A Tour Through TREE_RCU’s Data Structures [LWN.net] — The Linux Kernel 6.14.0-gnu-rt3-parabola1-1-rt documentation

# A Tour Through TREE_RCU’s Data Structures [LWN.net]

December 18, 2016

This article was contributed by Paul E. McKenney

## Introduction

This document describes RCU’s major data structures and their relationship to each other.

## Data-Structure Relationships

RCU is for all intents and purposes a large state machine, and its data structures maintain the state in such a way as to allow RCU readers to execute extremely quickly, while also processing the RCU grace periods requested by updaters in an efficient and extremely scalable fashion. The efficiency and scalability of RCU updaters is provided primarily by a combining tree, as shown below:

This diagram shows an enclosing `rcu_state` structure containing a tree of `rcu_node` structures. Each leaf node of the `rcu_node` tree has up to 16 `rcu_data` structures associated with it, so that there are `NR_CPUS` number of `rcu_data` structures, one for each possible CPU. This structure is adjusted at boot time, if needed, to handle the common case where `nr_cpu_ids` is much less than `NR_CPUs`. For example, a number of Linux distributions set `NR_CPUs=4096`, which results in a three-level `rcu_node` tree. If the actual hardware has only 16 CPUs, RCU will adjust itself at boot time, resulting in an `rcu_node` tree with only a single node.

The purpose of this combining tree is to allow per-CPU events such as quiescent states, dyntick-idle transitions, and CPU hotplug operations to be processed efficiently and scalably. Quiescent states are recorded by the per-CPU `rcu_data` structures, and other events are recorded by the leaf-level `rcu_node` structures. All of these events are combined at each level of the tree until finally grace periods are completed at the tree’s root `rcu_node` structure. A grace period can be completed at the root once every CPU (or, in the case of `CONFIG_PREEMPT_RCU`, task) has passed through a quiescent state. Once a grace period has completed, record of that fact is propagated back down the tree.

As can be seen from the diagram, on a 64-bit system a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout of 64 at the root and a fanout of 16 at the leaves.

Quick Quiz:
Why isn’t the fanout at the leaves also 64?
Answer:
Because there are more types of events that affect the leaf-level `rcu_node` structures than further up the tree. Therefore, if the leaf `rcu_node` structures have fanout of 64, the contention on these structures’ `->structures` becomes excessive. Experimentation on a wide variety of systems has shown that a fanout of 16 works well for the leaves of the `rcu_node` tree. Of course, further experience with systems having hundreds or thousands of CPUs may demonstrate that the fanout for the non-leaf `rcu_node` structures must also be reduced. Such reduction can be easily carried out when and if it proves necessary. In the meantime, if you are using such a system and running into contention problems on the non-leaf `rcu_node` structures, you may use the `CONFIG_RCU_FANOUT` kernel configuration parameter to reduce the non-leaf fanout as needed. Kernels built for systems with strong NUMA characteristics might also need to adjust `CONFIG_RCU_FANOUT` so that the domains of the `rcu_node` structures align with hardware boundaries. However, there has thus far been no need for this.

If your system has more than 1,024 CPUs (or more than 512 CPUs on a 32-bit system), then RCU will automatically add more levels to the tree. For example, if you are crazy enough to build a 64-bit system with 65,536 CPUs, RCU would configure the `rcu_node` tree as follows:

RCU currently permits up to a four-level tree, which on a 64-bit system accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for 32-bit systems. On the other hand, you can set both `CONFIG_RCU_FANOUT` and `CONFIG_RCU_FANOUT_LEAF` to be as small as 2, which would result in a 16-CPU test using a 4-level tree. This can be useful for testing large-system capabilities on small test machines.

This multi-level combining tree allows us to get most of the performance and scalability benefits of partitioning, even though RCU grace-period detection is inherently a global operation. The trick here is that only the last CPU to report a quiescent state into a given `rcu_node` structure need advance to the `rcu_node` structure at the next level up the tree. This means that at the leaf-level `rcu_node` structure, only one access out of sixteen will progress up the tree. For the internal `rcu_node` structures, the situation is even more extreme: Only one access out of sixty-four will progress up the tree. Because the vast majority of the CPUs do not progress up the tree, the lock contention remains roughly constant up the tree. No matter how many CPUs there are in the system, at most 64 quiescent-state reports per grace period will progress all the way to the root `rcu_node` structure, thus ensuring that the lock contention on that root `rcu_node` structure remains acceptably low.

In effect, the combining tree acts like a big shock absorber, keeping lock contention under control at all tree levels regardless of the level of loading on the system.

RCU updaters wait for normal grace periods by registering RCU callbacks, either directly via `call_rcu()` or indirectly via `synchronize_rcu()` and friends. RCU callbacks are represented by `rcu_head` structures, which are queued on `rcu_data` structures while they are waiting for a grace period to elapse, as shown in the following figure:

This figure shows how `TREE_RCU`’s and `PREEMPT_RCU`’s major data structures are related. Lesser data structures will be introduced with the algorithms that make use of them.

Note that each of the data structures in the above figure has its own synchronization:

1. Each `rcu_state` structures has a lock and a mutex, and some fields are protected by the corresponding root `rcu_node` structure’s lock.

2. Each `rcu_node` structure has a spinlock.

3. The fields in `rcu_data` are private to the corresponding CPU, although a few can be read and written by other CPUs.

It is important to note that different data structures can have very different ideas about the state of RCU at any given time. For but one example, awareness of the start or end of a given RCU grace period propagates slowly through the data structures. This slow propagation is absolutely necessary for RCU to have good read-side performance. If this balkanized implementation seems foreign to you, one useful trick is to consider each instance of these data structures to be a different person, each having the usual slightly different view of reality.

The general role of each of these data structures is as follows:

1. `rcu_state`: This structure forms the interconnection between the `rcu_node` and `rcu_data` structures, tracks grace periods, serves as short-term repository for callbacks orphaned by CPU-hotplug events, maintains `rcu_barrier()` state, tracks expedited grace-period state, and maintains state used to force quiescent states when grace periods extend too long,

2. `rcu_node`: This structure forms the combining tree that propagates quiescent-state information from the leaves to the root, and also propagates grace-period information from the root to the leaves. It provides local copies of the grace-period state in order to allow this information to be accessed in a synchronized manner without suffering the scalability limitations that would otherwise be imposed by global locking. In `CONFIG_PREEMPT_RCU` kernels, it manages the lists of tasks that have blocked while in their current RCU read-side critical section. In `CONFIG_PREEMPT_RCU` with `CONFIG_RCU_BOOST`, it manages the per-`rcu_node` priority-boosting kernel threads (kthreads) and state. Finally, it records CPU-hotplug state in order to determine which CPUs should be ignored during a given grace period.

3. `rcu_data`: This per-CPU structure is the focus of quiescent-state detection and RCU callback queuing. It also tracks its relationship to the corresponding leaf `rcu_node` structure to allow more-efficient propagation of quiescent states up the `rcu_node` combining tree. Like the `rcu_node` structure, it provides a local copy of the grace-period information to allow for-free synchronized access to this information from the corresponding CPU. Finally, this structure records past dyntick-idle state for the corresponding CPU and also tracks statistics.

4. `rcu_head`: This structure represents RCU callbacks, and is the only structure allocated and managed by RCU users. The `rcu_head` structure is normally embedded within the RCU-protected data structure.

If all you wanted from this article was a general notion of how RCU’s data structures are related, you are done. Otherwise, each of the following sections give more details on the `rcu_state`, `rcu_node` and `rcu_data` data structures.

### The `rcu_state` Structure

The `rcu_state` structure is the base structure that represents the state of RCU in the system. This structure forms the interconnection between the `rcu_node` and `rcu_data` structures, tracks grace periods, contains the lock used to synchronize with CPU-hotplug events, and maintains state used to force quiescent states when grace periods extend too long,

A few of the `rcu_state` structure’s fields are discussed, singly and in groups, in the following sections. The more specialized fields are covered in the discussion of their use.

#### Relationship to rcu_node and rcu_data Structures

This portion of the `rcu_state` structure is declared as follows:

    1   struct rcu_node node[NUM_RCU_NODES];
    2   struct rcu_node *level[NUM_RCU_LVLS + 1];
    3   struct rcu_data __percpu *rda;

Quick Quiz:
Wait a minute! You said that the `rcu_node` structures formed a tree, but they are declared as a flat array! What gives?
Answer:
The tree is laid out in the array. The first node In the array is the head, the next set of nodes in the array are children of the head node, and so on until the last set of nodes in the array are the leaves. See the following diagrams to see how this works.

The `rcu_node` tree is embedded into the `->node[]` array as shown in the following figure:

One interesting consequence of this mapping is that a breadth-first traversal of the tree is implemented as a simple linear scan of the array, which is in fact what the `rcu_for_each_node_breadth_first()` macro does. This macro is used at the beginning and ends of grace periods.

Each entry of the `->level` array references the first `rcu_node` structure on the corresponding level of the tree, for example, as shown below:

The zeroth element of the array references the root `rcu_node` structure, the first element references the first child of the root `rcu_node`, and finally the second element references the first leaf `rcu_node` structure.

For whatever it is worth, if you draw the tree to be tree-shaped rather than array-shaped, it is easy to draw a planar representation:

Finally, the `->rda` field references a per-CPU pointer to the corresponding CPU’s `rcu_data` structure.

All of these fields are constant once initialization is complete, and therefore need no protection.

#### Grace-Period Tracking

This portion of the `rcu_state` structure is declared as follows:

    1   unsigned long gp_seq;

RCU grace periods are numbered, and the `->gp_seq` field contains the current grace-period sequence number. The bottom two bits are the state of the current grace period, which can be zero for not yet started or one for in progress. In other words, if the bottom two bits of `->gp_seq` are zero, then RCU is idle. Any other value in the bottom two bits indicates that something is broken. This field is protected by the root `rcu_node` structure’s `->lock` field.

There are `->gp_seq` fields in the `rcu_node` and `rcu_data` structures as well. The fields in the `rcu_state` structure represent the most current value, and those of the other structures are compared in order to detect the beginnings and ends of grace periods in a distributed fashion. The values flow from `rcu_state` to `rcu_node` (down the tree from the root to the leaves) to `rcu_data`.

#### Miscellaneous

This portion of the `rcu_state` structure is declared as follows:

    1   unsigned long gp_max;
    2   char abbr;
    3   char *name;

The `->gp_max` field tracks the duration of the longest grace period in jiffies. It is protected by the root `rcu_node`’s `->lock`.

The `->name` and `->abbr` fields distinguish between preemptible RCU (“rcu_preempt” and “p”) and non-preemptible RCU (“rcu_sched” and “s”). These fields are used for diagnostic and tracing purposes.

### The `rcu_node` Structure

The `rcu_node` structures form the combining tree that propagates quiescent-state information from the leaves to the root and also that propagates grace-period information from the root down to the leaves. They provides local copies of the grace-period state in order to allow this information to be accessed in a synchronized manner without suffering the scalability limitations that would otherwise be imposed by global locking. In `CONFIG_PREEMPT_RCU` kernels, they manage the lists of tasks that have blocked while in their current RCU read-side critical section. In `CONFIG_PREEMPT_RCU` with `CONFIG_RCU_BOOST`, they manage the per-`rcu_node` priority-boosting kernel threads (kthreads) and state. Finally, they record CPU-hotplug state in order to determine which CPUs should be ignored during a given grace period.

The `rcu_node` structure’s fields are discussed, singly and in groups, in the following sections.

#### Connection to Combining Tree

This portion of the `rcu_node` structure is declared as follows:

    1   struct rcu_node *parent;
    2   u8 level;
    3   u8 grpnum;
    4   unsigned long grpmask;
    5   int grplo;
    6   int grphi;

The `->parent` pointer references the `rcu_node` one level up in the tree, and is `NULL` for the root `rcu_node`. The RCU implementation makes heavy use of this field to push quiescent states up the tree. The `->level` field gives the level in the tree, with the root being at level zero, its children at level one, and so on. The `->grpnum` field gives this node’s position within the children of its parent, so this number can range between 0 and 31 on 32-bit systems and between 0 and 63 on 64-bit systems. The `->level` and `->grpnum` fields are used only during initialization and for tracing. The `->grpmask` field is the bitmask counterpart of `->grpnum`, and therefore always has exactly one bit set. This mask is used to clear the bit corresponding to this `rcu_node` structure in its parent’s bitmasks, which are described later. Finally, the `->grplo` and `->grphi` fields contain the lowest and highest numbered CPU served by this `rcu_node` structure, respectively.

All of these fields are constant, and thus do not require any synchronization.

#### Synchronization

This field of the `rcu_node` structure is declared as follows:

    1   raw_spinlock_t lock;

This field is used to protect the remaining fields in this structure, unless otherwise stated. That said, all of the fields in this structure can be accessed without locking for tracing purposes. Yes, this can result in confusing traces, but better some tracing confusion than to be heisenbugged out of existence.

#### Grace-Period Tracking

This portion of the `rcu_node` structure is declared as follows:

    1   unsigned long gp_seq;
    2   unsigned long gp_seq_needed;

The `rcu_node` structures’ `->gp_seq` fields are the counterparts of the field of the same name in the `rcu_state` structure. They each may lag up to one step behind their `rcu_state` counterpart. If the bottom two bits of a given `rcu_node` structure’s `->gp_seq` field is zero, then this `rcu_node` structure believes that RCU is idle.

The `>gp_seq` field of each `rcu_node` structure is updated at the beginning and the end of each grace period.

The `->gp_seq_needed` fields record the furthest-in-the-future grace period request seen by the corresponding `rcu_node` structure. The request is considered fulfilled when the value of the `->gp_seq` field equals or exceeds that of the `->gp_seq_needed` field.

Quick Quiz:
Suppose that this `rcu_node` structure doesn’t see a request for a very long time. Won’t wrapping of the `->gp_seq` field cause problems?
Answer:
No, because if the `->gp_seq_needed` field lags behind the `->gp_seq` field, the `->gp_seq_needed` field will be updated at the end of the grace period. Modulo-arithmetic comparisons therefore will always get the correct answer, even with wrapping.

#### Quiescent-State Tracking

These fields manage the propagation of quiescent states up the combining tree.

This portion of the `rcu_node` structure has fields as follows:

    1   unsigned long qsmask;
    2   unsigned long expmask;
    3   unsigned long qsmaskinit;
    4   unsigned long expmaskinit;

The `->qsmask` field tracks which of this `rcu_node` structure’s children still need to report quiescent states for the current normal grace period. Such children will have a value of 1 in their corresponding bit. Note that the leaf `rcu_node` structures should be thought of as having `rcu_data` structures as their children. Similarly, the `->expmask` field tracks which of this `rcu_node` structure’s children still need to report quiescent states for the current expedited grace period. An expedited grace period has the same conceptual properties as a normal grace period, but the expedited implementation accepts extreme CPU overhead to obtain much lower grace-period latency, for example, consuming a few tens of microseconds worth of CPU time to reduce grace-period duration from milliseconds to tens of microseconds. The `->qsmaskinit` field tracks which of this `rcu_node` structure’s children cover for at least one online CPU. This mask is used to initialize `->qsmask`, and `->expmaskinit` is used to initialize `->expmask` and the beginning of the normal and expedited grace periods, respectively.

Quick Quiz:
Why are these bitmasks protected by locking? Come on, haven’t you heard of atomic instructions???
Answer:
Lockless grace-period computation! Such a tantalizing possibility! But consider the following sequence of events: CPU 0 has been in dyntick-idle mode for quite some time. When it wakes up, it notices that the current RCU grace period needs it to report in, so it sets a flag where the scheduling clock interrupt will find it. Meanwhile, CPU 1 is running `force_quiescent_state()`, and notices that CPU 0 has been in dyntick idle mode, which qualifies as an extended quiescent state. CPU 0’s scheduling clock interrupt fires in the middle of an RCU read-side critical section, and notices that the RCU core needs something, so commences RCU softirq processing. CPU 0’s softirq handler executes and is just about ready to report its quiescent state up the `rcu_node` tree. But CPU 1 beats it to the punch, completing the current grace period and starting a new one. CPU 0 now reports its quiescent state for the wrong grace period. That grace period might now end before the RCU read-side critical section. If that happens, disaster will ensue. So the locking is absolutely required in order to coordinate clearing of the bits with updating of the grace-period sequence number in `->gp_seq`.

#### Blocked-Task Management

`PREEMPT_RCU` allows tasks to be preempted in the midst of their RCU read-side critical sections, and these tasks must be tracked explicitly. The details of exactly why and how they are tracked will be covered in a separate article on RCU read-side processing. For now, it is enough to know that the `rcu_node` structure tracks them.

    1   struct list_head blkd_tasks;
    2   struct list_head *gp_tasks;
    3   struct list_head *exp_tasks;
    4   bool wait_blkd_tasks;

The `->blkd_tasks` field is a list header for the list of blocked and preempted tasks. As tasks undergo context switches within RCU read-side critical sections, their `task_struct` structures are enqueued (via the `task_struct`’s `->rcu_node_entry` field) onto the head of the `->blkd_tasks` list for the leaf `rcu_node` structure corresponding to the CPU on which the outgoing context switch executed. As these tasks later exit their RCU read-side critical sections, they remove themselves from the list. This list is therefore in reverse time order, so that if one of the tasks is blocking the current grace period, all subsequent tasks must also be blocking that same grace period. Therefore, a single pointer into this list suffices to track all tasks blocking a given grace period. That pointer is stored in `->gp_tasks` for normal grace periods and in `->exp_tasks` for expedited grace periods. These last two fields are `NULL` if either there is no grace period in flight or if there are no blocked tasks preventing that grace period from completing. If either of these two pointers is referencing a task that removes itself from the `->blkd_tasks` list, then that task must advance the pointer to the next task on the list, or set the pointer to `NULL` if there are no subsequent tasks on the list.

For example, suppose that tasks T1, T2, and T3 are all hard-affinitied to the largest-numbered CPU in the system. Then if task T1 blocked in an RCU read-side critical section, then an expedited grace period started, then task T2 blocked in an RCU read-side critical section, then a normal grace period started, and finally task 3 blocked in an RCU read-side critical section, then the state of the last leaf `rcu_node` structure’s blocked-task list would be as shown below:

Task T1 is blocking both grace periods, task T2 is blocking only the normal grace period, and task T3 is blocking neither grace period. Note that these tasks will not remove themselves from this list immediately upon resuming execution. They will instead remain on the list until they execute the outermost `rcu_read_unlock()` that ends their RCU read-side critical section.

The `->wait_blkd_tasks` field indicates whether or not the current grace period is waiting on a blocked task.

#### Sizing the `rcu_node` Array

The `rcu_node` array is sized via a series of C-preprocessor expressions as follows:

     1 #ifdef CONFIG_RCU_FANOUT
     2 #define RCU_FANOUT CONFIG_RCU_FANOUT
     3 #else
     4 # ifdef CONFIG_64BIT
     5 # define RCU_FANOUT 64
     6 # else
     7 # define RCU_FANOUT 32
     8 # endif
     9 #endif
    10
    11 #ifdef CONFIG_RCU_FANOUT_LEAF
    12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
    13 #else
    14 # ifdef CONFIG_64BIT
    15 # define RCU_FANOUT_LEAF 64
    16 # else
    17 # define RCU_FANOUT_LEAF 32
    18 # endif
    19 #endif
    20
    21 #define RCU_FANOUT_1        (RCU_FANOUT_LEAF)
    22 #define RCU_FANOUT_2        (RCU_FANOUT_1 * RCU_FANOUT)
    23 #define RCU_FANOUT_3        (RCU_FANOUT_2 * RCU_FANOUT)
    24 #define RCU_FANOUT_4        (RCU_FANOUT_3 * RCU_FANOUT)
    25
    26 #if NR_CPUS <= RCU_FANOUT_1
    27 #  define RCU_NUM_LVLS        1
    28 #  define NUM_RCU_LVL_0        1
    29 #  define NUM_RCU_NODES        NUM_RCU_LVL_0
    30 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
    31 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
    32 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
    33 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
    34 #elif NR_CPUS <= RCU_FANOUT_2
    35 #  define RCU_NUM_LVLS        2
    36 #  define NUM_RCU_LVL_0        1
    37 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
    38 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
    39 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
    40 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
    41 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
    42 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
    43 #elif NR_CPUS <= RCU_FANOUT_3
    44 #  define RCU_NUM_LVLS        3
    45 #  define NUM_RCU_LVL_0        1
    46 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
    47 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
    48 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
    49 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
    50 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
    51 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
    52 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
    53 #elif NR_CPUS <= RCU_FANOUT_4
    54 #  define RCU_NUM_LVLS        4
    55 #  define NUM_RCU_LVL_0        1
    56 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
    57 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
    58 #  define NUM_RCU_LVL_3        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
    59 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
    60 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
    61 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
    62 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
    63 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
    64 #else
    65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
    66 #endif

The maximum number of levels in the `rcu_node` structure is currently limited to four, as specified by lines 21-24 and the structure of the subsequent “if” statement. For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which should be sufficient for the next few years at least. For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which should see us through the next decade or so. This four-level tree also allows kernels built with `CONFIG_RCU_FANOUT=8` to support up to 4096 CPUs, which might be useful in very large systems having eight CPUs per socket (but please note that no one has yet shown any measurable performance degradation due to misaligned socket and `rcu_node` boundaries). In addition, building kernels with a full four levels of `rcu_node` tree permits better testing of RCU’s combining-tree code.

The `RCU_FANOUT` symbol controls how many children are permitted at each non-leaf level of the `rcu_node` tree. If the `CONFIG_RCU_FANOUT` Kconfig option is not specified, it is set based on the word size of the system, which is also the Kconfig default.

The `RCU_FANOUT_LEAF` symbol controls how many CPUs are handled by each leaf `rcu_node` structure. Experience has shown that allowing a given leaf `rcu_node` structure to handle 64 CPUs, as permitted by the number of bits in the `->qsmask` field on a 64-bit system, results in excessive contention for the leaf `rcu_node` structures’ `->lock` fields. The number of CPUs per leaf `rcu_node` structure is therefore limited to 16 given the default value of `CONFIG_RCU_FANOUT_LEAF`. If `CONFIG_RCU_FANOUT_LEAF` is unspecified, the value selected is based on the word size of the system, just as for `CONFIG_RCU_FANOUT`. Lines 11-19 perform this computation.

Lines 21-24 compute the maximum number of CPUs supported by a single-level (which contains a single `rcu_node` structure), two-level, three-level, and four-level `rcu_node` tree, respectively, given the fanout specified by `RCU_FANOUT` and `RCU_FANOUT_LEAF`. These numbers of CPUs are retained in the `RCU_FANOUT_1`, `RCU_FANOUT_2`, `RCU_FANOUT_3`, and `RCU_FANOUT_4` C-preprocessor variables, respectively.

These variables are used to control the C-preprocessor `#if` statement spanning lines 26-66 that computes the number of `rcu_node` structures required for each level of the tree, as well as the number of levels required. The number of levels is placed in the `NUM_RCU_LVLS` C-preprocessor variable by lines 27, 35, 44, and 54. The number of `rcu_node` structures for the topmost level of the tree is always exactly one, and this value is unconditionally placed into `NUM_RCU_LVL_0` by lines 28, 36, 45, and 55. The rest of the levels (if any) of the `rcu_node` tree are computed by dividing the maximum number of CPUs by the fanout supported by the number of levels from the current level down, rounding up. This computation is performed by lines 37, 46-47, and 56-58. Lines 31-33, 40-42, 50-52, and 62-63 create initializers for lockdep lock-class names. Finally, lines 64-66 produce an error if the maximum number of CPUs is too large for the specified fanout.

### The `rcu_segcblist` Structure

The `rcu_segcblist` structure maintains a segmented list of callbacks as follows:

     1 #define RCU_DONE_TAIL        0
     2 #define RCU_WAIT_TAIL        1
     3 #define RCU_NEXT_READY_TAIL  2
     4 #define RCU_NEXT_TAIL        3
     5 #define RCU_CBLIST_NSEGS     4
     6
     7 struct rcu_segcblist {
     8   struct rcu_head *head;
     9   struct rcu_head **tails[RCU_CBLIST_NSEGS];
    10   unsigned long gp_seq[RCU_CBLIST_NSEGS];
    11   long len;
    12   long len_lazy;
    13 };

The segments are as follows:

1. `RCU_DONE_TAIL`: Callbacks whose grace periods have elapsed. These callbacks are ready to be invoked.

2. `RCU_WAIT_TAIL`: Callbacks that are waiting for the current grace period. Note that different CPUs can have different ideas about which grace period is current, hence the `->gp_seq` field.

3. `RCU_NEXT_READY_TAIL`: Callbacks waiting for the next grace period to start.

4. `RCU_NEXT_TAIL`: Callbacks that have not yet been associated with a grace period.

The `->head` pointer references the first callback or is `NULL` if the list contains no callbacks (which is not the same as being empty). Each element of the `->tails[]` array references the `->next` pointer of the last callback in the corresponding segment of the list, or the list’s `->head` pointer if that segment and all previous segments are empty. If the corresponding segment is empty but some previous segment is not empty, then the array element is identical to its predecessor. Older callbacks are closer to the head of the list, and new callbacks are added at the tail. This relationship between the `->head` pointer, the `->tails[]` array, and the callbacks is shown in this diagram:

In this figure, the `->head` pointer references the first RCU callback in the list. The `->tails[RCU_DONE_TAIL]` array element references the `->head` pointer itself, indicating that none of the callbacks is ready to invoke. The `->tails[RCU_WAIT_TAIL]` array element references callback CB 2’s `->next` pointer, which indicates that CB 1 and CB 2 are both waiting on the current grace period, give or take possible disagreements about exactly which grace period is the current one. The `->tails[RCU_NEXT_READY_TAIL]` array element references the same RCU callback that `->tails[RCU_WAIT_TAIL]` does, which indicates that there are no callbacks waiting on the next RCU grace period. The `->tails[RCU_NEXT_TAIL]` array element references CB 4’s `->next` pointer, indicating that all the remaining RCU callbacks have not yet been assigned to an RCU grace period. Note that the `->tails[RCU_NEXT_TAIL]` array element always references the last RCU callback’s `->next` pointer unless the callback list is empty, in which case it references the `->head` pointer.

There is one additional important special case for the `->tails[RCU_NEXT_TAIL]` array element: It can be `NULL` when this list is disabled. Lists are disabled when the corresponding CPU is offline or when the corresponding CPU’s callbacks are offloaded to a kthread, both of which are described elsewhere.

CPUs advance their callbacks from the `RCU_NEXT_TAIL` to the `RCU_NEXT_READY_TAIL` to the `RCU_WAIT_TAIL` to the `RCU_DONE_TAIL` list segments as grace periods advance.

The `->gp_seq[]` array records grace-period numbers corresponding to the list segments. This is what allows different CPUs to have different ideas as to which is the current grace period while still avoiding premature invocation of their callbacks. In particular, this allows CPUs that go idle for extended periods to determine which of their callbacks are ready to be invoked after reawakening.

The `->len` counter contains the number of callbacks in `->head`, and the `->len_lazy` contains the number of those callbacks that are known to only free memory, and whose invocation can therefore be safely deferred.

Important

It is the `->len` field that determines whether or not there are callbacks associated with this `rcu_segcblist` structure, not the `->head` pointer. The reason for this is that all the ready-to-invoke callbacks (that is, those in the `RCU_DONE_TAIL` segment) are extracted all at once at callback-invocation time (`rcu_do_batch`), due to which `->head` may be set to NULL if there are no not-done callbacks remaining in the `rcu_segcblist`. If callback invocation must be postponed, for example, because a high-priority process just woke up on this CPU, then the remaining callbacks are placed back on the `RCU_DONE_TAIL` segment and `->head` once again points to the start of the segment. In short, the head field can briefly be `NULL` even though the CPU has callbacks present the entire time. Therefore, it is not appropriate to test the `->head` pointer for `NULL`.

In contrast, the `->len` and `->len_lazy` counts are adjusted only after the corresponding callbacks have been invoked. This means that the `->len` count is zero only if the `rcu_segcblist` structure really is devoid of callbacks. Of course, off-CPU sampling of the `->len` count requires careful use of appropriate synchronization, for example, memory barriers. This synchronization can be a bit subtle, particularly in the case of `rcu_barrier()`.

### The `rcu_data` Structure

The `rcu_data` maintains the per-CPU state for the RCU subsystem. The fields in this structure may be accessed only from the corresponding CPU (and from tracing) unless otherwise stated. This structure is the focus of quiescent-state detection and RCU callback queuing. It also tracks its relationship to the corresponding leaf `rcu_node` structure to allow more-efficient propagation of quiescent states up the `rcu_node` combining tree. Like the `rcu_node` structure, it provides a local copy of the grace-period information to allow for-free synchronized access to this information from the corresponding CPU. Finally, this structure records past dyntick-idle state for the corresponding CPU and also tracks statistics.

The `rcu_data` structure’s fields are discussed, singly and in groups, in the following sections.

#### Connection to Other Data Structures

This portion of the `rcu_data` structure is declared as follows:

    1   int cpu;
    2   struct rcu_node *mynode;
    3   unsigned long grpmask;
    4   bool beenonline;

The `->cpu` field contains the number of the corresponding CPU and the `->mynode` field references the corresponding `rcu_node` structure. The `->mynode` is used to propagate quiescent states up the combining tree. These two fields are constant and therefore do not require synchronization.

The `->grpmask` field indicates the bit in the `->mynode->qsmask` corresponding to this `rcu_data` structure, and is also used when propagating quiescent states. The `->beenonline` flag is set whenever the corresponding CPU comes online, which means that the debugfs tracing need not dump out any `rcu_data` structure for which this flag is not set.

#### Quiescent-State and Grace-Period Tracking

This portion of the `rcu_data` structure is declared as follows:

    1   unsigned long gp_seq;
    2   unsigned long gp_seq_needed;
    3   bool cpu_no_qs;
    4   bool core_needs_qs;
    5   bool gpwrap;

The `->gp_seq` field is the counterpart of the field of the same name in the `rcu_state` and `rcu_node` structures. The `->gp_seq_needed` field is the counterpart of the field of the same name in the rcu_node structure. They may each lag up to one behind their `rcu_node` counterparts, but in `CONFIG_NO_HZ_IDLE` and `CONFIG_NO_HZ_FULL` kernels can lag arbitrarily far behind for CPUs in dyntick-idle mode (but these counters will catch up upon exit from dyntick-idle mode). If the lower two bits of a given `rcu_data` structure’s `->gp_seq` are zero, then this `rcu_data` structure believes that RCU is idle.

Quick Quiz:
All this replication of the grace period numbers can only cause massive confusion. Why not just keep a global sequence number and be done with it???
Answer:
Because if there was only a single global sequence numbers, there would need to be a single global lock to allow safely accessing and updating it. And if we are not going to have a single global lock, we need to carefully manage the numbers on a per-node basis. Recall from the answer to a previous Quick Quiz that the consequences of applying a previously sampled quiescent state to the wrong grace period are quite severe.

The `->cpu_no_qs` flag indicates that the CPU has not yet passed through a quiescent state, while the `->core_needs_qs` flag indicates that the RCU core needs a quiescent state from the corresponding CPU. The `->gpwrap` field indicates that the corresponding CPU has remained idle for so long that the `gp_seq` counter is in danger of overflow, which will cause the CPU to disregard the values of its counters on its next exit from idle.

#### RCU Callback Handling

In the absence of CPU-hotplug events, RCU callbacks are invoked by the same CPU that registered them. This is strictly a cache-locality optimization: callbacks can and do get invoked on CPUs other than the one that registered them. After all, if the CPU that registered a given callback has gone offline before the callback can be invoked, there really is no other choice.

This portion of the `rcu_data` structure is declared as follows:

    1 struct rcu_segcblist cblist;
    2 long qlen_last_fqs_check;
    3 unsigned long n_cbs_invoked;
    4 unsigned long n_nocbs_invoked;
    5 unsigned long n_cbs_orphaned;
    6 unsigned long n_cbs_adopted;
    7 unsigned long n_force_qs_snap;
    8 long blimit;

The `->cblist` structure is the segmented callback list described earlier. The CPU advances the callbacks in its `rcu_data` structure whenever it notices that another RCU grace period has completed. The CPU detects the completion of an RCU grace period by noticing that the value of its `rcu_data` structure’s `->gp_seq` field differs from that of its leaf `rcu_node` structure. Recall that each `rcu_node` structure’s `->gp_seq` field is updated at the beginnings and ends of each grace period.

The `->qlen_last_fqs_check` and `->n_force_qs_snap` coordinate the forcing of quiescent states from `call_rcu()` and friends when callback lists grow excessively long.

The `->n_cbs_invoked`, `->n_cbs_orphaned`, and `->n_cbs_adopted` fields count the number of callbacks invoked, sent to other CPUs when this CPU goes offline, and received from other CPUs when those other CPUs go offline. The `->n_nocbs_invoked` is used when the CPU’s callbacks are offloaded to a kthread.

Finally, the `->blimit` counter is the maximum number of RCU callbacks that may be invoked at a given time.

#### Dyntick-Idle Handling

This portion of the `rcu_data` structure is declared as follows:

    1   int watching_snap;
    2   unsigned long dynticks_fqs;

The `->watching_snap` field is used to take a snapshot of the corresponding CPU’s dyntick-idle state when forcing quiescent states, and is therefore accessed from other CPUs. Finally, the `->dynticks_fqs` field is used to count the number of times this CPU is determined to be in dyntick-idle state, and is used for tracing and debugging purposes.

This portion of the rcu_data structure is declared as follows:

    1   long nesting;
    2   long nmi_nesting;
    3   atomic_t dynticks;
    4   bool rcu_need_heavy_qs;
    5   bool rcu_urgent_qs;

These fields in the rcu_data structure maintain the per-CPU dyntick-idle state for the corresponding CPU. The fields may be accessed only from the corresponding CPU (and from tracing) unless otherwise stated.

The `->nesting` field counts the nesting depth of process execution, so that in normal circumstances this counter has value zero or one. NMIs, irqs, and tracers are counted by the `->nmi_nesting` field. Because NMIs cannot be masked, changes to this variable have to be undertaken carefully using an algorithm provided by Andy Lutomirski. The initial transition from idle adds one, and nested transitions add two, so that a nesting level of five is represented by a `->nmi_nesting` value of nine. This counter can therefore be thought of as counting the number of reasons why this CPU cannot be permitted to enter dyntick-idle mode, aside from process-level transitions.

However, it turns out that when running in non-idle kernel context, the Linux kernel is fully capable of entering interrupt handlers that never exit and perhaps also vice versa. Therefore, whenever the `->nesting` field is incremented up from zero, the `->nmi_nesting` field is set to a large positive number, and whenever the `->nesting` field is decremented down to zero, the `->nmi_nesting` field is set to zero. Assuming that the number of misnested interrupts is not sufficient to overflow the counter, this approach corrects the `->nmi_nesting` field every time the corresponding CPU enters the idle loop from process context.

The `->dynticks` field counts the corresponding CPU’s transitions to and from either dyntick-idle or user mode, so that this counter has an even value when the CPU is in dyntick-idle mode or user mode and an odd value otherwise. The transitions to/from user mode need to be counted for user mode adaptive-ticks support (see NO_HZ: Reducing Scheduling-Clock Ticks).

The `->rcu_need_heavy_qs` field is used to record the fact that the RCU core code would really like to see a quiescent state from the corresponding CPU, so much so that it is willing to call for heavy-weight dyntick-counter operations. This flag is checked by RCU’s context-switch and `cond_resched()` code, which provide a momentary idle sojourn in response.

Finally, the `->rcu_urgent_qs` field is used to record the fact that the RCU core code would really like to see a quiescent state from the corresponding CPU, with the various other fields indicating just how badly RCU wants this quiescent state. This flag is checked by RCU’s context-switch path (`rcu_note_context_switch`) and the cond_resched code.

Quick Quiz:
Why not simply combine the `->nesting` and `->nmi_nesting` counters into a single counter that just counts the number of reasons that the corresponding CPU is non-idle?
Answer:
Because this would fail in the presence of interrupts whose handlers never return and of handlers that manage to return from a made-up interrupt.

Additional fields are present for some special-purpose builds, and are discussed separately.

### The `rcu_head` Structure

Each `rcu_head` structure represents an RCU callback. These structures are normally embedded within RCU-protected data structures whose algorithms use asynchronous grace periods. In contrast, when using algorithms that block waiting for RCU grace periods, RCU users need not provide `rcu_head` structures.

The `rcu_head` structure has fields as follows:

    1   struct rcu_head *next;
    2   void (*func)(struct rcu_head *head);

The `->next` field is used to link the `rcu_head` structures together in the lists within the `rcu_data` structures. The `->func` field is a pointer to the function to be called when the callback is ready to be invoked, and this function is passed a pointer to the `rcu_head` structure. However, `kfree_rcu()` uses the `->func` field to record the offset of the `rcu_head` structure within the enclosing RCU-protected data structure.

Both of these fields are used internally by RCU. From the viewpoint of RCU users, this structure is an opaque “cookie”.

Quick Quiz:
Given that the callback function `->func` is passed a pointer to the `rcu_head` structure, how is that function supposed to find the beginning of the enclosing RCU-protected data structure?
Answer:
In actual practice, there is a separate callback function per type of RCU-protected data structure. The callback function can therefore use the `container_of()` macro in the Linux kernel (or other pointer-manipulation facilities in other software environments) to find the beginning of the enclosing structure.

### RCU-Specific Fields in the `task_struct` Structure

The `CONFIG_PREEMPT_RCU` implementation uses some additional fields in the `task_struct` structure:

     1 #ifdef CONFIG_PREEMPT_RCU
     2   int rcu_read_lock_nesting;
     3   union rcu_special rcu_read_unlock_special;
     4   struct list_head rcu_node_entry;
     5   struct rcu_node *rcu_blocked_node;
     6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
     7 #ifdef CONFIG_TASKS_RCU
     8   unsigned long rcu_tasks_nvcsw;
     9   bool rcu_tasks_holdout;
    10   struct list_head rcu_tasks_holdout_list;
    11   int rcu_tasks_idle_cpu;
    12 #endif /* #ifdef CONFIG_TASKS_RCU */

The `->rcu_read_lock_nesting` field records the nesting level for RCU read-side critical sections, and the `->rcu_read_unlock_special` field is a bitmask that records special conditions that require `rcu_read_unlock()` to do additional work. The `->rcu_node_entry` field is used to form lists of tasks that have blocked within preemptible-RCU read-side critical sections and the `->rcu_blocked_node` field references the `rcu_node` structure whose list this task is a member of, or `NULL` if it is not blocked within a preemptible-RCU read-side critical section.

The `->rcu_tasks_nvcsw` field tracks the number of voluntary context switches that this task had undergone at the beginning of the current tasks-RCU grace period, `->rcu_tasks_holdout` is set if the current tasks-RCU grace period is waiting on this task, `->rcu_tasks_holdout_list` is a list element enqueuing this task on the holdout list, and `->rcu_tasks_idle_cpu` tracks which CPU this idle task is running, but only if the task is currently running, that is, if the CPU is currently idle.

### Accessor Functions

The following listing shows the `rcu_get_root()`, `rcu_for_each_node_breadth_first` and `rcu_for_each_leaf_node()` function and macros:

     1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
     2 {
     3   return &rsp->node[0];
     4 }
     5
     6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
     7   for ((rnp) = &(rsp)->node[0]; \
     8        (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
     9
    10 #define rcu_for_each_leaf_node(rsp, rnp) \
    11   for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
    12        (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)

The `rcu_get_root()` simply returns a pointer to the first element of the specified `rcu_state` structure’s `->node[]` array, which is the root `rcu_node` structure.

As noted earlier, the `rcu_for_each_node_breadth_first()` macro takes advantage of the layout of the `rcu_node` structures in the `rcu_state` structure’s `->node[]` array, performing a breadth-first traversal by simply traversing the array in order. Similarly, the `rcu_for_each_leaf_node()` macro traverses only the last part of the array, thus traversing only the leaf `rcu_node` structures.

Quick Quiz:
What does `rcu_for_each_leaf_node()` do if the `rcu_node` tree contains only a single node?
Answer:
In the single-node case, `rcu_for_each_leaf_node()` traverses the single node.

### Summary

So the state of RCU is represented by an `rcu_state` structure, which contains a combining tree of `rcu_node` and `rcu_data` structures. Finally, in `CONFIG_NO_HZ_IDLE` kernels, each CPU’s dyntick-idle state is tracked by dynticks-related fields in the `rcu_data` structure. If you made it this far, you are well prepared to read the code walkthroughs in the other articles in this series.

### Acknowledgments

I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn for helping me get this document into a more human-readable state.

### Legal Statement

This work represents the view of the author and does not necessarily represent the view of IBM.

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