linux-kernel - Re: Instrumentation and RCU

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Message-ID: <20200309144427.0ce0eabc@gandalf.local.home>
Date:   Mon, 9 Mar 2020 14:44:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        paulmck <paulmck@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: Instrumentation and RCU

On Mon, 9 Mar 2020 14:37:40 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> So I think we could go back to plain RCU for rcuidle tracepoints if we do
> the cheaper "rcu_is_watching()" check rather than invoking
> rcu_irq_{enter,exit}_irqson() unconditionally.

You mean something like this?

-- Steve

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 5f4de82ffa0f..1904dbb3a921 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -164,7 +164,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		struct tracepoint_func *it_func_ptr;			\
 		void *it_func;						\
 		void *__data;						\
-		int __maybe_unused __idx = 0;				\
+		int __maybe_unused __idx = -1;				\
 									\
 		if (!(cond))						\
 			return;						\
@@ -179,8 +179,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		 * For rcuidle callers, use srcu since sched-rcu	\
 		 * doesn't work from the idle path.			\
 		 */							\
-		if (rcuidle)						\
-			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
+		if (rcuidle && !rcu_is_watching())			\
+			__idx = srcu_read_lock_notrace(&tracepoint_srcu); \
 									\
 		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
 									\
@@ -199,7 +199,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 			} while ((++it_func_ptr)->func);		\
 		}							\
 									\
-		if (rcuidle)						\
+		if (rcuidle && __idx != -1)				\
 			rcu_irq_exit_irqson();				\
 									\
 		preempt_enable_notrace();				\