[PATCH v3 2/5] rcu/tree: Add better tracing for dyntick-idle

[Joel Fernandes (Google)]

The dyntick-idle traces are a bit confusing. This patch makes it simpler
and adds some missing cases such as EQS-enter due to user vs idle mode.

Following are the changes:
(1) Add a new context field to trace_rcu_dyntick tracepoint. This
    context field can be "USER", "IDLE" or "IRQ".

(2) Remove the "++=" and "--=" strings and replace them with
   "StillNonIdle". This is much easier on the eyes, and the -- and ++
   are easily apparent in the dynticks_nesting counters we are printing
   anyway.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 include/trace/events/rcu.h | 29 ++++++++++++++++-------------
 kernel/rcu/tree.c          | 20 +++++++++++++-------
 2 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index f9a7811148e2a..164c43b72ca29 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -435,26 +435,28 @@ TRACE_EVENT_RCU(rcu_fqs,
 #endif /* #if defined(CONFIG_TREE_RCU) */
 
 /*
- * Tracepoint for dyntick-idle entry/exit events.  These take a string
- * as argument: "Start" for entering dyntick-idle mode, "Startirq" for
- * entering it from irq/NMI, "End" for leaving it, "Endirq" for leaving it
- * to irq/NMI, "--=" for events moving towards idle, and "++=" for events
- * moving away from idle.
+ * Tracepoint for dyntick-idle entry/exit events.  These take 2 strings
+ * as argument:
+ * polarilty: "Start", "End", "StillIdle" for entering, exiting or still being
+ * in dyntick-idle mode.
+ * context: "USER" or "KERNEL" or "IRQ".
+ * NMIs nested in IRQs are inferred with dynticks_nesting > 1 in IRQ context.
  *
  * These events also take a pair of numbers, which indicate the nesting
  * depth before and after the event of interest, and a third number that is
- * the ->dynticks counter.  Note that task-related and interrupt-related
- * events use two separate counters, and that the "++=" and "--=" events
- * for irq/NMI will change the counter by two, otherwise by one.
+ * the ->dynticks counter. During NMI nesting within IRQs, the dynticks_nesting
+ * counter changes by two, otherwise one.
  */
 TRACE_EVENT_RCU(rcu_dyntick,
 
-	TP_PROTO(const char *polarity, long oldnesting, long newnesting, int dynticks),
+	TP_PROTO(const char *polarity, const char *context, long oldnesting,
+		 long newnesting, int dynticks),
 
-	TP_ARGS(polarity, oldnesting, newnesting, dynticks),
+	TP_ARGS(polarity, context, oldnesting, newnesting, dynticks),
 
 	TP_STRUCT__entry(
 		__field(const char *, polarity)
+		__field(const char *, context)
 		__field(long, oldnesting)
 		__field(long, newnesting)
 		__field(int, dynticks)
@@ -462,14 +464,15 @@ TRACE_EVENT_RCU(rcu_dyntick,
 
 	TP_fast_assign(
 		__entry->polarity = polarity;
+		__entry->context = context;
 		__entry->oldnesting = oldnesting;
 		__entry->newnesting = newnesting;
 		__entry->dynticks = dynticks;
 	),
 
-	TP_printk("%s %lx %lx %#3x", __entry->polarity,
-		  __entry->oldnesting, __entry->newnesting,
-		  __entry->dynticks & 0xfff)
+	TP_printk("%s %s %lx %lx %#3x", __entry->polarity,
+		__entry->context, __entry->oldnesting, __entry->newnesting,
+		__entry->dynticks & 0xfff)
 );
 
 /*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1ec7b1d4a03c4..eb7a4d90b3b91 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -573,7 +573,8 @@ static void rcu_eqs_enter(bool user)
 	}
 
 	lockdep_assert_irqs_disabled();
-	trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
+	trace_rcu_dyntick(TPS("Start"), (user ? TPS("USER") : TPS("IDLE")),
+			  rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
 	rdp = this_cpu_ptr(&rcu_data);
 	do_nocb_deferred_wakeup(rdp);
@@ -648,15 +649,17 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
 	 * leave it in non-RCU-idle state.
 	 */
 	if (rdp->dynticks_nmi_nesting != 1) {
-		trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2,
-				  atomic_read(&rdp->dynticks));
+		trace_rcu_dyntick(TPS("StillNonIdle"), TPS("IRQ"),
+				  rdp->dynticks_nmi_nesting,
+				  rdp->dynticks_nmi_nesting - 2, atomic_read(&rdp->dynticks));
 		WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
 			   rdp->dynticks_nmi_nesting - 2);
 		return;
 	}
 
 	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
-	trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
+	trace_rcu_dyntick(TPS("Start"), TPS("IRQ"), rdp->dynticks_nmi_nesting,
+			  0, atomic_read(&rdp->dynticks));
 	WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
 
 	if (irq)
@@ -748,7 +751,8 @@ static void rcu_eqs_exit(bool user)
 	rcu_dynticks_eqs_exit();
 	// ... but is watching here.
 	rcu_cleanup_after_idle();
-	trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
+	trace_rcu_dyntick(TPS("End"), (user ? TPS("USER") : TPS("IDLE")),
+			  rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
 	WRITE_ONCE(rdp->dynticks_nesting, 1);
 	WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
@@ -850,9 +854,11 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 		}
 		raw_spin_unlock_rcu_node(rdp->mynode);
 	}
-	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
-			  rdp->dynticks_nmi_nesting,
+
+	trace_rcu_dyntick(incby == 1 ? TPS("End") : TPS("StillNonIdle"),
+			  TPS("IRQ"), rdp->dynticks_nmi_nesting,
 			  rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));
+
 	WRITE_ONCE(rdp->dynticks_nmi_nesting, /* Prevent store tearing. */
 		   rdp->dynticks_nmi_nesting + incby);
 	barrier();
-- 
2.26.2.526.g744177e7f7-goog