From 3af3999b9a325d462c9353389b7507c4b7bc5428 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 6 Oct 2017 13:48:14 -0700 Subject: doc: Update dyntick-idle design documentation for NMI/irq consolidation Signed-off-by: Paul E. McKenney --- .../Design/Data-Structures/Data-Structures.html | 46 +++++++++++++++------- 1 file changed, 32 insertions(+), 14 deletions(-) (limited to 'Documentation/RCU/Design/Data-Structures') 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: 
-  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:

The ->dynticks_nesting 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 ->dynticks_nmi_nesting +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 +->dynticks_nmi_nesting 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 ->dynticks_nmi_nesting -field, except that NMIs that interrupt non-dyntick-idle execution -are not counted. +mode, aside from process-level transitions. + +

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 ->dynticks_nesting field is +incremented up from zero, the ->dynticks_nmi_nesting field +is set to a large positive number, and whenever the +->dynticks_nesting field is decremented down to zero, +the the ->dynticks_nmi_nesting field is set to zero. +Assuming that the number of misnested interrupts is not sufficient +to overflow the counter, this approach corrects the +->dynticks_nmi_nesting field every time the corresponding +CPU enters the idle loop from process context.

The ->dynticks field counts the corresponding CPU's transitions to and from dyntick-idle mode, so that this counter @@ -1231,14 +1247,16 @@ in response.   Quick Quiz: - Why not just count all NMIs? - Wouldn't that be simpler and less error prone? + Why not simply combine the ->dynticks_nesting + and ->dynticks_nmi_nesting counters into a + single counter that just counts the number of reasons that + the corresponding CPU is non-idle? Answer: - It seems simpler only until you think hard about how to go about - updating the rcu_dynticks structure's - ->dynticks 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.   -- cgit