[2/5] doc: rcu: Remove rcu_dynticks from Data-Structures

From: Joel Fernandes (Google)
Date: Tue Sep 25 2018 - 14:26:35 EST

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rcu_dynticks was folded into rcu_data structure. Update the data
structures RCU document accordingly.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
.../BigTreeClassicRCUBHdyntick.svg | 695 ------------------
.../Data-Structures/Data-Structures.html | 92 +--
2 files changed, 25 insertions(+), 762 deletions(-)
delete mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg

diff --git a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg
deleted file mode 100644
index 21ba7823479d..000000000000
--- a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg
+++ /dev/null
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diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 476b1ac38e4c..f58cd3912918 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -23,8 +23,6 @@ to each other.
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_dynticks Structure">
- The <tt>rcu_dynticks</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">
@@ -173,17 +171,9 @@ CPUs whose scheduling-clock interrupts have been turned off are
said to be in <i>dyntick-idle mode</i>.
RCU must handle dyntick-idle CPUs specially
because RCU would otherwise wake up each CPU on every grace period,
-which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
-RCU uses the <tt>rcu_dynticks</tt> structure to track
-which CPUs are in dyntick idle mode, as shown below:
-
-</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
-
-</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
-for all flavors of RCU.
-Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
-CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
-that <tt>rcu_dynticks</tt>, as shown in the figure.
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>. RCU uses
+the dynticks related fields in the <tt>rcu_data</tt> structure to track which
+CPUs are in dyntick idle mode.

</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
@@ -216,9 +206,6 @@ its own synchronization:
<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.
-<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private
- to the corresponding CPU, although a few can be read by
- other CPUs.
</ol>

<p>It is important to note that different data structures can have
@@ -274,11 +261,6 @@ follows:
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_dynticks</tt>:
- This per-CPU structure tracks the current dyntick-idle
- state for the corresponding CPU.
- Unlike the other three structures, the <tt>rcu_dynticks</tt>
- structure is not replicated per RCU flavor.
<li> <tt>rcu_head</tt>:
This structure represents RCU callbacks, and is the
only structure allocated and managed by RCU users.
@@ -289,8 +271,8 @@ follows:
<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>, <tt>rcu_data</tt>,
-and <tt>rcu_dynticks</tt> data structures.
+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>
@@ -1017,30 +999,18 @@ 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;
+ 2 struct rcu_node *mynode;
+ 3 unsigned long grpmask;
+ 4 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.
+corresponding CPU 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
+These two fields are constant and therefore donot require synchronization.
+<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.
@@ -1181,26 +1151,22 @@ 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:
+<p>
+This portion of the rcu_data structure is declared 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;
+ 5 bool rcu_urgent_qs;
</pre>

+<p>These fields in the rcu_data structure maintain the per-CPU dyntick-idle
+state for the corresponding CPU.
+The fields may be accessed only from the corresponding CPU (and from tracing)
+unless otherwise stated.
+
<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.
@@ -1242,19 +1208,11 @@ 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.
+the fact that the RCU core code would really like to see a quiescent state from
+the corresponding CPU, with the various other fields indicating just how badly
+RCU wants this quiescent state. This flag is checked by RCU's context-switch path
+(<tt>rcu_note_context_switch</tt>) and the cond_resched code.

<table>
<tr><th> </th></tr>
@@ -1431,7 +1389,7 @@ 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.
+state is tracked by dynticks-related fields in the <tt>rcu_data</tt> structure.

If you made it this far, you are well prepared to read the code
walkthroughs in the other articles in this series.
--
2.19.0.444.g18242da7ef-goog

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