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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-10-06 13:48:14 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-11-28 15:51:22 -0800
commit3af3999b9a325d462c9353389b7507c4b7bc5428 (patch)
treeda4c339294d84236b04daeca78c9d85da247ed27 /Documentation/RCU/Design
parente68bbb266dcfed201d8d54a2828ef820d747f083 (diff)
doc: Update dyntick-idle design documentation for NMI/irq consolidation
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'Documentation/RCU/Design')
-rw-r--r--Documentation/RCU/Design/Data-Structures/Data-Structures.html46
1 files changed, 32 insertions, 14 deletions
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 38d6d800761f..1ac011de606e 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -1182,8 +1182,8 @@ CPU (and from tracing) unless otherwise stated.
Its fields are as follows:
<pre>
- 1 int dynticks_nesting;
- 2 int dynticks_nmi_nesting;
+ 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;
@@ -1191,15 +1191,31 @@ Its fields are as follows:
</pre>
<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
-nesting depth of normal interrupts.
-In addition, this counter is incremented when exiting dyntick-idle
-mode and decremented when entering it.
+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>-&gt;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>-&gt;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 non-maskable interrupts (NMIs).
-NMIs are counted by the <tt>-&gt;dynticks_nmi_nesting</tt>
-field, except that NMIs that interrupt non-dyntick-idle execution
-are not counted.
+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>-&gt;dynticks_nesting</tt> field is
+incremented up from zero, the <tt>-&gt;dynticks_nmi_nesting</tt> field
+is set to a large positive number, and whenever the
+<tt>-&gt;dynticks_nesting</tt> field is decremented down to zero,
+the the <tt>-&gt;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>-&gt;dynticks_nmi_nesting</tt> field every time the corresponding
+CPU enters the idle loop from process context.
</p><p>The <tt>-&gt;dynticks</tt> field counts the corresponding
CPU's transitions to and from dyntick-idle mode, so that this counter
@@ -1231,14 +1247,16 @@ in response.
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
- Why not just count all NMIs?
- Wouldn't that be simpler and less error prone?
+ Why not simply combine the <tt>-&gt;dynticks_nesting</tt>
+ and <tt>-&gt;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">
- It seems simpler only until you think hard about how to go about
- updating the <tt>rcu_dynticks</tt> structure's
- <tt>-&gt;dynticks</tt> field.
+ 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>&nbsp;</td></tr>
</table>