[PATCH] tracepoint: rcuidle: use rcu_is_watching() and tree-rcu

[Mathieu Desnoyers]

commit e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use
SRCU") aimed at improving performance of rcuidle tracepoints by using
SRCU rather than temporarily enabling tree-rcu every time.

commit 865e63b04e9b ("tracing: Add back in rcu_irq_enter/exit_irqson()
for rcuidle tracepoints") adds back the high-overhead enabling of
tree-rcu because perf expects RCU to be watching when called from
rcuidle tracepoints.

It turns out that by using "rcu_is_watching()" and conditionally
calling the high-overhead rcu_irq_enter/exit_irqson(), the original
motivation for using SRCU in the first place disappears.

I suspect that the original benchmarks justifying the introduction
of SRCU to handle rcuidle tracepoints was caused by preempt/irq
tracepoints, which are typically invoked from contexts that have
RCU watching.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
CC: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/tracepoint.h | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 1fb11daa5c53..8e0e94fee29a 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -165,25 +165,22 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		void *it_func;						\
 		void *__data;						\
 		int __maybe_unused __idx = 0;				\
+		bool __exit_rcu = false;				\
 									\
 		if (!(cond))						\
 			return;						\
 									\
-		/* srcu can't be used from NMI */			\
-		WARN_ON_ONCE(rcuidle && in_nmi());			\
-									\
-		/* keep srcu and sched-rcu usage consistent */		\
-		preempt_disable_notrace();				\
-									\
 		/*							\
-		 * For rcuidle callers, use srcu since sched-rcu	\
-		 * doesn't work from the idle path.			\
+		 * For rcuidle callers, temporarily enable RCU if	\
+		 * it is not currently watching.			\
 		 */							\
-		if (rcuidle) {						\
-			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
+		if (rcuidle && !rcu_is_watching()) {			\
 			rcu_irq_enter_irqson();				\
+			__exit_rcu = true;				\
 		}							\
 									\
+		preempt_disable_notrace();				\
+									\
 		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
 									\
 		if (it_func_ptr) {					\
@@ -194,12 +191,10 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 			} while ((++it_func_ptr)->func);		\
 		}							\
 									\
-		if (rcuidle) {						\
-			rcu_irq_exit_irqson();				\
-			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
-		}							\
-									\
 		preempt_enable_notrace();				\
+									\
+		if (__exit_rcu)						\
+			rcu_irq_exit_irqson();				\
 	} while (0)
 
 #ifndef MODULE
-- 
2.17.1