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doc: Update dyntick-idle design documentation for NMI/irq consolidation
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -1182,8 +1182,8 @@ CPU (and from tracing) unless otherwise stated.
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Its fields are as follows:
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<pre>
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1 int dynticks_nesting;
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2 int dynticks_nmi_nesting;
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1 long dynticks_nesting;
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2 long dynticks_nmi_nesting;
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3 atomic_t dynticks;
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4 bool rcu_need_heavy_qs;
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5 unsigned long rcu_qs_ctr;
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@ -1191,15 +1191,31 @@ Its fields are as follows:
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</pre>
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<p>The <tt>->dynticks_nesting</tt> field counts the
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nesting depth of normal interrupts.
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In addition, this counter is incremented when exiting dyntick-idle
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mode and decremented when entering it.
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nesting depth of process execution, so that in normal circumstances
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this counter has value zero or one.
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NMIs, irqs, and tracers are counted by the <tt>->dynticks_nmi_nesting</tt>
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field.
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Because NMIs cannot be masked, changes to this variable have to be
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undertaken carefully using an algorithm provided by Andy Lutomirski.
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The initial transition from idle adds one, and nested transitions
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add two, so that a nesting level of five is represented by a
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<tt>->dynticks_nmi_nesting</tt> value of nine.
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This counter can therefore be thought of as counting the number
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of reasons why this CPU cannot be permitted to enter dyntick-idle
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mode, aside from non-maskable interrupts (NMIs).
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NMIs are counted by the <tt>->dynticks_nmi_nesting</tt>
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field, except that NMIs that interrupt non-dyntick-idle execution
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are not counted.
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mode, aside from process-level transitions.
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<p>However, it turns out that when running in non-idle kernel context,
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the Linux kernel is fully capable of entering interrupt handlers that
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never exit and perhaps also vice versa.
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Therefore, whenever the <tt>->dynticks_nesting</tt> field is
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incremented up from zero, the <tt>->dynticks_nmi_nesting</tt> field
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is set to a large positive number, and whenever the
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<tt>->dynticks_nesting</tt> field is decremented down to zero,
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the the <tt>->dynticks_nmi_nesting</tt> field is set to zero.
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Assuming that the number of misnested interrupts is not sufficient
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to overflow the counter, this approach corrects the
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<tt>->dynticks_nmi_nesting</tt> field every time the corresponding
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CPU enters the idle loop from process context.
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</p><p>The <tt>->dynticks</tt> field counts the corresponding
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CPU's transitions to and from dyntick-idle mode, so that this counter
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@ -1231,14 +1247,16 @@ in response.
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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 not just count all NMIs?
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Wouldn't that be simpler and less error prone?
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Why not simply combine the <tt>->dynticks_nesting</tt>
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and <tt>->dynticks_nmi_nesting</tt> counters into a
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single counter that just counts the number of reasons that
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the corresponding CPU is non-idle?
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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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It seems simpler only until you think hard about how to go about
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updating the <tt>rcu_dynticks</tt> structure's
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<tt>->dynticks</tt> field.
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Because this would fail in the presence of interrupts whose
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handlers never return and of handlers that manage to return
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from a made-up interrupt.
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</font></td></tr>
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<tr><td> </td></tr>
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</table>
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