks blocking a given grace period.
638 That pointer is stored in <tt>->gp_tasks</tt> for normal
639 grace periods and in <tt>->exp_tasks</tt> for expedited
641 These last two fields are <tt>NULL</tt> if either there is
642 no grace period in flight or if there are no blocked tasks
643 preventing that grace period from completing.
644 If either of these two pointers is referencing a task that
645 removes itself from the <tt>->blkd_tasks</tt> list,
646 then that task must advance the pointer to the next task on
647 the list, or set the pointer to <tt>NULL</tt> if there
648 are no subsequent tasks on the list.
650 </p><p>For example, suppose that tasks T1, T2, and T3 are
651 all hard-affinitied to the largest-numbered CPU in the system.
652 Then if task T1 blocked in an RCU read-side
653 critical section, then an expedited grace period started,
654 then task T2 blocked in an RCU read-side critical section,
655 then a normal grace period started, and finally task 3 blocked
656 in an RCU read-side critical section, then the state of the
657 last leaf <tt>rcu_node</tt> structure's blocked-task list
658 would be as shown below:
660 </p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
662 </p><p>Task T1 is blocking both grace periods, task T2 is
663 blocking only the normal grace period, and task T3 is blocking
664 neither grace period.
665 Note that these tasks will not remove themselves from this list
666 immediately upon resuming execution.
667 They will instead remain on the list until they execute the outermost
668 <tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
672 The <tt>->wait_blkd_tasks</tt> field indicates whether or not
673 the current grace period is waiting on a blocked task.
675 <h5>Sizing the <tt>rcu_node</tt> Array</h5>
677 <p>The <tt>rcu_node</tt> array is sized via a series of
678 C-preprocessor expressions as follows:
681 1 #ifdef CONFIG_RCU_FANOUT
682 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
684 4 # ifdef CONFIG_64BIT
685 5 # define RCU_FANOUT 64
687 7 # define RCU_FANOUT 32
691 11 #ifdef CONFIG_RCU_FANOUT_LEAF
692 12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
694 14 # ifdef CONFIG_64BIT
695 15 # define RCU_FANOUT_LEAF 64
697 17 # define RCU_FANOUT_LEAF 32
701 21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
702 22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
703 23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
704 24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
706 26 #if NR_CPUS <= RCU_FANOUT_1
707 27 # define RCU_NUM_LVLS 1
708 28 # define NUM_RCU_LVL_0 1
709 29 # define NUM_RCU_NODES NUM_RCU_LVL_0
710 30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
711 31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
712 32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
713 33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
714 34 #elif NR_CPUS <= RCU_FANOUT_2
715 35 # define RCU_NUM_LVLS 2
716 36 # define NUM_RCU_LVL_0 1
717 37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
718 38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
719 39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
720 40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
721 41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
722 42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
723 43 #elif NR_CPUS <= RCU_FANOUT_3
724 44 # define RCU_NUM_LVLS 3
725 45 # define NUM_RCU_LVL_0 1
726 46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
727 47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
728 48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
729 49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
730 50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
731 51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
732 52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
733 53 #elif NR_CPUS <= RCU_FANOUT_4
734 54 # define RCU_NUM_LVLS 4
735 55 # define NUM_RCU_LVL_0 1
736 56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
737 57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
738 58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
739 59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
740 60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
741 61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
742 62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
743 63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
745 65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
749 <p>The maximum number of levels in the <tt>rcu_node</tt> structure
750 is currently limited to four, as specified by lines 21-24
751 and the structure of the subsequent “if” statement.
752 For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
753 should be sufficient for the next few years at least.
754 For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
755 should see us through the next decade or so.
756 This four-level tree also allows kernels built with
757 <tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
758 which might be useful in very large systems having eight CPUs per
759 socket (but please note that no one has yet shown any measurable
760 performance degradation due to misaligned socket and <tt>rcu_node</tt>
762 In addition, building kernels with a full four levels of <tt>rcu_node</tt>
763 tree permits better testing of RCU's combining-tree code.
765 </p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
766 are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
767 If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
768 it is set based on the word size of the system, which is also
771 </p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
772 handled by each leaf <tt>rcu_node</tt> structure.
773 Experience has shown that allowing a given leaf <tt>rcu_node</tt>
774 structure to handle 64 CPUs, as permitted by the number of bits in
775 the <tt>->qsmask</tt> field on a 64-bit system, results in
776 excessive contention for the leaf <tt>rcu_node</tt> structures'
777 <tt>->lock</tt> fields.
778 The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
779 limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
780 If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
781 selected is based on the word size of the system, just as for
782 <tt>CONFIG_RCU_FANOUT</tt>.
783 Lines 11-19 perform this computation.
785 </p><p>Lines 21-24 compute the maximum number of CPUs supported by
786 a single-level (which contains a single <tt>rcu_node</tt> structure),
787 two-level, three-level, and four-level <tt>rcu_node</tt> tree,
788 respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
789 and <tt>RCU_FANOUT_LEAF</tt>.
790 These numbers of CPUs are retained in the
791 <tt>RCU_FANOUT_1</tt>,
792 <tt>RCU_FANOUT_2</tt>,
793 <tt>RCU_FANOUT_3</tt>, and
794 <tt>RCU_FANOUT_4</tt>
795 C-preprocessor variables, respectively.
797 </p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
798 statement spanning lines 26-66 that computes the number of
799 <tt>rcu_node</tt> structures required for each level of the tree,
800 as well as the number of levels required.
801 The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
802 C-preprocessor variable by lines 27, 35, 44, and 54.
803 The number of <tt>rcu_node</tt> structures for the topmost level
804 of the tree is always exactly one, and this value is unconditionally
805 placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55.
806 The rest of the levels (if any) of the <tt>rcu_node</tt> tree
807 are computed by dividing the maximum number of CPUs by the
808 fanout supported by the number of levels from the current level down,
809 rounding up. This computation is performed by lines 37,
810 46-47, and 56-58.
811 Lines 31-33, 40-42, 50-52, and 62-63 create initializers
812 for lockdep lock-class names.
813 Finally, lines 64-66 produce an error if the maximum number of
814 CPUs is too large for the specified fanout.
816 <h3><a name="The rcu_segcblist Structure">
817 The <tt>rcu_segcblist</tt> Structure</a></h3>
819 The <tt>rcu_segcblist</tt> structure maintains a segmented list of
820 callbacks as follows:
823 1 #define RCU_DONE_TAIL 0
824 2 #define RCU_WAIT_TAIL 1
825 3 #define RCU_NEXT_READY_TAIL 2
826 4 #define RCU_NEXT_TAIL 3
827 5 #define RCU_CBLIST_NSEGS 4
829 7 struct rcu_segcblist {
830 8 struct rcu_head *head;
831 9 struct rcu_head **tails[RCU_CBLIST_NSEGS];
832 10 unsigned long gp_seq[RCU_CBLIST_NSEGS];
839 The segments are as follows:
842 <li> <tt>RCU_DONE_TAIL</tt>: Callbacks whose grace periods have elapsed.
843 These callbacks are ready to be invoked.
844 <li> <tt>RCU_WAIT_TAIL</tt>: Callbacks that are waiting for the
845 current grace period.
846 Note that different CPUs can have different ideas about which
847 grace period is current, hence the <tt>->gp_seq</tt> field.
848 <li> <tt>RCU_NEXT_READY_TAIL</tt>: Callbacks waiting for the next
849 grace period to start.
850 <li> <tt>RCU_NEXT_TAIL</tt>: Callbacks that have not yet been
851 associated with a grace period.
855 The <tt>->head</tt> pointer references the first callback or
856 is <tt>NULL</tt> if the list contains no callbacks (which is
857 <i>not</i> the same as being empty).
858 Each element of the <tt>->tails[]</tt> array references the
859 <tt>->next</tt> pointer of the last callback in the corresponding
860 segment of the list, or the list's <tt>->head</tt> pointer if
861 that segment and all previous segments are empty.
862 If the corresponding segment is empty but some previous segment is
863 not empty, then the array element is identical to its predecessor.
864 Older callbacks are closer to the head of the list, and new callbacks
865 are added at the tail.
866 This relationship between the <tt>->head</tt> pointer, the
867 <tt>->tails[]</tt> array, and the callbacks is shown in this
870 </p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
872 </p><p>In this figure, the <tt>->head</tt> pointer references the
874 RCU callback in the list.
875 The <tt>->tails[RCU_DONE_TAIL]</tt> array element references
876 the <tt>->head</tt> pointer itself, indicating that none
877 of the callbacks is ready to invoke.
878 The <tt>->tails[RCU_WAIT_TAIL]</tt> array element references callback
879 CB 2's <tt>->next</tt> pointer, which indicates that
880 CB 1 and CB 2 are both waiting on the current grace period,
881 give or take possible disagreements about exactly which grace period
883 The <tt>->tails[RCU_NEXT_READY_TAIL]</tt> array element
884 references the same RCU callback that <tt>->tails[RCU_WAIT_TAIL]</tt>
885 does, which indicates that there are no callbacks waiting on the next
887 The <tt>->tails[RCU_NEXT_TAIL]</tt> array element references
888 CB 4's <tt>->next</tt> pointer, indicating that all the
889 remaining RCU callbacks have not yet been assigned to an RCU grace
891 Note that the <tt>->tails[RCU_NEXT_TAIL]</tt> array element
892 always references the last RCU callback's <tt>->next</tt> pointer
893 unless the callback list is empty, in which case it references
894 the <tt>->head</tt> pointer.
897 There is one additional important special case for the
898 <tt>->tails[RCU_NEXT_TAIL]</tt> array element: It can be <tt>NULL</tt>
899 when this list is <i>disabled</i>.
900 Lists are disabled when the corresponding CPU is offline or when
901 the corresponding CPU's callbacks are offloaded to a kthread,
902 both of which are described elsewhere.
904 </p><p>CPUs advance their callbacks from the
905 <tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
906 <tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
907 as grace periods advance.
909 </p><p>The <tt>->gp_seq[]</tt> array records grace-period
910 numbers corresponding to the list segments.
911 This is what allows different CPUs to have different ideas as to
912 which is the current grace period while still avoiding premature
913 invocation of their callbacks.
914 In particular, this allows CPUs that go idle for extended periods
915 to determine which of their callbacks are ready to be invoked after
918 </p><p>The <tt>->len</tt> counter contains the number of
919 callbacks in <tt>->head</tt>, and the
920 <tt>->len_lazy</tt> contains the number of those callbacks that
921 are known to only free memory, and whose invocation can therefore
924 <p><b>Important note</b>: It is the <tt>->len</tt> field that
925 determines whether or not there are callbacks associated with
926 this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>->head</tt>
928 The reason for this is that all the ready-to-invoke callbacks
929 (that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
930 all at once at callback-invocation time (<tt>rcu_do_batch</tt>), due
931 to which <tt>->head</tt> may be set to NULL if there are no not-done
932 callbacks remaining in the <tt>rcu_segcblist</tt>.
933 If callback invocation must be postponed, for example, because a
934 high-priority process just woke up on this CPU, then the remaining
935 callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment and
936 <tt>->head</tt> once again points to the start of the segment.
937 In short, the head field can briefly be <tt>NULL</tt> even though the
938 CPU has callbacks present the entire time.
939 Therefore, it is not appropriate to test the <tt>->head</tt> pointer
942 <p>In contrast, the <tt>->len</tt> and <tt>->len_lazy</tt> counts
943 are adjusted only after the corresponding callbacks have been invoked.
944 This means that the <tt>->len</tt> count is zero only if
945 the <tt>rcu_segcblist</tt> structure really is devoid of callbacks.
946 Of course, off-CPU sampling of the <tt>->len</tt> count requires
947 careful use of appropriate synchronization, for example, memory barriers.
948 This synchronization can be a bit subtle, particularly in the case
949 of <tt>rcu_barrier()</tt>.
951 <h3><a name="The rcu_data Structure">
952 The <tt>rcu_data</tt> Structure</a></h3>
954 <p>The <tt>rcu_data</tt> maintains the per-CPU state for the RCU subsystem.
955 The fields in this structure may be accessed only from the corresponding
956 CPU (and from tracing) unless otherwise stated.
957 This structure is the
958 focus of quiescent-state detection and RCU callback queuing.
959 It also tracks its relationship to the corresponding leaf
960 <tt>rcu_node</tt> structure to allow more-efficient
961 propagation of quiescent states up the <tt>rcu_node</tt>
963 Like the <tt>rcu_node</tt> structure, it provides a local
964 copy of the grace-period information to allow for-free
966 access to this information from the corresponding CPU.
967 Finally, this structure records past dyntick-idle state
968 for the corresponding CPU and also tracks statistics.
970 </p><p>The <tt>rcu_data</tt> structure's fields are discussed,
971 singly and in groups, in the following sections.
973 <h5>Connection to Other Data Structures</h5>
975 <p>This portion of the <tt>rcu_data</tt> structure is declared
980 2 struct rcu_node *mynode;
981 3 unsigned long grpmask;
985 <p>The <tt>->cpu</tt> field contains the number of the
986 corresponding CPU and the <tt>->mynode</tt> field references the
987 corresponding <tt>rcu_node</tt> structure.
988 The <tt>->mynode</tt> is used to propagate quiescent states
989 up the combining tree.
990 These two fields are constant and therefore do not require synchronization.
992 <p>The <tt>->grpmask</tt> field indicates the bit in
993 the <tt>->mynode->qsmask</tt> corresponding to this
994 <tt>rcu_data</tt> structure, and is also used when propagating
996 The <tt>->beenonline</tt> flag is set whenever the corresponding
997 CPU comes online, which means that the debugfs tracing need not dump
998 out any <tt>rcu_data</tt> structure for which this flag is not set.
1000 <h5>Quiescent-State and Grace-Period Tracking</h5>
1002 <p>This portion of the <tt>rcu_data</tt> structure is declared
1006 1 unsigned long gp_seq;
1007 2 unsigned long gp_seq_needed;
1009 4 bool core_needs_qs;
1013 <p>The <tt>->gp_seq</tt> field is the counterpart of the field of the same
1014 name in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures. The
1015 <tt>->gp_seq_needed</tt> field is the counterpart of the field of the same
1016 name in the rcu_node</tt> structure.
1017 They may each lag up to one behind their <tt>rcu_node</tt>
1018 counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
1019 <tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
1020 arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
1021 will catch up upon exit from dyntick-idle mode).
1022 If the lower two bits of a given <tt>rcu_data</tt> structure's
1023 <tt>->gp_seq</tt> are zero, then this <tt>rcu_data</tt>
1024 structure believes that RCU is idle.
1027 <tr><th> </th></tr>
1028 <tr><th align="left">Quick Quiz:</th></tr>
1030 All this replication of the grace period numbers can only cause
1032 Why not just keep a global sequence number and be done with it???
1034 <tr><th align="left">Answer:</th></tr>
1035 <tr><td bgcolor="#ffffff"><font color="ffffff">
1036 Because if there was only a single global sequence
1037 numbers, there would need to be a single global lock to allow
1038 safely accessing and updating it.
1039 And if we are not going to have a single global lock, we need
1040 to carefully manage the numbers on a per-node basis.
1041 Recall from the answer to a previous Quick Quiz that the consequences
1042 of applying a previously sampled quiescent state to the wrong
1043 grace period are quite severe.
1045 <tr><td> </td></tr>
1048 <p>The <tt>->cpu_no_qs</tt> flag indicates that the
1049 CPU has not yet passed through a quiescent state,
1050 while the <tt>->core_needs_qs</tt> flag indicates that the
1051 RCU core needs a quiescent state from the corresponding CPU.
1052 The <tt>->gpwrap</tt> field indicates that the corresponding
1053 CPU has remained idle for so long that the
1054 <tt>gp_seq</tt> counter is in danger of overflow, which
1055 will cause the CPU to disregard the values of its counters on
1056 its next exit from idle.
1058 <h5>RCU Callback Handling</h5>
1060 <p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
1061 the same CPU that registered them.
1062 This is strictly a cache-locality optimization: callbacks can and
1063 do get invoked on CPUs other than the one that registered them.
1064 After all, if the CPU that registered a given callback has gone
1065 offline before the callback can be invoked, there really is no other
1068 </p><p>This portion of the <tt>rcu_data</tt> structure is declared
1072 1 struct rcu_segcblist cblist;
1073 2 long qlen_last_fqs_check;
1074 3 unsigned long n_cbs_invoked;
1075 4 unsigned long n_nocbs_invoked;
1076 5 unsigned long n_cbs_orphaned;
1077 6 unsigned long n_cbs_adopted;
1078 7 unsigned long n_force_qs_snap;
1082 <p>The <tt>->cblist</tt> structure is the segmented callback list
1084 The CPU advances the callbacks in its <tt>rcu_data</tt> structure
1085 whenever it notices that another RCU grace period has completed.
1086 The CPU detects the completion of an RCU grace period by noticing
1087 that the value of its <tt>rcu_data</tt> structure's
1088 <tt>->gp_seq</tt> field differs from that of its leaf
1089 <tt>rcu_node</tt> structure.
1090 Recall that each <tt>rcu_node</tt> structure's
1091 <tt>->gp_seq</tt> field is updated at the beginnings and ends of each
1095 The <tt>->qlen_last_fqs_check</tt> and
1096 <tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent
1097 states from <tt>call_rcu()</tt> and friends when callback
1098 lists grow excessively long.
1100 </p><p>The <tt>->n_cbs_invoked</tt>,
1101 <tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt>
1102 fields count the number of callbacks invoked,
1103 sent to other CPUs when this CPU goes offline,
1104 and received from other CPUs when those other CPUs go offline.
1105 The <tt>->n_nocbs_invoked</tt> is used when the CPU's callbacks
1106 are offloaded to a kthread.
1109 Finally, the <tt>->blimit</tt> counter is the maximum number of
1110 RCU callbacks that may be invoked at a given time.
1112 <h5>Dyntick-Idle Handling</h5>
1114 <p>This portion of the <tt>rcu_data</tt> structure is declared
1118 1 int dynticks_snap;
1119 2 unsigned long dynticks_fqs;
1122 The <tt>->dynticks_snap</tt> field is used to take a snapshot
1123 of the corresponding CPU's dyntick-idle state when forcing
1124 quiescent states, and is therefore accessed from other CPUs.
1125 Finally, the <tt>->dynticks_fqs</tt> field is used to
1126 count the number of times this CPU is determined to be in
1127 dyntick-idle state, and is used for tracing and debugging purposes.
1130 This portion of the rcu_data structure is declared as follows:
1133 1 long dynticks_nesting;
1134 2 long dynticks_nmi_nesting;
1135 3 atomic_t dynticks;
1136 4 bool rcu_need_heavy_qs;
1137 5 bool rcu_urgent_qs;
1140 <p>These fields in the rcu_data structure maintain the per-CPU dyntick-idle
1141 state for the corresponding CPU.
1142 The fields may be accessed only from the corresponding CPU (and from tracing)
1143 unless otherwise stated.
1145 <p>The <tt>->dynticks_nesting</tt> field counts the
1146 nesting depth of process execution, so that in normal circumstances
1147 this counter has value zero or one.
1148 NMIs, irqs, and tracers are counted by the <tt>->dynticks_nmi_nesting</tt>
1150 Because NMIs cannot be masked, changes to this variable have to be
1151 undertaken carefully using an algorithm provided by Andy Lutomirski.
1152 The initial transition from idle adds one, and nested transitions
1153 add two, so that a nesting level of five is represented by a
1154 <tt>->dynticks_nmi_nesting</tt> value of nine.
1155 This counter can therefore be thought of as counting the number
1156 of reasons why this CPU cannot be permitted to enter dyntick-idle
1157 mode, aside from process-level transitions.
1159 <p>However, it turns out that when running in non-idle kernel context,
1160 the Linux kernel is fully capable of entering interrupt handlers that
1161 never exit and perhaps also vice versa.
1162 Therefore, whenever the <tt>->dynticks_nesting</tt> field is
1163 incremented up from zero, the <tt>->dynticks_nmi_nesting</tt> field
1164 is set to a large positive number, and whenever the
1165 <tt>->dynticks_nesting</tt> field is decremented down to zero,
1166 the the <tt>->dynticks_nmi_nesting</tt> field is set to zero.
1167 Assuming that the number of misnested interrupts is not sufficient
1168 to overflow the counter, this approach corrects the
1169 <tt>->dynticks_nmi_nesting</tt> field every time the corresponding
1170 CPU enters the idle loop from process context.
1172 </p><p>The <tt>->dynticks</tt> field counts the corresponding
1173 CPU's transitions to and from either dyntick-idle or user mode, so
1174 that this counter has an even value when the CPU is in dyntick-idle
1175 mode or user mode and an odd value otherwise. The transitions to/from
1176 user mode need to be counted for user mode adaptive-ticks support
1177 (see timers/NO_HZ.txt).
1179 </p><p>The <tt>->rcu_need_heavy_qs</tt> field is used
1180 to record the fact that the RCU core code would really like to
1181 see a quiescent state from the corresponding CPU, so much so that
1182 it is willing to call for heavy-weight dyntick-counter operations.
1183 This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
1184 code, which provide a momentary idle sojourn in response.
1186 </p><p>Finally, the <tt>->rcu_urgent_qs</tt> field is used to record
1187 the fact that the RCU core code would really like to see a quiescent state from
1188 the corresponding CPU, with the various other fields indicating just how badly
1189 RCU wants this quiescent state.
1190 This flag is checked by RCU's context-switch path
1191 (<tt>rcu_note_context_switch</tt>) and the cond_resched code.
1194 <tr><th> </th></tr>
1195 <tr><th align="left">Quick Quiz:</th></tr>
1197 Why not simply combine the <tt>->dynticks_nesting</tt>
1198 and <tt>->dynticks_nmi_nesting</tt> counters into a
1199 single counter that just counts the number of reasons that
1200 the corresponding CPU is non-idle?
1202 <tr><th align="left">Answer:</th></tr>
1203 <tr><td bgcolor="#ffffff"><font color="ffffff">
1204 Because this would fail in the presence of interrupts whose
1205 handlers never return and of handlers that manage to return
1206 from a made-up interrupt.
1208 <tr><td> </td></tr>
1211 <p>Additional fields are present for some special-purpose
1212 builds, and are discussed separately.
1214 <h3><a name="The rcu_head Structure">
1215 The <tt>rcu_head</tt> Structure</a></h3>
1217 <p>Each <tt>rcu_head</tt> structure represents an RCU callback.
1218 These structures are normally embedded within RCU-protected data
1219 structures whose algorithms use asynchronous grace periods.
1220 In contrast, when using algorithms that block waiting for RCU grace periods,
1221 RCU users need not provide <tt>rcu_head</tt> structures.
1223 </p><p>The <tt>rcu_head</tt> structure has fields as follows:
1226 1 struct rcu_head *next;
1227 2 void (*func)(struct rcu_head *head);
1230 <p>The <tt>->next</tt> field is used
1231 to link the <tt>rcu_head</tt> structures together in the
1232 lists within the <tt>rcu_data</tt> structures.
1233 The <tt>->func</tt> field is a pointer to the function
1234 to be called when the callback is ready to be invoked, and
1235 this function is passed a pointer to the <tt>rcu_head</tt>
1237 However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt>
1238 field to record the offset of the <tt>rcu_head</tt>
1239 structure within the enclosing RCU-protected data structure.
1241 </p><p>Both of these fields are used internally by RCU.
1242 From the viewpoint of RCU users, this structure is an
1243 opaque “cookie”.
1246 <tr><th> </th></tr>
1247 <tr><th align="left">Quick Quiz:</th></tr>
1249 Given that the callback function <tt>->func</tt>
1250 is passed a pointer to the <tt>rcu_head</tt> structure,
1251 how is that function supposed to find the beginning of the
1252 enclosing RCU-protected data structure?
1254 <tr><th align="left">Answer:</th></tr>
1255 <tr><td bgcolor="#ffffff"><font color="ffffff">
1256 In actual practice, there is a separate callback function per
1257 type of RCU-protected data structure.
1258 The callback function can therefore use the <tt>container_of()</tt>
1259 macro in the Linux kernel (or other pointer-manipulation facilities
1260 in other software environments) to find the beginning of the
1261 enclosing structure.
1263 <tr><td> </td></tr>
1266 <h3><a name="RCU-Specific Fields in the task_struct Structure">
1267 RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
1269 <p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
1270 additional fields in the <tt>task_struct</tt> structure:
1273 1 #ifdef CONFIG_PREEMPT_RCU
1274 2 int rcu_read_lock_nesting;
1275 3 union rcu_special rcu_read_unlock_special;
1276 4 struct list_head rcu_node_entry;
1277 5 struct rcu_node *rcu_blocked_node;
1278 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
1279 7 #ifdef CONFIG_TASKS_RCU
1280 8 unsigned long rcu_tasks_nvcsw;
1281 9 bool rcu_tasks_holdout;
1282 10 struct list_head rcu_tasks_holdout_list;
1283 11 int rcu_tasks_idle_cpu;
1284 12 #endif /* #ifdef CONFIG_TASKS_RCU */
1287 <p>The <tt>->rcu_read_lock_nesting</tt> field records the
1288 nesting level for RCU read-side critical sections, and
1289 the <tt>->rcu_read_unlock_special</tt> field is a bitmask
1290 that records special conditions that require <tt>rcu_read_unlock()</tt>
1291 to do additional work.
1292 The <tt>->rcu_node_entry</tt> field is used to form lists of
1293 tasks that have blocked within preemptible-RCU read-side critical
1294 sections and the <tt>->rcu_blocked_node</tt> field references
1295 the <tt>rcu_node</tt> structure whose list this task is a member of,
1296 or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
1297 read-side critical section.
1299 <p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of
1300 voluntary context switches that this task had undergone at the
1301 beginning of the current tasks-RCU grace period,
1302 <tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU
1303 grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt>
1304 is a list element enqueuing this task on the holdout list,
1305 and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this
1306 idle task is running, but only if the task is currently running,
1307 that is, if the CPU is currently idle.
1309 <h3><a name="Accessor Functions">
1310 Accessor Functions</a></h3>
1312 <p>The following listing shows the
1313 <tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt> and
1314 <tt>rcu_for_each_leaf_node()</tt> function and macros:
1317 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
1319 3 return &rsp->node[0];
1322 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
1323 7 for ((rnp) = &(rsp)->node[0]; \
1324 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
1326 10 #define rcu_for_each_leaf_node(rsp, rnp) \
1327 11 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
1328 12 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
1331 <p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
1332 first element of the specified <tt>rcu_state</tt> structure's
1333 <tt>->node[]</tt> array, which is the root <tt>rcu_node</tt>
1336 </p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
1337 macro takes advantage of the layout of the <tt>rcu_node</tt>
1338 structures in the <tt>rcu_state</tt> structure's
1339 <tt>->node[]</tt> array, performing a breadth-first traversal by
1340 simply traversing the array in order.
1341 Similarly, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
1342 the last part of the array, thus traversing only the leaf
1343 <tt>rcu_node</tt> structures.
1346 <tr><th> </th></tr>
1347 <tr><th align="left">Quick Quiz:</th></tr>
1350 <tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
1351 contains only a single node?
1353 <tr><th align="left">Answer:</th></tr>
1354 <tr><td bgcolor="#ffffff"><font color="ffffff">
1355 In the single-node case,
1356 <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
1358 <tr><td> </td></tr>
1361 <h3><a name="Summary">
1364 So the state of RCU is represented by an <tt>rcu_state</tt> structure,
1365 which contains a combining tree of <tt>rcu_node</tt> and
1366 <tt>rcu_data</tt> structures.
1367 Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
1368 state is tracked by dynticks-related fields in the <tt>rcu_data</tt> structure.
1370 If you made it this far, you are well prepared to read the code
1371 walkthroughs in the other articles in this series.
1373 <h3><a name="Acknowledgments">
1374 Acknowledgments</a></h3>
1376 I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
1377 Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
1378 for helping me get this document into a more human-readable state.
1380 <h3><a name="Legal Statement">
1381 Legal Statement</a></h3>
1383 <p>This work represents the view of the author and does not necessarily
1384 represent the view of IBM.
1386 </p><p>Linux is a registered trademark of Linus Torvalds.
1388 </p><p>Other company, product, and service names may be trademarks or
1389 service marks of others.