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doc: Remove rcu_dynticks from Data-Structures 

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

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Joel Fernandes (Google) 2018-10-03 17:37:25 -07:00 committed by Paul E. McKenney
parent 5cc379a42a
commit c9b6f899e1
2 changed files with 25 additions and 760 deletions
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@ -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">
@ -174,16 +172,8 @@ 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.
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 +999,19 @@ 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>-&gt;cpu</tt> field contains the number of the
corresponding CPU, the <tt>-&gt;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>-&gt;mynode</tt> field references the corresponding
<tt>rcu_node</tt> structure.
corresponding CPU and the <tt>-&gt;mynode</tt> field references the
corresponding <tt>rcu_node</tt> structure.
The <tt>-&gt;mynode</tt> is used to propagate quiescent states
up the combining tree.
<p>The <tt>-&gt;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.
These two fields are constant and therefore do not require synchronization.
</p><p>The <tt>-&gt;grpmask</tt> field indicates the bit in
<p>The <tt>-&gt;grpmask</tt> field indicates the bit in
the <tt>-&gt;mynode-&gt;qsmask</tt> corresponding to this
<tt>rcu_data</tt> structure, and is also used when propagating
quiescent states.
@ -1181,26 +1152,22 @@ Finally, the <tt>-&gt;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>-&gt;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 +1209,12 @@ 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>-&gt;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>-&gt;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>-&gt;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>&nbsp;</th></tr>
@ -1431,7 +1391,7 @@ 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.