1463 lines
58 KiB
HTML
1463 lines
58 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
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<meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<p>December 18, 2016</p>
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<p>This article was contributed by Paul E. McKenney</p>
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<h3>Introduction</h3>
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This document describes RCU's major data structures and their relationship
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to each other.
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<ol>
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<li> <a href="#Data-Structure Relationships">
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Data-Structure Relationships</a>
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<li> <a href="#The rcu_state Structure">
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The <tt>rcu_state</tt> Structure</a>
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<li> <a href="#The rcu_node Structure">
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The <tt>rcu_node</tt> Structure</a>
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<li> <a href="#The rcu_segcblist Structure">
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The <tt>rcu_segcblist</tt> Structure</a>
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<li> <a href="#The rcu_data Structure">
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The <tt>rcu_data</tt> Structure</a>
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<li> <a href="#The rcu_dynticks Structure">
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The <tt>rcu_dynticks</tt> Structure</a>
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<li> <a href="#The rcu_head Structure">
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The <tt>rcu_head</tt> Structure</a>
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<li> <a href="#RCU-Specific Fields in the task_struct Structure">
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RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
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<li> <a href="#Accessor Functions">
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Accessor Functions</a>
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</ol>
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<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
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<p>RCU is for all intents and purposes a large state machine, and its
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data structures maintain the state in such a way as to allow RCU readers
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to execute extremely quickly, while also processing the RCU grace periods
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requested by updaters in an efficient and extremely scalable fashion.
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The efficiency and scalability of RCU updaters is provided primarily
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by a combining tree, as shown below:
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</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
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</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
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containing a tree of <tt>rcu_node</tt> structures.
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Each leaf node of the <tt>rcu_node</tt> tree has up to 16
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<tt>rcu_data</tt> structures associated with it, so that there
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are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
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one for each possible CPU.
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This structure is adjusted at boot time, if needed, to handle the
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common case where <tt>nr_cpu_ids</tt> is much less than
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<tt>NR_CPUs</tt>.
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For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
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which results in a three-level <tt>rcu_node</tt> tree.
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If the actual hardware has only 16 CPUs, RCU will adjust itself
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at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
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</p><p>The purpose of this combining tree is to allow per-CPU events
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such as quiescent states, dyntick-idle transitions,
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and CPU hotplug operations to be processed efficiently
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and scalably.
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Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
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and other events are recorded by the leaf-level <tt>rcu_node</tt>
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structures.
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All of these events are combined at each level of the tree until finally
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grace periods are completed at the tree's root <tt>rcu_node</tt>
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structure.
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A grace period can be completed at the root once every CPU
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(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
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has passed through a quiescent state.
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Once a grace period has completed, record of that fact is propagated
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back down the tree.
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</p><p>As can be seen from the diagram, on a 64-bit system
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a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
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of 64 at the root and a fanout of 16 at the leaves.
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<table>
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<tr><th> </th></tr>
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<tr><th align="left">Quick Quiz:</th></tr>
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<tr><td>
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Why isn't the fanout at the leaves also 64?
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</td></tr>
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<tr><th align="left">Answer:</th></tr>
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<tr><td bgcolor="#ffffff"><font color="ffffff">
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Because there are more types of events that affect the leaf-level
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<tt>rcu_node</tt> structures than further up the tree.
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Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
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64, the contention on these structures' <tt>->structures</tt>
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becomes excessive.
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Experimentation on a wide variety of systems has shown that a fanout
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of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
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</font>
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<p><font color="ffffff">Of course, further experience with
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systems having hundreds or thousands of CPUs may demonstrate
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that the fanout for the non-leaf <tt>rcu_node</tt> structures
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must also be reduced.
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Such reduction can be easily carried out when and if it proves
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necessary.
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In the meantime, if you are using such a system and running into
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contention problems on the non-leaf <tt>rcu_node</tt> structures,
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you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
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parameter to reduce the non-leaf fanout as needed.
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</font>
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<p><font color="ffffff">Kernels built for systems with
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strong NUMA characteristics might also need to adjust
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<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
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<tt>rcu_node</tt> structures align with hardware boundaries.
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However, there has thus far been no need for this.
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</font></td></tr>
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<tr><td> </td></tr>
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</table>
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<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
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a 32-bit system), then RCU will automatically add more levels to the
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tree.
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For example, if you are crazy enough to build a 64-bit system with 65,536
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CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
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</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
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</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
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accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
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32-bit systems.
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On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
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as small as 2 if you wish, which would permit only 16 CPUs, which
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is useful for testing.
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</p><p>This multi-level combining tree allows us to get most of the
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performance and scalability
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benefits of partitioning, even though RCU grace-period detection is
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inherently a global operation.
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The trick here is that only the last CPU to report a quiescent state
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into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
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structure at the next level up the tree.
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This means that at the leaf-level <tt>rcu_node</tt> structure, only
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one access out of sixteen will progress up the tree.
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For the internal <tt>rcu_node</tt> structures, the situation is even
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more extreme: Only one access out of sixty-four will progress up
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the tree.
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Because the vast majority of the CPUs do not progress up the tree,
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the lock contention remains roughly constant up the tree.
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No matter how many CPUs there are in the system, at most 64 quiescent-state
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reports per grace period will progress all the way to the root
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<tt>rcu_node</tt> structure, thus ensuring that the lock contention
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on that root <tt>rcu_node</tt> structure remains acceptably low.
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</p><p>In effect, the combining tree acts like a big shock absorber,
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keeping lock contention under control at all tree levels regardless
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of the level of loading on the system.
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</p><p>The Linux kernel actually supports multiple flavors of RCU
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running concurrently, so RCU builds separate data structures for each
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flavor.
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For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
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rcu_sched and rcu_bh, as shown below:
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</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
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</p><p>Energy efficiency is increasingly important, and for that
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reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
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turns off the scheduling-clock interrupts on idle CPUs, which in
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turn allows those CPUs to attain deeper sleep states and to consume
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less energy.
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CPUs whose scheduling-clock interrupts have been turned off are
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said to be in <i>dyntick-idle mode</i>.
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RCU must handle dyntick-idle CPUs specially
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because RCU would otherwise wake up each CPU on every grace period,
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which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
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RCU uses the <tt>rcu_dynticks</tt> structure to track
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which CPUs are in dyntick idle mode, as shown below:
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</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
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</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
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for all flavors of RCU.
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Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
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CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
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that <tt>rcu_dynticks</tt>, as shown in the figure.
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</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
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rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
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</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
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</p><p>RCU updaters wait for normal grace periods by registering
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RCU callbacks, either directly via <tt>call_rcu()</tt> and
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friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
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there being a separate interface per flavor of RCU)
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or indirectly via <tt>synchronize_rcu()</tt> and friends.
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RCU callbacks are represented by <tt>rcu_head</tt> structures,
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which are queued on <tt>rcu_data</tt> structures while they are
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waiting for a grace period to elapse, as shown in the following figure:
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</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
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</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
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<tt>PREEMPT_RCU</tt>'s major data structures are related.
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Lesser data structures will be introduced with the algorithms that
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make use of them.
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</p><p>Note that each of the data structures in the above figure has
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its own synchronization:
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<p><ol>
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<li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
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and some fields are protected by the corresponding root
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<tt>rcu_node</tt> structure's lock.
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<li> Each <tt>rcu_node</tt> structure has a spinlock.
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<li> The fields in <tt>rcu_data</tt> are private to the corresponding
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CPU, although a few can be read and written by other CPUs.
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<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private
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to the corresponding CPU, although a few can be read by
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other CPUs.
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</ol>
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<p>It is important to note that different data structures can have
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very different ideas about the state of RCU at any given time.
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For but one example, awareness of the start or end of a given RCU
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grace period propagates slowly through the data structures.
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This slow propagation is absolutely necessary for RCU to have good
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read-side performance.
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If this balkanized implementation seems foreign to you, one useful
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trick is to consider each instance of these data structures to be
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a different person, each having the usual slightly different
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view of reality.
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</p><p>The general role of each of these data structures is as
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follows:
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</p><ol>
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<li> <tt>rcu_state</tt>:
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This structure forms the interconnection between the
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<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
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tracks grace periods, serves as short-term repository
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for callbacks orphaned by CPU-hotplug events,
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maintains <tt>rcu_barrier()</tt> state,
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tracks expedited grace-period state,
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and maintains state used to force quiescent states when
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grace periods extend too long,
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<li> <tt>rcu_node</tt>: This structure forms the combining
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tree that propagates quiescent-state
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information from the leaves to the root, and also propagates
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grace-period information from the root to the leaves.
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It provides local copies of the grace-period state in order
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to allow this information to be accessed in a synchronized
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manner without suffering the scalability limitations that
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would otherwise be imposed by global locking.
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In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
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of tasks that have blocked while in their current
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RCU read-side critical section.
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In <tt>CONFIG_PREEMPT_RCU</tt> with
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<tt>CONFIG_RCU_BOOST</tt>, it manages the
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per-<tt>rcu_node</tt> priority-boosting
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kernel threads (kthreads) and state.
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Finally, it records CPU-hotplug state in order to determine
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which CPUs should be ignored during a given grace period.
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<li> <tt>rcu_data</tt>: This per-CPU structure is the
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focus of quiescent-state detection and RCU callback queuing.
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It also tracks its relationship to the corresponding leaf
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<tt>rcu_node</tt> structure to allow more-efficient
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propagation of quiescent states up the <tt>rcu_node</tt>
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combining tree.
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Like the <tt>rcu_node</tt> structure, it provides a local
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copy of the grace-period information to allow for-free
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synchronized
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access to this information from the corresponding CPU.
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Finally, this structure records past dyntick-idle state
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for the corresponding CPU and also tracks statistics.
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<li> <tt>rcu_dynticks</tt>:
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This per-CPU structure tracks the current dyntick-idle
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state for the corresponding CPU.
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Unlike the other three structures, the <tt>rcu_dynticks</tt>
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structure is not replicated per RCU flavor.
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<li> <tt>rcu_head</tt>:
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This structure represents RCU callbacks, and is the
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only structure allocated and managed by RCU users.
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The <tt>rcu_head</tt> structure is normally embedded
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within the RCU-protected data structure.
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</ol>
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<p>If all you wanted from this article was a general notion of how
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RCU's data structures are related, you are done.
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Otherwise, each of the following sections give more details on
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the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
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and <tt>rcu_dynticks</tt> data structures.
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<h3><a name="The rcu_state Structure">
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The <tt>rcu_state</tt> Structure</a></h3>
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<p>The <tt>rcu_state</tt> structure is the base structure that
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represents a flavor of RCU.
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This structure forms the interconnection between the
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<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
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tracks grace periods, contains the lock used to
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synchronize with CPU-hotplug events,
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and maintains state used to force quiescent states when
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grace periods extend too long,
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</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
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singly and in groups, in the following sections.
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The more specialized fields are covered in the discussion of their
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use.
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<h5>Relationship to rcu_node and rcu_data Structures</h5>
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This portion of the <tt>rcu_state</tt> structure is declared
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as follows:
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<pre>
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1 struct rcu_node node[NUM_RCU_NODES];
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2 struct rcu_node *level[NUM_RCU_LVLS + 1];
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3 struct rcu_data __percpu *rda;
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</pre>
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<table>
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<tr><th> </th></tr>
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<tr><th align="left">Quick Quiz:</th></tr>
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<tr><td>
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Wait a minute!
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You said that the <tt>rcu_node</tt> structures formed a tree,
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but they are declared as a flat array!
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What gives?
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</td></tr>
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<tr><th align="left">Answer:</th></tr>
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<tr><td bgcolor="#ffffff"><font color="ffffff">
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The tree is laid out in the array.
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The first node In the array is the head, the next set of nodes in the
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array are children of the head node, and so on until the last set of
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nodes in the array are the leaves.
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</font>
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<p><font color="ffffff">See the following diagrams to see how
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this works.
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</font></td></tr>
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<tr><td> </td></tr>
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</table>
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<p>The <tt>rcu_node</tt> tree is embedded into the
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<tt>->node[]</tt> array as shown in the following figure:
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</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
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</p><p>One interesting consequence of this mapping is that a
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breadth-first traversal of the tree is implemented as a simple
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linear scan of the array, which is in fact what the
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<tt>rcu_for_each_node_breadth_first()</tt> macro does.
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This macro is used at the beginning and ends of grace periods.
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</p><p>Each entry of the <tt>->level</tt> array references
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the first <tt>rcu_node</tt> structure on the corresponding level
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of the tree, for example, as shown below:
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</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
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</p><p>The zero<sup>th</sup> element of the array references the root
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<tt>rcu_node</tt> structure, the first element references the
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first child of the root <tt>rcu_node</tt>, and finally the second
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element references the first leaf <tt>rcu_node</tt> structure.
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</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
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rather than array-shaped, it is easy to draw a planar representation:
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</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
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</p><p>Finally, the <tt>->rda</tt> field references a per-CPU
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pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
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</p><p>All of these fields are constant once initialization is complete,
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and therefore need no protection.
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<h5>Grace-Period Tracking</h5>
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<p>This portion of the <tt>rcu_state</tt> structure is declared
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as follows:
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<pre>
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1 unsigned long gp_seq;
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</pre>
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<p>RCU grace periods are numbered, and
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the <tt>->gp_seq</tt> field contains the current grace-period
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sequence number.
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The bottom two bits are the state of the current grace period,
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which can be zero for not yet started or one for in progress.
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In other words, if the bottom two bits of <tt>->gp_seq</tt> are
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zero, the corresponding flavor of RCU is idle.
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Any other value in the bottom two bits indicates that something is broken.
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This field is protected by the root <tt>rcu_node</tt> structure's
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<tt>->lock</tt> field.
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</p><p>There are <tt>->gp_seq</tt> fields
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in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
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as well.
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The fields in the <tt>rcu_state</tt> structure represent the
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most current value, and those of the other structures are compared
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in order to detect the beginnings and ends of grace periods in a distributed
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fashion.
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The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
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(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
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<h5>Miscellaneous</h5>
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<p>This portion of the <tt>rcu_state</tt> structure is declared
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as follows:
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<pre>
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1 unsigned long gp_max;
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2 char abbr;
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3 char *name;
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</pre>
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<p>The <tt>->gp_max</tt> field tracks the duration of the longest
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grace period in jiffies.
|
|
It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>.
|
|
|
|
<p>The <tt>->name</tt> field points to the name of the RCU flavor
|
|
(for example, “rcu_sched”), and is constant.
|
|
The <tt>->abbr</tt> field contains a one-character abbreviation,
|
|
for example, “s” for RCU-sched.
|
|
|
|
<h3><a name="The rcu_node Structure">
|
|
The <tt>rcu_node</tt> Structure</a></h3>
|
|
|
|
<p>The <tt>rcu_node</tt> structures form the combining
|
|
tree that propagates quiescent-state
|
|
information from the leaves to the root and also that propagates
|
|
grace-period information from the root down to the leaves.
|
|
They provides local copies of the grace-period state in order
|
|
to allow this information to be accessed in a synchronized
|
|
manner without suffering the scalability limitations that
|
|
would otherwise be imposed by global locking.
|
|
In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
|
|
of tasks that have blocked while in their current
|
|
RCU read-side critical section.
|
|
In <tt>CONFIG_PREEMPT_RCU</tt> with
|
|
<tt>CONFIG_RCU_BOOST</tt>, they manage the
|
|
per-<tt>rcu_node</tt> priority-boosting
|
|
kernel threads (kthreads) and state.
|
|
Finally, they record CPU-hotplug state in order to determine
|
|
which CPUs should be ignored during a given grace period.
|
|
|
|
</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
|
|
singly and in groups, in the following sections.
|
|
|
|
<h5>Connection to Combining Tree</h5>
|
|
|
|
<p>This portion of the <tt>rcu_node</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 struct rcu_node *parent;
|
|
2 u8 level;
|
|
3 u8 grpnum;
|
|
4 unsigned long grpmask;
|
|
5 int grplo;
|
|
6 int grphi;
|
|
</pre>
|
|
|
|
<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt>
|
|
one level up in the tree, and is <tt>NULL</tt> for the root
|
|
<tt>rcu_node</tt>.
|
|
The RCU implementation makes heavy use of this field to push quiescent
|
|
states up the tree.
|
|
The <tt>->level</tt> field gives the level in the tree, with
|
|
the root being at level zero, its children at level one, and so on.
|
|
The <tt>->grpnum</tt> field gives this node's position within
|
|
the children of its parent, so this number can range between 0 and 31
|
|
on 32-bit systems and between 0 and 63 on 64-bit systems.
|
|
The <tt>->level</tt> and <tt>->grpnum</tt> fields are
|
|
used only during initialization and for tracing.
|
|
The <tt>->grpmask</tt> field is the bitmask counterpart of
|
|
<tt>->grpnum</tt>, and therefore always has exactly one bit set.
|
|
This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
|
|
structure in its parent's bitmasks, which are described later.
|
|
Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields
|
|
contain the lowest and highest numbered CPU served by this
|
|
<tt>rcu_node</tt> structure, respectively.
|
|
|
|
</p><p>All of these fields are constant, and thus do not require any
|
|
synchronization.
|
|
|
|
<h5>Synchronization</h5>
|
|
|
|
<p>This field of the <tt>rcu_node</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 raw_spinlock_t lock;
|
|
</pre>
|
|
|
|
<p>This field is used to protect the remaining fields in this structure,
|
|
unless otherwise stated.
|
|
That said, all of the fields in this structure can be accessed without
|
|
locking for tracing purposes.
|
|
Yes, this can result in confusing traces, but better some tracing confusion
|
|
than to be heisenbugged out of existence.
|
|
|
|
<h5>Grace-Period Tracking</h5>
|
|
|
|
<p>This portion of the <tt>rcu_node</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 unsigned long gp_seq;
|
|
2 unsigned long gp_seq_needed;
|
|
</pre>
|
|
|
|
<p>The <tt>rcu_node</tt> structures' <tt>->gp_seq</tt> fields are
|
|
the counterparts of the field of the same name in the <tt>rcu_state</tt>
|
|
structure.
|
|
They each may lag up to one step behind their <tt>rcu_state</tt>
|
|
counterpart.
|
|
If the bottom two bits of a given <tt>rcu_node</tt> structure's
|
|
<tt>->gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
|
|
structure believes that RCU is idle.
|
|
</p><p>The <tt>>gp_seq</tt> field of each <tt>rcu_node</tt>
|
|
structure is updated at the beginning and the end
|
|
of each grace period.
|
|
|
|
<p>The <tt>->gp_seq_needed</tt> fields record the
|
|
furthest-in-the-future grace period request seen by the corresponding
|
|
<tt>rcu_node</tt> structure. The request is considered fulfilled when
|
|
the value of the <tt>->gp_seq</tt> field equals or exceeds that of
|
|
the <tt>->gp_seq_needed</tt> field.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
Suppose that this <tt>rcu_node</tt> structure doesn't see
|
|
a request for a very long time.
|
|
Won't wrapping of the <tt>->gp_seq</tt> field cause
|
|
problems?
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
No, because if the <tt>->gp_seq_needed</tt> field lags behind the
|
|
<tt>->gp_seq</tt> field, the <tt>->gp_seq_needed</tt> field
|
|
will be updated at the end of the grace period.
|
|
Modulo-arithmetic comparisons therefore will always get the
|
|
correct answer, even with wrapping.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<h5>Quiescent-State Tracking</h5>
|
|
|
|
<p>These fields manage the propagation of quiescent states up the
|
|
combining tree.
|
|
|
|
</p><p>This portion of the <tt>rcu_node</tt> structure has fields
|
|
as follows:
|
|
|
|
<pre>
|
|
1 unsigned long qsmask;
|
|
2 unsigned long expmask;
|
|
3 unsigned long qsmaskinit;
|
|
4 unsigned long expmaskinit;
|
|
</pre>
|
|
|
|
<p>The <tt>->qsmask</tt> field tracks which of this
|
|
<tt>rcu_node</tt> structure's children still need to report
|
|
quiescent states for the current normal grace period.
|
|
Such children will have a value of 1 in their corresponding bit.
|
|
Note that the leaf <tt>rcu_node</tt> structures should be
|
|
thought of as having <tt>rcu_data</tt> structures as their
|
|
children.
|
|
Similarly, the <tt>->expmask</tt> field tracks which
|
|
of this <tt>rcu_node</tt> structure's children still need to report
|
|
quiescent states for the current expedited grace period.
|
|
An expedited grace period has
|
|
the same conceptual properties as a normal grace period, but the
|
|
expedited implementation accepts extreme CPU overhead to obtain
|
|
much lower grace-period latency, for example, consuming a few
|
|
tens of microseconds worth of CPU time to reduce grace-period
|
|
duration from milliseconds to tens of microseconds.
|
|
The <tt>->qsmaskinit</tt> field tracks which of this
|
|
<tt>rcu_node</tt> structure's children cover for at least
|
|
one online CPU.
|
|
This mask is used to initialize <tt>->qsmask</tt>,
|
|
and <tt>->expmaskinit</tt> is used to initialize
|
|
<tt>->expmask</tt> and the beginning of the
|
|
normal and expedited grace periods, respectively.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
Why are these bitmasks protected by locking?
|
|
Come on, haven't you heard of atomic instructions???
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
Lockless grace-period computation! Such a tantalizing possibility!
|
|
</font>
|
|
|
|
<p><font color="ffffff">But consider the following sequence of events:
|
|
</font>
|
|
|
|
<ol>
|
|
<li> <font color="ffffff">CPU 0 has been in dyntick-idle
|
|
mode for quite some time.
|
|
When it wakes up, it notices that the current RCU
|
|
grace period needs it to report in, so it sets a
|
|
flag where the scheduling clock interrupt will find it.
|
|
</font><p>
|
|
<li> <font color="ffffff">Meanwhile, CPU 1 is running
|
|
<tt>force_quiescent_state()</tt>,
|
|
and notices that CPU 0 has been in dyntick idle mode,
|
|
which qualifies as an extended quiescent state.
|
|
</font><p>
|
|
<li> <font color="ffffff">CPU 0's scheduling clock
|
|
interrupt fires in the
|
|
middle of an RCU read-side critical section, and notices
|
|
that the RCU core needs something, so commences RCU softirq
|
|
processing.
|
|
</font>
|
|
<p>
|
|
<li> <font color="ffffff">CPU 0's softirq handler
|
|
executes and is just about ready
|
|
to report its quiescent state up the <tt>rcu_node</tt>
|
|
tree.
|
|
</font><p>
|
|
<li> <font color="ffffff">But CPU 1 beats it to the punch,
|
|
completing the current
|
|
grace period and starting a new one.
|
|
</font><p>
|
|
<li> <font color="ffffff">CPU 0 now reports its quiescent
|
|
state for the wrong
|
|
grace period.
|
|
That grace period might now end before the RCU read-side
|
|
critical section.
|
|
If that happens, disaster will ensue.
|
|
</font>
|
|
</ol>
|
|
|
|
<p><font color="ffffff">So the locking is absolutely required in
|
|
order to coordinate clearing of the bits with updating of the
|
|
grace-period sequence number in <tt>->gp_seq</tt>.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<h5>Blocked-Task Management</h5>
|
|
|
|
<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
|
|
midst of their RCU read-side critical sections, and these tasks
|
|
must be tracked explicitly.
|
|
The details of exactly why and how they are tracked will be covered
|
|
in a separate article on RCU read-side processing.
|
|
For now, it is enough to know that the <tt>rcu_node</tt>
|
|
structure tracks them.
|
|
|
|
<pre>
|
|
1 struct list_head blkd_tasks;
|
|
2 struct list_head *gp_tasks;
|
|
3 struct list_head *exp_tasks;
|
|
4 bool wait_blkd_tasks;
|
|
</pre>
|
|
|
|
<p>The <tt>->blkd_tasks</tt> field is a list header for
|
|
the list of blocked and preempted tasks.
|
|
As tasks undergo context switches within RCU read-side critical
|
|
sections, their <tt>task_struct</tt> structures are enqueued
|
|
(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt>
|
|
field) onto the head of the <tt>->blkd_tasks</tt> list for the
|
|
leaf <tt>rcu_node</tt> structure corresponding to the CPU
|
|
on which the outgoing context switch executed.
|
|
As these tasks later exit their RCU read-side critical sections,
|
|
they remove themselves from the list.
|
|
This list is therefore in reverse time order, so that if one of the tasks
|
|
is blocking the current grace period, all subsequent tasks must
|
|
also be blocking that same grace period.
|
|
Therefore, a single pointer into this list suffices to track
|
|
all tasks blocking a given grace period.
|
|
That pointer is stored in <tt>->gp_tasks</tt> for normal
|
|
grace periods and in <tt>->exp_tasks</tt> for expedited
|
|
grace periods.
|
|
These last two fields are <tt>NULL</tt> if either there is
|
|
no grace period in flight or if there are no blocked tasks
|
|
preventing that grace period from completing.
|
|
If either of these two pointers is referencing a task that
|
|
removes itself from the <tt>->blkd_tasks</tt> list,
|
|
then that task must advance the pointer to the next task on
|
|
the list, or set the pointer to <tt>NULL</tt> if there
|
|
are no subsequent tasks on the list.
|
|
|
|
</p><p>For example, suppose that tasks T1, T2, and T3 are
|
|
all hard-affinitied to the largest-numbered CPU in the system.
|
|
Then if task T1 blocked in an RCU read-side
|
|
critical section, then an expedited grace period started,
|
|
then task T2 blocked in an RCU read-side critical section,
|
|
then a normal grace period started, and finally task 3 blocked
|
|
in an RCU read-side critical section, then the state of the
|
|
last leaf <tt>rcu_node</tt> structure's blocked-task list
|
|
would be as shown below:
|
|
|
|
</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
|
|
|
|
</p><p>Task T1 is blocking both grace periods, task T2 is
|
|
blocking only the normal grace period, and task T3 is blocking
|
|
neither grace period.
|
|
Note that these tasks will not remove themselves from this list
|
|
immediately upon resuming execution.
|
|
They will instead remain on the list until they execute the outermost
|
|
<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
|
|
section.
|
|
|
|
<p>
|
|
The <tt>->wait_blkd_tasks</tt> field indicates whether or not
|
|
the current grace period is waiting on a blocked task.
|
|
|
|
<h5>Sizing the <tt>rcu_node</tt> Array</h5>
|
|
|
|
<p>The <tt>rcu_node</tt> array is sized via a series of
|
|
C-preprocessor expressions as follows:
|
|
|
|
<pre>
|
|
1 #ifdef CONFIG_RCU_FANOUT
|
|
2 #define RCU_FANOUT CONFIG_RCU_FANOUT
|
|
3 #else
|
|
4 # ifdef CONFIG_64BIT
|
|
5 # define RCU_FANOUT 64
|
|
6 # else
|
|
7 # define RCU_FANOUT 32
|
|
8 # endif
|
|
9 #endif
|
|
10
|
|
11 #ifdef CONFIG_RCU_FANOUT_LEAF
|
|
12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
|
|
13 #else
|
|
14 # ifdef CONFIG_64BIT
|
|
15 # define RCU_FANOUT_LEAF 64
|
|
16 # else
|
|
17 # define RCU_FANOUT_LEAF 32
|
|
18 # endif
|
|
19 #endif
|
|
20
|
|
21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
|
|
22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
|
|
23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
|
|
24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
|
|
25
|
|
26 #if NR_CPUS <= RCU_FANOUT_1
|
|
27 # define RCU_NUM_LVLS 1
|
|
28 # define NUM_RCU_LVL_0 1
|
|
29 # define NUM_RCU_NODES NUM_RCU_LVL_0
|
|
30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
|
|
31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
|
|
32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
|
|
33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
|
|
34 #elif NR_CPUS <= RCU_FANOUT_2
|
|
35 # define RCU_NUM_LVLS 2
|
|
36 # define NUM_RCU_LVL_0 1
|
|
37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
|
|
38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
|
|
39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
|
|
40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
|
|
41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
|
|
42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
|
|
43 #elif NR_CPUS <= RCU_FANOUT_3
|
|
44 # define RCU_NUM_LVLS 3
|
|
45 # define NUM_RCU_LVL_0 1
|
|
46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
|
|
47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
|
|
48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
|
|
49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
|
|
50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
|
|
51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
|
|
52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
|
|
53 #elif NR_CPUS <= RCU_FANOUT_4
|
|
54 # define RCU_NUM_LVLS 4
|
|
55 # define NUM_RCU_LVL_0 1
|
|
56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
|
|
57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
|
|
58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
|
|
59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
|
|
60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
|
|
61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
|
|
62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
|
|
63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
|
|
64 #else
|
|
65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
|
|
66 #endif
|
|
</pre>
|
|
|
|
<p>The maximum number of levels in the <tt>rcu_node</tt> structure
|
|
is currently limited to four, as specified by lines 21-24
|
|
and the structure of the subsequent “if” statement.
|
|
For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
|
|
should be sufficient for the next few years at least.
|
|
For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
|
|
should see us through the next decade or so.
|
|
This four-level tree also allows kernels built with
|
|
<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
|
|
which might be useful in very large systems having eight CPUs per
|
|
socket (but please note that no one has yet shown any measurable
|
|
performance degradation due to misaligned socket and <tt>rcu_node</tt>
|
|
boundaries).
|
|
In addition, building kernels with a full four levels of <tt>rcu_node</tt>
|
|
tree permits better testing of RCU's combining-tree code.
|
|
|
|
</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
|
|
are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
|
|
If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
|
|
it is set based on the word size of the system, which is also
|
|
the Kconfig default.
|
|
|
|
</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
|
|
handled by each leaf <tt>rcu_node</tt> structure.
|
|
Experience has shown that allowing a given leaf <tt>rcu_node</tt>
|
|
structure to handle 64 CPUs, as permitted by the number of bits in
|
|
the <tt>->qsmask</tt> field on a 64-bit system, results in
|
|
excessive contention for the leaf <tt>rcu_node</tt> structures'
|
|
<tt>->lock</tt> fields.
|
|
The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
|
|
limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
|
|
If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
|
|
selected is based on the word size of the system, just as for
|
|
<tt>CONFIG_RCU_FANOUT</tt>.
|
|
Lines 11-19 perform this computation.
|
|
|
|
</p><p>Lines 21-24 compute the maximum number of CPUs supported by
|
|
a single-level (which contains a single <tt>rcu_node</tt> structure),
|
|
two-level, three-level, and four-level <tt>rcu_node</tt> tree,
|
|
respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
|
|
and <tt>RCU_FANOUT_LEAF</tt>.
|
|
These numbers of CPUs are retained in the
|
|
<tt>RCU_FANOUT_1</tt>,
|
|
<tt>RCU_FANOUT_2</tt>,
|
|
<tt>RCU_FANOUT_3</tt>, and
|
|
<tt>RCU_FANOUT_4</tt>
|
|
C-preprocessor variables, respectively.
|
|
|
|
</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
|
|
statement spanning lines 26-66 that computes the number of
|
|
<tt>rcu_node</tt> structures required for each level of the tree,
|
|
as well as the number of levels required.
|
|
The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
|
|
C-preprocessor variable by lines 27, 35, 44, and 54.
|
|
The number of <tt>rcu_node</tt> structures for the topmost level
|
|
of the tree is always exactly one, and this value is unconditionally
|
|
placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55.
|
|
The rest of the levels (if any) of the <tt>rcu_node</tt> tree
|
|
are computed by dividing the maximum number of CPUs by the
|
|
fanout supported by the number of levels from the current level down,
|
|
rounding up. This computation is performed by lines 37,
|
|
46-47, and 56-58.
|
|
Lines 31-33, 40-42, 50-52, and 62-63 create initializers
|
|
for lockdep lock-class names.
|
|
Finally, lines 64-66 produce an error if the maximum number of
|
|
CPUs is too large for the specified fanout.
|
|
|
|
<h3><a name="The rcu_segcblist Structure">
|
|
The <tt>rcu_segcblist</tt> Structure</a></h3>
|
|
|
|
The <tt>rcu_segcblist</tt> structure maintains a segmented list of
|
|
callbacks as follows:
|
|
|
|
<pre>
|
|
1 #define RCU_DONE_TAIL 0
|
|
2 #define RCU_WAIT_TAIL 1
|
|
3 #define RCU_NEXT_READY_TAIL 2
|
|
4 #define RCU_NEXT_TAIL 3
|
|
5 #define RCU_CBLIST_NSEGS 4
|
|
6
|
|
7 struct rcu_segcblist {
|
|
8 struct rcu_head *head;
|
|
9 struct rcu_head **tails[RCU_CBLIST_NSEGS];
|
|
10 unsigned long gp_seq[RCU_CBLIST_NSEGS];
|
|
11 long len;
|
|
12 long len_lazy;
|
|
13 };
|
|
</pre>
|
|
|
|
<p>
|
|
The segments are as follows:
|
|
|
|
<ol>
|
|
<li> <tt>RCU_DONE_TAIL</tt>: Callbacks whose grace periods have elapsed.
|
|
These callbacks are ready to be invoked.
|
|
<li> <tt>RCU_WAIT_TAIL</tt>: Callbacks that are waiting for the
|
|
current grace period.
|
|
Note that different CPUs can have different ideas about which
|
|
grace period is current, hence the <tt>->gp_seq</tt> field.
|
|
<li> <tt>RCU_NEXT_READY_TAIL</tt>: Callbacks waiting for the next
|
|
grace period to start.
|
|
<li> <tt>RCU_NEXT_TAIL</tt>: Callbacks that have not yet been
|
|
associated with a grace period.
|
|
</ol>
|
|
|
|
<p>
|
|
The <tt>->head</tt> pointer references the first callback or
|
|
is <tt>NULL</tt> if the list contains no callbacks (which is
|
|
<i>not</i> the same as being empty).
|
|
Each element of the <tt>->tails[]</tt> array references the
|
|
<tt>->next</tt> pointer of the last callback in the corresponding
|
|
segment of the list, or the list's <tt>->head</tt> pointer if
|
|
that segment and all previous segments are empty.
|
|
If the corresponding segment is empty but some previous segment is
|
|
not empty, then the array element is identical to its predecessor.
|
|
Older callbacks are closer to the head of the list, and new callbacks
|
|
are added at the tail.
|
|
This relationship between the <tt>->head</tt> pointer, the
|
|
<tt>->tails[]</tt> array, and the callbacks is shown in this
|
|
diagram:
|
|
|
|
</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
|
|
|
|
</p><p>In this figure, the <tt>->head</tt> pointer references the
|
|
first
|
|
RCU callback in the list.
|
|
The <tt>->tails[RCU_DONE_TAIL]</tt> array element references
|
|
the <tt>->head</tt> pointer itself, indicating that none
|
|
of the callbacks is ready to invoke.
|
|
The <tt>->tails[RCU_WAIT_TAIL]</tt> array element references callback
|
|
CB 2's <tt>->next</tt> pointer, which indicates that
|
|
CB 1 and CB 2 are both waiting on the current grace period,
|
|
give or take possible disagreements about exactly which grace period
|
|
is the current one.
|
|
The <tt>->tails[RCU_NEXT_READY_TAIL]</tt> array element
|
|
references the same RCU callback that <tt>->tails[RCU_WAIT_TAIL]</tt>
|
|
does, which indicates that there are no callbacks waiting on the next
|
|
RCU grace period.
|
|
The <tt>->tails[RCU_NEXT_TAIL]</tt> array element references
|
|
CB 4's <tt>->next</tt> pointer, indicating that all the
|
|
remaining RCU callbacks have not yet been assigned to an RCU grace
|
|
period.
|
|
Note that the <tt>->tails[RCU_NEXT_TAIL]</tt> array element
|
|
always references the last RCU callback's <tt>->next</tt> pointer
|
|
unless the callback list is empty, in which case it references
|
|
the <tt>->head</tt> pointer.
|
|
|
|
<p>
|
|
There is one additional important special case for the
|
|
<tt>->tails[RCU_NEXT_TAIL]</tt> array element: It can be <tt>NULL</tt>
|
|
when this list is <i>disabled</i>.
|
|
Lists are disabled when the corresponding CPU is offline or when
|
|
the corresponding CPU's callbacks are offloaded to a kthread,
|
|
both of which are described elsewhere.
|
|
|
|
</p><p>CPUs advance their callbacks from the
|
|
<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
|
|
<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
|
|
as grace periods advance.
|
|
|
|
</p><p>The <tt>->gp_seq[]</tt> array records grace-period
|
|
numbers corresponding to the list segments.
|
|
This is what allows different CPUs to have different ideas as to
|
|
which is the current grace period while still avoiding premature
|
|
invocation of their callbacks.
|
|
In particular, this allows CPUs that go idle for extended periods
|
|
to determine which of their callbacks are ready to be invoked after
|
|
reawakening.
|
|
|
|
</p><p>The <tt>->len</tt> counter contains the number of
|
|
callbacks in <tt>->head</tt>, and the
|
|
<tt>->len_lazy</tt> contains the number of those callbacks that
|
|
are known to only free memory, and whose invocation can therefore
|
|
be safely deferred.
|
|
|
|
<p><b>Important note</b>: It is the <tt>->len</tt> field that
|
|
determines whether or not there are callbacks associated with
|
|
this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>->head</tt>
|
|
pointer.
|
|
The reason for this is that all the ready-to-invoke callbacks
|
|
(that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
|
|
all at once at callback-invocation time.
|
|
If callback invocation must be postponed, for example, because a
|
|
high-priority process just woke up on this CPU, then the remaining
|
|
callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment.
|
|
Either way, the <tt>->len</tt> and <tt>->len_lazy</tt> counts
|
|
are adjusted after the corresponding callbacks have been invoked, and so
|
|
again it is the <tt>->len</tt> count that accurately reflects whether
|
|
or not there are callbacks associated with this <tt>rcu_segcblist</tt>
|
|
structure.
|
|
Of course, off-CPU sampling of the <tt>->len</tt> count requires
|
|
the use of appropriate synchronization, for example, memory barriers.
|
|
This synchronization can be a bit subtle, particularly in the case
|
|
of <tt>rcu_barrier()</tt>.
|
|
|
|
<h3><a name="The rcu_data Structure">
|
|
The <tt>rcu_data</tt> Structure</a></h3>
|
|
|
|
<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
|
|
corresponding flavor of RCU.
|
|
The fields in this structure may be accessed only from the corresponding
|
|
CPU (and from tracing) unless otherwise stated.
|
|
This structure is the
|
|
focus of quiescent-state detection and RCU callback queuing.
|
|
It also tracks its relationship to the corresponding leaf
|
|
<tt>rcu_node</tt> structure to allow more-efficient
|
|
propagation of quiescent states up the <tt>rcu_node</tt>
|
|
combining tree.
|
|
Like the <tt>rcu_node</tt> structure, it provides a local
|
|
copy of the grace-period information to allow for-free
|
|
synchronized
|
|
access to this information from the corresponding CPU.
|
|
Finally, this structure records past dyntick-idle state
|
|
for the corresponding CPU and also tracks statistics.
|
|
|
|
</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
|
|
singly and in groups, in the following sections.
|
|
|
|
<h5>Connection to Other Data Structures</h5>
|
|
|
|
<p>This portion of the <tt>rcu_data</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 int cpu;
|
|
2 struct rcu_state *rsp;
|
|
3 struct rcu_node *mynode;
|
|
4 struct rcu_dynticks *dynticks;
|
|
5 unsigned long grpmask;
|
|
6 bool beenonline;
|
|
</pre>
|
|
|
|
<p>The <tt>->cpu</tt> field contains the number of the
|
|
corresponding CPU, the <tt>->rsp</tt> pointer references
|
|
the corresponding <tt>rcu_state</tt> structure (and is most frequently
|
|
used to locate the name of the corresponding flavor of RCU for tracing),
|
|
and the <tt>->mynode</tt> field references the corresponding
|
|
<tt>rcu_node</tt> structure.
|
|
The <tt>->mynode</tt> is used to propagate quiescent states
|
|
up the combining tree.
|
|
<p>The <tt>->dynticks</tt> pointer references the
|
|
<tt>rcu_dynticks</tt> structure corresponding to this
|
|
CPU.
|
|
Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
|
|
structure is shared among all flavors of RCU.
|
|
These first four fields are constant and therefore require not
|
|
synchronization.
|
|
|
|
</p><p>The <tt>->grpmask</tt> field indicates the bit in
|
|
the <tt>->mynode->qsmask</tt> corresponding to this
|
|
<tt>rcu_data</tt> structure, and is also used when propagating
|
|
quiescent states.
|
|
The <tt>->beenonline</tt> flag is set whenever the corresponding
|
|
CPU comes online, which means that the debugfs tracing need not dump
|
|
out any <tt>rcu_data</tt> structure for which this flag is not set.
|
|
|
|
<h5>Quiescent-State and Grace-Period Tracking</h5>
|
|
|
|
<p>This portion of the <tt>rcu_data</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 unsigned long gp_seq;
|
|
2 unsigned long gp_seq_needed;
|
|
3 bool cpu_no_qs;
|
|
4 bool core_needs_qs;
|
|
5 bool gpwrap;
|
|
6 unsigned long rcu_qs_ctr_snap;
|
|
</pre>
|
|
|
|
<p>The <tt>->gp_seq</tt> and <tt>->gp_seq_needed</tt>
|
|
fields are the counterparts of the fields of the same name
|
|
in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
|
|
They may each lag up to one behind their <tt>rcu_node</tt>
|
|
counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
|
|
<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
|
|
arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
|
|
will catch up upon exit from dyntick-idle mode).
|
|
If the lower two bits of a given <tt>rcu_data</tt> structure's
|
|
<tt>->gp_seq</tt> are zero, then this <tt>rcu_data</tt>
|
|
structure believes that RCU is idle.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
All this replication of the grace period numbers can only cause
|
|
massive confusion.
|
|
Why not just keep a global sequence number and be done with it???
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
Because if there was only a single global sequence
|
|
numbers, there would need to be a single global lock to allow
|
|
safely accessing and updating it.
|
|
And if we are not going to have a single global lock, we need
|
|
to carefully manage the numbers on a per-node basis.
|
|
Recall from the answer to a previous Quick Quiz that the consequences
|
|
of applying a previously sampled quiescent state to the wrong
|
|
grace period are quite severe.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<p>The <tt>->cpu_no_qs</tt> flag indicates that the
|
|
CPU has not yet passed through a quiescent state,
|
|
while the <tt>->core_needs_qs</tt> flag indicates that the
|
|
RCU core needs a quiescent state from the corresponding CPU.
|
|
The <tt>->gpwrap</tt> field indicates that the corresponding
|
|
CPU has remained idle for so long that the
|
|
<tt>gp_seq</tt> counter is in danger of overflow, which
|
|
will cause the CPU to disregard the values of its counters on
|
|
its next exit from idle.
|
|
Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
|
|
cases where a given operation has resulted in a quiescent state
|
|
for all flavors of RCU, for example, <tt>cond_resched()</tt>
|
|
when RCU has indicated a need for quiescent states.
|
|
|
|
<h5>RCU Callback Handling</h5>
|
|
|
|
<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
|
|
the same CPU that registered them.
|
|
This is strictly a cache-locality optimization: callbacks can and
|
|
do get invoked on CPUs other than the one that registered them.
|
|
After all, if the CPU that registered a given callback has gone
|
|
offline before the callback can be invoked, there really is no other
|
|
choice.
|
|
|
|
</p><p>This portion of the <tt>rcu_data</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 struct rcu_segcblist cblist;
|
|
2 long qlen_last_fqs_check;
|
|
3 unsigned long n_cbs_invoked;
|
|
4 unsigned long n_nocbs_invoked;
|
|
5 unsigned long n_cbs_orphaned;
|
|
6 unsigned long n_cbs_adopted;
|
|
7 unsigned long n_force_qs_snap;
|
|
8 long blimit;
|
|
</pre>
|
|
|
|
<p>The <tt>->cblist</tt> structure is the segmented callback list
|
|
described earlier.
|
|
The CPU advances the callbacks in its <tt>rcu_data</tt> structure
|
|
whenever it notices that another RCU grace period has completed.
|
|
The CPU detects the completion of an RCU grace period by noticing
|
|
that the value of its <tt>rcu_data</tt> structure's
|
|
<tt>->gp_seq</tt> field differs from that of its leaf
|
|
<tt>rcu_node</tt> structure.
|
|
Recall that each <tt>rcu_node</tt> structure's
|
|
<tt>->gp_seq</tt> field is updated at the beginnings and ends of each
|
|
grace period.
|
|
|
|
<p>
|
|
The <tt>->qlen_last_fqs_check</tt> and
|
|
<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent
|
|
states from <tt>call_rcu()</tt> and friends when callback
|
|
lists grow excessively long.
|
|
|
|
</p><p>The <tt>->n_cbs_invoked</tt>,
|
|
<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt>
|
|
fields count the number of callbacks invoked,
|
|
sent to other CPUs when this CPU goes offline,
|
|
and received from other CPUs when those other CPUs go offline.
|
|
The <tt>->n_nocbs_invoked</tt> is used when the CPU's callbacks
|
|
are offloaded to a kthread.
|
|
|
|
<p>
|
|
Finally, the <tt>->blimit</tt> counter is the maximum number of
|
|
RCU callbacks that may be invoked at a given time.
|
|
|
|
<h5>Dyntick-Idle Handling</h5>
|
|
|
|
<p>This portion of the <tt>rcu_data</tt> structure is declared
|
|
as follows:
|
|
|
|
<pre>
|
|
1 int dynticks_snap;
|
|
2 unsigned long dynticks_fqs;
|
|
</pre>
|
|
|
|
The <tt>->dynticks_snap</tt> field is used to take a snapshot
|
|
of the corresponding CPU's dyntick-idle state when forcing
|
|
quiescent states, and is therefore accessed from other CPUs.
|
|
Finally, the <tt>->dynticks_fqs</tt> field is used to
|
|
count the number of times this CPU is determined to be in
|
|
dyntick-idle state, and is used for tracing and debugging purposes.
|
|
|
|
<h3><a name="The rcu_dynticks Structure">
|
|
The <tt>rcu_dynticks</tt> Structure</a></h3>
|
|
|
|
<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
|
|
for the corresponding CPU.
|
|
Unlike the other structures, <tt>rcu_dynticks</tt> is not
|
|
replicated over the different flavors of RCU.
|
|
The fields in this structure may be accessed only from the corresponding
|
|
CPU (and from tracing) unless otherwise stated.
|
|
Its fields are as follows:
|
|
|
|
<pre>
|
|
1 long dynticks_nesting;
|
|
2 long dynticks_nmi_nesting;
|
|
3 atomic_t dynticks;
|
|
4 bool rcu_need_heavy_qs;
|
|
5 unsigned long rcu_qs_ctr;
|
|
6 bool rcu_urgent_qs;
|
|
</pre>
|
|
|
|
<p>The <tt>->dynticks_nesting</tt> field counts the
|
|
nesting depth of process execution, so that in normal circumstances
|
|
this counter has value zero or one.
|
|
NMIs, irqs, and tracers are counted by the <tt>->dynticks_nmi_nesting</tt>
|
|
field.
|
|
Because NMIs cannot be masked, changes to this variable have to be
|
|
undertaken carefully using an algorithm provided by Andy Lutomirski.
|
|
The initial transition from idle adds one, and nested transitions
|
|
add two, so that a nesting level of five is represented by a
|
|
<tt>->dynticks_nmi_nesting</tt> value of nine.
|
|
This counter can therefore be thought of as counting the number
|
|
of reasons why this CPU cannot be permitted to enter dyntick-idle
|
|
mode, aside from process-level transitions.
|
|
|
|
<p>However, it turns out that when running in non-idle kernel context,
|
|
the Linux kernel is fully capable of entering interrupt handlers that
|
|
never exit and perhaps also vice versa.
|
|
Therefore, whenever the <tt>->dynticks_nesting</tt> field is
|
|
incremented up from zero, the <tt>->dynticks_nmi_nesting</tt> field
|
|
is set to a large positive number, and whenever the
|
|
<tt>->dynticks_nesting</tt> field is decremented down to zero,
|
|
the the <tt>->dynticks_nmi_nesting</tt> field is set to zero.
|
|
Assuming that the number of misnested interrupts is not sufficient
|
|
to overflow the counter, this approach corrects the
|
|
<tt>->dynticks_nmi_nesting</tt> field every time the corresponding
|
|
CPU enters the idle loop from process context.
|
|
|
|
</p><p>The <tt>->dynticks</tt> field counts the corresponding
|
|
CPU's transitions to and from dyntick-idle mode, so that this counter
|
|
has an even value when the CPU is in dyntick-idle mode and an odd
|
|
value otherwise.
|
|
|
|
</p><p>The <tt>->rcu_need_heavy_qs</tt> field is used
|
|
to record the fact that the RCU core code would really like to
|
|
see a quiescent state from the corresponding CPU, so much so that
|
|
it is willing to call for heavy-weight dyntick-counter operations.
|
|
This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
|
|
code, which provide a momentary idle sojourn in response.
|
|
|
|
</p><p>The <tt>->rcu_qs_ctr</tt> field is used to record
|
|
quiescent states from <tt>cond_resched()</tt>.
|
|
Because <tt>cond_resched()</tt> can execute quite frequently, this
|
|
must be quite lightweight, as in a non-atomic increment of this
|
|
per-CPU field.
|
|
|
|
</p><p>Finally, the <tt>->rcu_urgent_qs</tt> field is used to record
|
|
the fact that the RCU core code would really like to see a quiescent
|
|
state from the corresponding CPU, with the various other fields indicating
|
|
just how badly RCU wants this quiescent state.
|
|
This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
|
|
code, which, if nothing else, non-atomically increment <tt>->rcu_qs_ctr</tt>
|
|
in response.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
Why not simply combine the <tt>->dynticks_nesting</tt>
|
|
and <tt>->dynticks_nmi_nesting</tt> counters into a
|
|
single counter that just counts the number of reasons that
|
|
the corresponding CPU is non-idle?
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
Because this would fail in the presence of interrupts whose
|
|
handlers never return and of handlers that manage to return
|
|
from a made-up interrupt.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<p>Additional fields are present for some special-purpose
|
|
builds, and are discussed separately.
|
|
|
|
<h3><a name="The rcu_head Structure">
|
|
The <tt>rcu_head</tt> Structure</a></h3>
|
|
|
|
<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
|
|
These structures are normally embedded within RCU-protected data
|
|
structures whose algorithms use asynchronous grace periods.
|
|
In contrast, when using algorithms that block waiting for RCU grace periods,
|
|
RCU users need not provide <tt>rcu_head</tt> structures.
|
|
|
|
</p><p>The <tt>rcu_head</tt> structure has fields as follows:
|
|
|
|
<pre>
|
|
1 struct rcu_head *next;
|
|
2 void (*func)(struct rcu_head *head);
|
|
</pre>
|
|
|
|
<p>The <tt>->next</tt> field is used
|
|
to link the <tt>rcu_head</tt> structures together in the
|
|
lists within the <tt>rcu_data</tt> structures.
|
|
The <tt>->func</tt> field is a pointer to the function
|
|
to be called when the callback is ready to be invoked, and
|
|
this function is passed a pointer to the <tt>rcu_head</tt>
|
|
structure.
|
|
However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt>
|
|
field to record the offset of the <tt>rcu_head</tt>
|
|
structure within the enclosing RCU-protected data structure.
|
|
|
|
</p><p>Both of these fields are used internally by RCU.
|
|
From the viewpoint of RCU users, this structure is an
|
|
opaque “cookie”.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
Given that the callback function <tt>->func</tt>
|
|
is passed a pointer to the <tt>rcu_head</tt> structure,
|
|
how is that function supposed to find the beginning of the
|
|
enclosing RCU-protected data structure?
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
In actual practice, there is a separate callback function per
|
|
type of RCU-protected data structure.
|
|
The callback function can therefore use the <tt>container_of()</tt>
|
|
macro in the Linux kernel (or other pointer-manipulation facilities
|
|
in other software environments) to find the beginning of the
|
|
enclosing structure.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<h3><a name="RCU-Specific Fields in the task_struct Structure">
|
|
RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
|
|
|
|
<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
|
|
additional fields in the <tt>task_struct</tt> structure:
|
|
|
|
<pre>
|
|
1 #ifdef CONFIG_PREEMPT_RCU
|
|
2 int rcu_read_lock_nesting;
|
|
3 union rcu_special rcu_read_unlock_special;
|
|
4 struct list_head rcu_node_entry;
|
|
5 struct rcu_node *rcu_blocked_node;
|
|
6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
|
|
7 #ifdef CONFIG_TASKS_RCU
|
|
8 unsigned long rcu_tasks_nvcsw;
|
|
9 bool rcu_tasks_holdout;
|
|
10 struct list_head rcu_tasks_holdout_list;
|
|
11 int rcu_tasks_idle_cpu;
|
|
12 #endif /* #ifdef CONFIG_TASKS_RCU */
|
|
</pre>
|
|
|
|
<p>The <tt>->rcu_read_lock_nesting</tt> field records the
|
|
nesting level for RCU read-side critical sections, and
|
|
the <tt>->rcu_read_unlock_special</tt> field is a bitmask
|
|
that records special conditions that require <tt>rcu_read_unlock()</tt>
|
|
to do additional work.
|
|
The <tt>->rcu_node_entry</tt> field is used to form lists of
|
|
tasks that have blocked within preemptible-RCU read-side critical
|
|
sections and the <tt>->rcu_blocked_node</tt> field references
|
|
the <tt>rcu_node</tt> structure whose list this task is a member of,
|
|
or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
|
|
read-side critical section.
|
|
|
|
<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of
|
|
voluntary context switches that this task had undergone at the
|
|
beginning of the current tasks-RCU grace period,
|
|
<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU
|
|
grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt>
|
|
is a list element enqueuing this task on the holdout list,
|
|
and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this
|
|
idle task is running, but only if the task is currently running,
|
|
that is, if the CPU is currently idle.
|
|
|
|
<h3><a name="Accessor Functions">
|
|
Accessor Functions</a></h3>
|
|
|
|
<p>The following listing shows the
|
|
<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
|
|
<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
|
|
<tt>rcu_for_each_leaf_node()</tt> function and macros:
|
|
|
|
<pre>
|
|
1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
|
|
2 {
|
|
3 return &rsp->node[0];
|
|
4 }
|
|
5
|
|
6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
|
|
7 for ((rnp) = &(rsp)->node[0]; \
|
|
8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
|
|
9
|
|
10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
|
|
11 for ((rnp) = &(rsp)->node[0]; \
|
|
12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++)
|
|
13
|
|
14 #define rcu_for_each_leaf_node(rsp, rnp) \
|
|
15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
|
|
16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
|
|
</pre>
|
|
|
|
<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
|
|
first element of the specified <tt>rcu_state</tt> structure's
|
|
<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt>
|
|
structure.
|
|
|
|
</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
|
|
macro takes advantage of the layout of the <tt>rcu_node</tt>
|
|
structures in the <tt>rcu_state</tt> structure's
|
|
<tt>->node[]</tt> array, performing a breadth-first traversal by
|
|
simply traversing the array in order.
|
|
The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
|
|
similarly, but traverses only the first part of the array, thus excluding
|
|
the leaf <tt>rcu_node</tt> structures.
|
|
Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
|
|
the last part of the array, thus traversing only the leaf
|
|
<tt>rcu_node</tt> structures.
|
|
|
|
<table>
|
|
<tr><th> </th></tr>
|
|
<tr><th align="left">Quick Quiz:</th></tr>
|
|
<tr><td>
|
|
What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
|
|
<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
|
|
contains only a single node?
|
|
</td></tr>
|
|
<tr><th align="left">Answer:</th></tr>
|
|
<tr><td bgcolor="#ffffff"><font color="ffffff">
|
|
In the single-node case,
|
|
<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
|
|
and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
|
|
</font></td></tr>
|
|
<tr><td> </td></tr>
|
|
</table>
|
|
|
|
<h3><a name="Summary">
|
|
Summary</a></h3>
|
|
|
|
So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
|
|
which contains a combining tree of <tt>rcu_node</tt> and
|
|
<tt>rcu_data</tt> structures.
|
|
Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
|
|
state is tracked by an <tt>rcu_dynticks</tt> structure.
|
|
|
|
If you made it this far, you are well prepared to read the code
|
|
walkthroughs in the other articles in this series.
|
|
|
|
<h3><a name="Acknowledgments">
|
|
Acknowledgments</a></h3>
|
|
|
|
I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
|
|
Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
|
|
for helping me get this document into a more human-readable state.
|
|
|
|
<h3><a name="Legal Statement">
|
|
Legal Statement</a></h3>
|
|
|
|
<p>This work represents the view of the author and does not necessarily
|
|
represent the view of IBM.
|
|
|
|
</p><p>Linux is a registered trademark of Linus Torvalds.
|
|
|
|
</p><p>Other company, product, and service names may be trademarks or
|
|
service marks of others.
|
|
|
|
</body></html>
|