Commit-ID: 3af3999b9a325d462c9353389b7507c4b7bc5428 Gitweb: https://git.kernel.org/tip/3af3999b9a325d462c9353389b7507c4b7bc5428 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Fri, 6 Oct 2017 13:48:14 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Tue, 28 Nov 2017 15:51:22 -0800 doc: Update dyntick-idle design documentation for NMI/irq consolidation Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- .../Design/Data-Structures/Data-Structures.html | 46 +++++++++++++++------- 1 file 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 38d6d80..1ac011d 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>->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>->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 non-maskable interrupts (NMIs). -NMIs are counted by the <tt>->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>->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 @@ -1231,14 +1247,16 @@ in response. <tr><th> </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>->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"> - It seems simpler only until you think hard about how to go about - updating the <tt>rcu_dynticks</tt> structure's - <tt>->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> </td></tr> </table> -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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