diff options
| author | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-08-01 17:39:18 -0400 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@kernel.org> | 2019-10-29 02:48:12 -0700 |
| commit | ccc9971e2147225cb2d25e5543d6757988dd0a1d (patch) | |
| tree | e356b268517ca2813ec518c59457f9d61dd60d78 /Documentation/RCU/Design | |
| parent | c07e6f36bc321d70e8255c30d327e299b8301146 (diff) | |
| download | linux-ccc9971e2147225cb2d25e5543d6757988dd0a1d.tar.gz linux-ccc9971e2147225cb2d25e5543d6757988dd0a1d.tar.bz2 linux-ccc9971e2147225cb2d25e5543d6757988dd0a1d.zip | |
docs: rcu: convert some articles from html to ReST
There are 4 RCU articles that are written on html format.
The way they are, they can't be part of the Linux Kernel
documentation body nor share the styles and pdf output.
So, convert them to ReST format.
This way, make htmldocs and make pdfdocs will produce a
documentation output that will be like the original ones, but
will be part of the Linux Kernel documentation body.
Part of the conversion was done with the help of pandoc, but
the result had some broken things that had to be manually
fixed.
Following are manual changes Mauro made when doing the automatic conversion:
Quoting from: https://lore.kernel.org/rcu/20190726154550.5eeae294@coco.lan/
> > At least the pandoc's version I used here has a bug: its conversion
> > from html to ReST on those files only start after a <body> tag - or
> > when the first quiz table starts. I only discovered that adding a
> > <body> at the beginning of the file solve this book at the last
> > conversions.
> >
> > So, for most html->ReST conversions, I manually converted the first
> > part of the document, basically stripping html paragraph tags and
> > by replacing highlights by the ReST syntax.
> >
> > Also, all the quiz tables seem to assume some javascript macro or
> > css style that would be hiding the answer part until the mouse moves
> > to it. Such macro/css was not there at the kernel tree. So, the quiz
> > answers have the same color as the background, making them invisible.
> > Even if we had such macro/css, this is not portable for pdf/LaTeX output
> > (and I'm not sure if this would work with ePub).
> >
> > So, I ended by manually doing the table conversion.
> >
> > Finally, I double-checked if the conversions ended ok, addressing any
> > issues that might have heppened.
> >
> > So, after both automatic conversion and manual fixes, I opened both the
> > html files produced by Sphinx and the original ones and compared them
> > line per line (except for the indexes, as Sphinx produces them
> > automatically), in order to see if all information from the original
> > files will be there on a format close to what we have on other ReST
> > files, fixing any pending issues if any.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'Documentation/RCU/Design')
9 files changed, 4971 insertions, 6102 deletions
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html deleted file mode 100644 index c30c1957c7e6..000000000000 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html +++ /dev/null @@ -1,1391 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" - "http://www.w3.org/TR/html4/loose.dtd"> - <html> - <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title> - <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> - - <p>December 18, 2016</p> - <p>This article was contributed by Paul E. McKenney</p> - -<h3>Introduction</h3> - -This document describes RCU's major data structures and their relationship -to each other. - -<ol> -<li> <a href="#Data-Structure Relationships"> - Data-Structure Relationships</a> -<li> <a href="#The rcu_state Structure"> - The <tt>rcu_state</tt> Structure</a> -<li> <a href="#The rcu_node Structure"> - The <tt>rcu_node</tt> Structure</a> -<li> <a href="#The rcu_segcblist Structure"> - The <tt>rcu_segcblist</tt> Structure</a> -<li> <a href="#The rcu_data Structure"> - The <tt>rcu_data</tt> Structure</a> -<li> <a href="#The rcu_head Structure"> - The <tt>rcu_head</tt> Structure</a> -<li> <a href="#RCU-Specific Fields in the task_struct Structure"> - RCU-Specific Fields in the <tt>task_struct</tt> Structure</a> -<li> <a href="#Accessor Functions"> - Accessor Functions</a> -</ol> - -<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3> - -<p>RCU is for all intents and purposes a large state machine, and its -data structures maintain the state in such a way as to allow RCU readers -to execute extremely quickly, while also processing the RCU grace periods -requested by updaters in an efficient and extremely scalable fashion. -The efficiency and scalability of RCU updaters is provided primarily -by a combining tree, as shown below: - -</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%"> - -</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure -containing a tree of <tt>rcu_node</tt> structures. -Each leaf node of the <tt>rcu_node</tt> tree has up to 16 -<tt>rcu_data</tt> structures associated with it, so that there -are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures, -one for each possible CPU. -This structure is adjusted at boot time, if needed, to handle the -common case where <tt>nr_cpu_ids</tt> is much less than -<tt>NR_CPUs</tt>. -For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>, -which results in a three-level <tt>rcu_node</tt> tree. -If the actual hardware has only 16 CPUs, RCU will adjust itself -at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node. - -</p><p>The purpose of this combining tree is to allow per-CPU events -such as quiescent states, dyntick-idle transitions, -and CPU hotplug operations to be processed efficiently -and scalably. -Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures, -and other events are recorded by the leaf-level <tt>rcu_node</tt> -structures. -All of these events are combined at each level of the tree until finally -grace periods are completed at the tree's root <tt>rcu_node</tt> -structure. -A grace period can be completed at the root once every CPU -(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task) -has passed through a quiescent state. -Once a grace period has completed, record of that fact is propagated -back down the tree. - -</p><p>As can be seen from the diagram, on a 64-bit system -a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout -of 64 at the root and a fanout of 16 at the leaves. - -<table> -<tr><th> </th></tr> -<tr><th align="left">Quick Quiz:</th></tr> -<tr><td> - Why isn't the fanout at the leaves also 64? -</td></tr> -<tr><th align="left">Answer:</th></tr> -<tr><td bgcolor="#ffffff"><font color="ffffff"> - Because there are more types of events that affect the leaf-level - <tt>rcu_node</tt> structures than further up the tree. - Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of - 64, the contention on these structures' <tt>->structures</tt> - becomes excessive. - Experimentation on a wide variety of systems has shown that a fanout - of 16 works well for the leaves of the <tt>rcu_node</tt> tree. - </font> - - <p><font color="ffffff">Of course, further experience with - systems having hundreds or thousands of CPUs may demonstrate - that the fanout for the non-leaf <tt>rcu_node</tt> structures - must also be reduced. - Such reduction can be easily carried out when and if it proves - necessary. - In the meantime, if you are using such a system and running into - contention problems on the non-leaf <tt>rcu_node</tt> structures, - you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration - parameter to reduce the non-leaf fanout as needed. - </font> - - <p><font color="ffffff">Kernels built for systems with - strong NUMA characteristics might also need to adjust - <tt>CONFIG_RCU_FANOUT</tt> so that the domains of the - <tt>rcu_node</tt> structures align with hardware boundaries. - However, there has thus far been no need for this. -</font></td></tr> -<tr><td> </td></tr> -</table> - -<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on -a 32-bit system), then RCU will automatically add more levels to the -tree. -For example, if you are crazy enough to build a 64-bit system with 65,536 -CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows: - -</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%"> - -</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system -accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for -32-bit systems. -On the other hand, you can set both <tt>CONFIG_RCU_FANOUT</tt> and -<tt>CONFIG_RCU_FANOUT_LEAF</tt> to be as small as 2, which would result -in a 16-CPU test using a 4-level tree. -This can be useful for testing large-system capabilities on small test -machines. - -</p><p>This multi-level combining tree allows us to get most of the -performance and scalability -benefits of partitioning, even though RCU grace-period detection is -inherently a global operation. -The trick here is that only the last CPU to report a quiescent state -into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt> -structure at the next level up the tree. -This means that at the leaf-level <tt>rcu_node</tt> structure, only -one access out of sixteen will progress up the tree. -For the internal <tt>rcu_node</tt> structures, the situation is even -more extreme: Only one access out of sixty-four will progress up -the tree. -Because the vast majority of the CPUs do not progress up the tree, -the lock contention remains roughly constant up the tree. -No matter how many CPUs there are in the system, at most 64 quiescent-state -reports per grace period will progress all the way to the root -<tt>rcu_node</tt> structure, thus ensuring that the lock contention -on that root <tt>rcu_node</tt> structure remains acceptably low. - -</p><p>In effect, the combining tree acts like a big shock absorber, -keeping lock contention under control at all tree levels regardless -of the level of loading on the system. - -</p><p>RCU updaters wait for normal grace periods by registering -RCU callbacks, either directly via <tt>call_rcu()</tt> -or indirectly via <tt>synchronize_rcu()</tt> and friends. -RCU callbacks are represented by <tt>rcu_head</tt> structures, -which are queued on <tt>rcu_data</tt> structures while they are -waiting for a grace period to elapse, as shown in the following figure: - -</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%"> - -</p><p>This figure shows how <tt>TREE_RCU</tt>'s and -<tt>PREEMPT_RCU</tt>'s major data structures are related. -Lesser data structures will be introduced with the algorithms that -make use of them. - -</p><p>Note that each of the data structures in the above figure has -its own synchronization: - -<p><ol> -<li> Each <tt>rcu_state</tt> structures has a lock and a mutex, - and some fields are protected by the corresponding root - <tt>rcu_node</tt> structure's lock. -<li> Each <tt>rcu_node</tt> structure has a spinlock. -<li> The fields in <tt>rcu_data</tt> are private to the corresponding - CPU, although a few can be read and written by other CPUs. -</ol> - -<p>It is important to note that different data structures can have -very different ideas about the state of RCU at any given time. -For but one example, awareness of the start or end of a given RCU -grace period propagates slowly through the data structures. -This slow propagation is absolutely necessary for RCU to have good -read-side performance. -If this balkanized implementation seems foreign to you, one useful -trick is to consider each instance of these data structures to be -a different person, each having the usual slightly different -view of reality. - -</p><p>The general role of each of these data structures is as -follows: - -</p><ol> -<li> <tt>rcu_state</tt>: - This structure forms the interconnection between the - <tt>rcu_node</tt> and <tt>rcu_data</tt> structures, - tracks grace periods, serves as short-term repository - for callbacks orphaned by CPU-hotplug events, - maintains <tt>rcu_barrier()</tt> state, - tracks expedited grace-period state, - and maintains state used to force quiescent states when - grace periods extend too long, -<li> <tt>rcu_node</tt>: This structure forms the combining - tree that propagates quiescent-state - information from the leaves to the root, and also propagates - grace-period information from the root to the leaves. - It provides local copies of the grace-period state in order - to allow this information to be accessed in a synchronized - manner without suffering the scalability limitations that - would otherwise be imposed by global locking. - In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists - of tasks that have blocked while in their current - RCU read-side critical section. - In <tt>CONFIG_PREEMPT_RCU</tt> with - <tt>CONFIG_RCU_BOOST</tt>, it manages the - per-<tt>rcu_node</tt> priority-boosting - kernel threads (kthreads) and state. - Finally, it records CPU-hotplug state in order to determine - which CPUs should be ignored during a given grace period. -<li> <tt>rcu_data</tt>: This per-CPU structure is the - focus of quiescent-state detection and RCU callback queuing. - It also tracks its relationship to the corresponding leaf - <tt>rcu_node</tt> structure to allow more-efficient - propagation of quiescent states up the <tt>rcu_node</tt> - combining tree. - Like the <tt>rcu_node</tt> structure, it provides a local - copy of the grace-period information to allow for-free - synchronized - access to this information from the corresponding CPU. - Finally, this structure records past dyntick-idle state - for the corresponding CPU and also tracks statistics. -<li> <tt>rcu_head</tt>: - This structure represents RCU callbacks, and is the - only structure allocated and managed by RCU users. - The <tt>rcu_head</tt> structure is normally embedded - within the RCU-protected data structure. -</ol> - -<p>If all you wanted from this article was a general notion of how -RCU's data structures are related, you are done. -Otherwise, each of the following sections give more details on -the <tt>rcu_state</tt>, <tt>rcu_node</tt> and <tt>rcu_data</tt> data -structures. - -<h3><a name="The rcu_state Structure"> -The <tt>rcu_state</tt> Structure</a></h3> - -<p>The <tt>rcu_state</tt> structure is the base structure that -represents the state of RCU in the system. -This structure forms the interconnection between the -<tt>rcu_node</tt> and <tt>rcu_data</tt> structures, -tracks grace periods, contains the lock used to -synchronize with CPU-hotplug events, -and maintains state used to force quiescent states when -grace periods extend too long, - -</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed, -singly and in groups, in the following sections. -The more specialized fields are covered in the discussion of their -use. - -<h5>Relationship to rcu_node and rcu_data Structures</h5> - -This portion of the <tt>rcu_state</tt> structure is declared -as follows: - -<pre> - 1 struct rcu_node node[NUM_RCU_NODES]; - 2 struct rcu_node *level[NUM_RCU_LVLS + 1]; - 3 struct rcu_data __percpu *rda; -</pre> - -<table> -<tr><th> </th></tr> -<tr><th align="left">Quick Quiz:</th></tr> -<tr><td> - Wait a minute! - You said that the <tt>rcu_node</tt> structures formed a tree, - but they are declared as a flat array! - What gives? -</td></tr> -<tr><th align="left">Answer:</th></tr> -<tr><td bgcolor="#ffffff"><font color="ffffff"> - The tree is laid out in the array. - The first node In the array is the head, the next set of nodes in the - array are children of the head node, and so on until the last set of - nodes in the array are the leaves. - </font> - - <p><font color="ffffff">See the following diagrams to see how - this works. -</font></td></tr> -<tr><td> </td></tr> -</table> - -<p>The <tt>rcu_node</tt> tree is embedded into the -<tt>->node[]</tt> array as shown in the following figure: - -</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%"> - -</p><p>One interesting consequence of this mapping is that a -breadth-first traversal of the tree is implemented as a simple -linear scan of the array, which is in fact what the -<tt>rcu_for_each_node_breadth_first()</tt> macro does. -This macro is used at the beginning and ends of grace periods. - -</p><p>Each entry of the <tt>->level</tt> array references -the first <tt>rcu_node</tt> structure on the corresponding level -of the tree, for example, as shown below: - -</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%"> - -</p><p>The zero<sup>th</sup> element of the array references the root -<tt>rcu_node</tt> structure, the first element references the -first child of the root <tt>rcu_node</tt>, and finally the second -element references the first leaf <tt>rcu_node</tt> structure. - -</p><p>For whatever it is worth, if you draw the tree to be tree-shaped -rather than array-shaped, it is easy to draw a planar representation: - -</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%"> - -</p><p>Finally, the <tt>->rda</tt> field references a per-CPU -pointer to the corresponding CPU's <tt>rcu_data</tt> structure. - -</p><p>All of these fields are constant once initialization is complete, -and therefore need no protection. - -<h5>Grace-Period Tracking</h5> - -<p>This portion of the <tt>rcu_state</tt> structure is declared -as follows: - -<pre> - 1 unsigned long gp_seq; -</pre> - -<p>RCU grace periods are numbered, and -the <tt>->gp_seq</tt> field contains the current grace-period -sequence number. -The bottom two bits are the state of the current grace period, -which can be zero for not yet started or one for in progress. -In other words, if the bottom two bits of <tt>->gp_seq</tt> are -zero, then RCU is idle. -Any other value in the bottom two bits indicates that something is broken. -This field is protected by the root <tt>rcu_node</tt> structure's -<tt>->lock</tt> field. - -</p><p>There are <tt>->gp_seq</tt> fields -in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures -as well. -The fields in the <tt>rcu_state</tt> structure represent the -most current value, and those of the other structures are compared -in order to detect the beginnings and ends of grace periods in a distributed -fashion. -The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt> -(down the tree from the root to the leaves) to <tt>rcu_data</tt>. - -<h5>Miscellaneous</h5> - -<p>This portion of the <tt>rcu_state</tt> structure is declared -as follows: - -<pre> - 1 unsigned long gp_max; - 2 char abbr; - 3 char *name; -</pre> - -<p>The <tt>->gp_max</tt> field tracks the duration of the longest -grace period in jiffies. -It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>. - -<p>The <tt>->name</tt> and <tt>->abbr</tt> fields distinguish -between preemptible RCU (“rcu_preempt” and “p”) -and non-preemptible RCU (“rcu_sched” and “s”). -These fields are used for diagnostic and tracing purposes. - -<h3><a name="The rcu_node Structure"> -The <tt>rcu_node</tt> Structure</a></h3> - -<p>The <tt>rcu_node</tt> structures form the combining -tree that propagates quiescent-state -information from the leaves to the root and also that propagates -grace-period information from the root down to the leaves. -They provides local copies of the grace-period state in order -to allow this information to be accessed in a synchronized -manner without suffering the scalability limitations that -would otherwise be imposed by global locking. -In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists -of tasks that have blocked while in their current -RCU read-side critical section. -In <tt>CONFIG_PREEMPT_RCU</tt> with -<tt>CONFIG_RCU_BOOST</tt>, they manage the -per-<tt>rcu_node</tt> priority-boosting -kernel threads (kthreads) and state. -Finally, they record CPU-hotplug state in order to determine -which CPUs should be ignored during a given grace period. - -</p><p>The <tt>rcu_node</tt> structure's fields are discussed, -singly and in groups, in the following sections. - -<h5>Connection to Combining Tree</h5> - -<p>This portion of the <tt>rcu_node</tt> structure is declared -as follows: - -<pre> - 1 struct rcu_node *parent; - 2 u8 level; - 3 u8 grpnum; - 4 unsigned long grpmask; - 5 int grplo; - 6 int grphi; -</pre> - -<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt> -one level up in the tree, and is <tt>NULL</tt> for the root -<tt>rcu_node</tt>. -The RCU implementation makes heavy use of this field to push quiescent -states up the tree. -The <tt>->level</tt> field gives the level in the tree, with -the root being at level zero, its children at level one, and so on. -The <tt>->grpnum</tt> field gives this node's position within -the children of its parent, so this number can range between 0 and 31 -on 32-bit systems and between 0 and 63 on 64-bit systems. -The <tt>->level</tt> and <tt>->grpnum</tt> fields are -used only during initialization and for tracing. -The <tt>->grpmask</tt> field is the bitmask counterpart of -<tt>->grpnum</tt>, and therefore always has exactly one bit set. -This mask is used to clear the bit corresponding to this <tt>rcu_node</tt> -structure in its parent's bitmasks, which are described later. -Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields -contain the lowest and highest numbered CPU served by this -<tt>rcu_node</tt> structure, respectively. - -</p><p>All of these fields are constant, and thus do not require any -synchronization. - -<h5>Synchronization</h5> - -<p>This field of the <tt>rcu_node</tt> structure is declared -as follows: - -<pre> - 1 raw_spinlock_t lock; -</pre> - -<p>This field is used to protect the remaining fields in this structure, -unless otherwise stated. -That said, all of the fields in this structure can be accessed without -locking for tracing purposes. -Yes, this can result in confusing traces, but better some tracing confusion -than to be heisenbugged out of existence. - -<h5>Grace-Period Tracking</h5> - -<p>This portion of the <tt>rcu_node</tt> structure is declared -as follows: - -<pre> - 1 unsigned long gp_seq; - 2 unsigned long gp_seq_needed; -</pre> - -<p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are -the counterparts of the field of the same name in the <tt>rcu_state</tt> -structure. -They each may lag up to one step behind their <tt>rcu_state</tt> -counterpart. -If the bottom two bits of a given <tt>rcu_node</tt> structure's -<tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt> -structure believes that RCU is idle. -</p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt> -structure is updated at the beginning and the end -of each grace period. - -<p>The <tt>->gp_seq_needed</tt> fields record the -furthest-in-the-future grace period request seen by the corresponding -<tt>rcu_node</tt> structure. The request is considered fulfilled when -the value of the <tt>->gp_seq</tt> field equals or exceeds that of -the <tt>->gp_seq_needed</tt> field. - -<table> -<tr><th> </th></tr> -<tr><th align="left">Quick Quiz:</th></tr> -<tr><td> - Suppose that this <tt>rcu_node</tt> structure doesn't see - a request for a very long time. - Won't wrapping of the <tt>->gp_seq</tt> field cause - problems? -</td></tr> -<tr><th align="left">Answer:</th></tr> -<tr><td bgcolor="#ffffff"><font color="ffffff"> - No, because if the <tt>->gp_seq_needed</tt> field lags behind the - <tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field - will be updated at the end of the grace period. - Modulo-arithmetic comparisons therefore will always get the - correct answer, even with wrapping. -</font></td></tr> -<tr><td> </td></tr> -</table> - -<h5>Quiescent-State Tracking</h5> - -<p>These fields manage the propagation of quiescent states up the -combining tree. - -</p><p>This portion of the <tt>rcu_node</tt> structure has fields -as follows: - -<pre> - 1 unsigned long qsmask; - 2 unsigned long expmask; - 3 unsigned long qsmaskinit; - 4 unsigned long expmaskinit; -</pre> - -<p>The <tt>->qsmask</tt> field tracks which of this -<tt>rcu_node</tt> structure's children still need to report -quiescent states for the current normal grace period. -Such children will have a value of 1 in their corresponding bit. -Note that the leaf <tt>rcu_node</tt> structures should be -thought of as having <tt>rcu_data</tt> structures as their -children. -Similarly, the <tt>->expmask</tt> field tracks which -of this <tt>rcu_node</tt> structure's children still need to report -quiescent states for the current expedited grace period. -An expedited grace period has -the same conceptual properties as a normal grace period, but the -expedited implementation accepts extreme CPU overhead to obtain -much lower grace-period latency, for example, consuming a few -tens of microseconds worth of CPU time to reduce grace-period -duration from milliseconds to tens of microseconds. -The <tt>->qsmaskinit</tt> field tracks which of this -<tt>rcu_node</tt> structure's children cover for at least -one online CPU. -This mask is used to initialize <tt>->qsmask</tt>, -and <tt>->expmaskinit</tt> is used to initialize -<tt>->expmask</tt> and the beginning of the -normal and expedited grace periods, respectively. - -<table> -<tr><th> </th></tr> -<tr><th align="left">Quick Quiz:</th></tr> -<tr><td> - Why are these bitmasks protected by locking? - Come on, haven't you heard of atomic instructions??? -</td></tr> -<tr><th align="left">Answer:</th></tr> -<tr><td bgcolor="#ffffff"><font color="ffffff"> - Lockless grace-period computation! Such a tantalizing possibility! - </font> - - <p><font color="ffffff">But consider the following sequence of events: - </font> - - <ol> - <li> <font color="ffffff">CPU 0 has been in dyntick-idle - mode for quite some time. - When it wakes up, it notices that the current RCU - grace period needs it to report in, so it sets a - flag where the scheduling clock interrupt will find it. - </font><p> - <li> <font color="ffffff">Meanwhile, CPU 1 is running - <tt>force_quiescent_state()</tt>, - and notices that CPU 0 has been in dyntick idle mode, - which qualifies as an extended quiescent state. - </font><p> - <li> <font color="ffffff">CPU 0's scheduling clock - interrupt fires in the - middle of an RCU read-side critical section, and notices - that the RCU core needs something, so commences RCU softirq - processing. - </font> - <p> - <li> <font color="ffffff">CPU 0's softirq handler - executes and is just about ready - to report its quiescent state up the <tt>rcu_node</tt> - tree. - </font><p> - <li> <font color="ffffff">But CPU 1 beats it to the punch, - completing the current - grace period and starting a new one. - </font><p> - <li> <font color="ffffff">CPU 0 now reports its quiescent - state for the wrong - grace period. - That grace period might now end before the RCU read-side - critical section. - If that happens, disaster will ensue. - </font> - </ol> - - <p><font color="ffffff">So the locking is absolutely required in - order to coordinate clearing of the bits with updating of the - grace-period sequence number in <tt>->gp_seq</tt>. -</font></td></tr> -<tr><td> </td></tr> -</table> - -<h5>Blocked-Task Management</h5> - -<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the -midst of their RCU read-side critical sections, and these tasks -must be tracked explicitly. -The details of exactly why and how they are tracked will be covered -in a separate article on RCU read-side processing. -For now, it is enough to know that the <tt>rcu_node</tt> -structure tracks them. - -<pre> - 1 struct list_head blkd_tasks; - 2 struct list_head *gp_tasks; - 3 struct list_head *exp_tasks; - 4 bool wait_blkd_tasks; -</pre> - -<p>The <tt>->blkd_tasks</tt> field is a list header for -the list of blocked and preempted tasks. -As tasks undergo context switches within RCU read-side critical -sections, their <tt>task_struct</tt> structures are enqueued -(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt> -field) onto the head of the <tt>->blkd_tasks</tt> list for the -leaf <tt>rcu_node</tt> structure corresponding to the CPU -on which the outgoing context switch executed. -As these tasks later exit their RCU read-side critical sections, -they remove themselves from the list. -This list is therefore in reverse time order, so that if one of the tasks -is blocking the current grace period, all subsequent tasks must -also be blocking that same grace period. -Therefore, a single pointer into this list suffices to track -all tasks blocking a given grace period. -That pointer is stored in <tt>->gp_tasks</tt> for normal -grace periods and in <tt>->exp_tasks</tt> for expedited -grace periods. -These last two fields are <tt>NULL</tt> if either there is -no grace period in flight or if there are no blocked tasks -preventing that grace period from completing. -If either of these two pointers is referencing a task that -removes itself from the <tt>->blkd_tasks</tt> list, -then that task must advance the pointer to the next task on -the list, or set the pointer to <tt>NULL</tt> if there -are no subsequent tasks on the list. - -</p><p>For example, suppose that tasks T1, T2, and T3 are -all hard-affinitied to the largest-numbered CPU in the system. -Then if task T1 blocked in an RCU read-side -critical section, then an expedited grace period started, -then task T2 blocked in an RCU read-side critical section, -then a normal grace period started, and finally task 3 blocked -in an RCU read-side critical section, then the state of the -last leaf <tt>rcu_node</tt> structure's blocked-task list -would be as shown below: - -</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%"> - -</p><p>Task T1 is blocking both grace periods, task T2 is -blocking only the normal grace period, and task T3 is blocking -neither grace period. -Note that these tasks will not remove themselves from this list -immediately upon resuming execution. -They will instead remain on the list until they execute the outermost -<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical -section. - -<p> -The <tt>->wait_blkd_tasks</tt> field indicates whether or not -the current grace period is waiting on a blocked task. - -<h5>Sizing the <tt>rcu_node</tt> Array</h5> - -<p>The <tt>rcu_node</tt> array is sized via a series of -C-preprocessor expressions as follows: - -<pre> - 1 #ifdef CONFIG_RCU_FANOUT - 2 #define RCU_FANOUT CONFIG_RCU_FANOUT - 3 #else - 4 # ifdef CONFIG_64BIT - 5 # define RCU_FANOUT 64 - 6 # else - 7 # define RCU_FANOUT 32 - 8 # endif - 9 #endif -10 -11 #ifdef CONFIG_RCU_FANOUT_LEAF -12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF -13 #else -14 # ifdef CONFIG_64BIT -15 # define RCU_FANOUT_LEAF 64 -16 # else -17 # define RCU_FANOUT_LEAF 32 -18 # endif -19 #endif -20 -21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF) -22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT) -23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT) -24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT) -25 -26 #if NR_CPUS <= RCU_FANOUT_1 -27 # define RCU_NUM_LVLS 1 -28 # define NUM_RCU_LVL_0 1 -29 # define NUM_RCU_NODES NUM_RCU_LVL_0 -30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 } -31 # define RCU_NODE_NAME_INIT { "rcu_node_0" } -32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" } -33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" } -34 #elif NR_CPUS <= RCU_FANOUT_2 -35 # define RCU_NUM_LVLS 2 -36 # define NUM_RCU_LVL_0 1 -37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) -38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1) -39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 } -40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" } -41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" } -42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" } -43 #elif NR_CPUS <= RCU_FANOUT_3 -44 # define RCU_NUM_LVLS 3 -45 # define NUM_RCU_LVL_0 1 -46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) -47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(N |