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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, paulmck <paulmck@kernel.org>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: Re: Instrumentation and RCU
Date: Mon, 9 Mar 2020 14:52:40 -0400 (EDT)
Message-ID: <1851876075.21840.1583779960064.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200309144427.0ce0eabc@gandalf.local.home>

----- On Mar 9, 2020, at 2:44 PM, rostedt rostedt@goodmis.org wrote:

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

I have a hard time applying this patch to any tree I can fetch, so
I will use caution and say: probably not. ;-)

And when I say "go back to plain RCU", I really mean removing use of SRCU
from the tracepoints until we have other purposes for it (e.g. taking
faults within specific tracepoint probes such as syscall enter/exit).

Thanks,

Mathieu

> 
> -- 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();				\

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 17:02 Thomas Gleixner
2020-03-09 18:15 ` Steven Rostedt
2020-03-09 18:42   ` Joel Fernandes
2020-03-09 19:07     ` Steven Rostedt
2020-03-09 19:20       ` Mathieu Desnoyers
2020-03-16 15:02       ` Joel Fernandes
2020-03-09 18:59   ` Thomas Gleixner
2020-03-10  8:09     ` Masami Hiramatsu
2020-03-10 11:43       ` Thomas Gleixner
2020-03-10 15:31         ` Mathieu Desnoyers
2020-03-10 15:46           ` Steven Rostedt
2020-03-10 16:21             ` Mathieu Desnoyers
2020-03-11  0:18               ` Masami Hiramatsu
2020-03-11  0:37                 ` Mathieu Desnoyers
2020-03-11  7:48                   ` Masami Hiramatsu
2020-03-10 16:06         ` Masami Hiramatsu
2020-03-12 13:53         ` Peter Zijlstra
2020-03-10 15:24       ` Mathieu Desnoyers
2020-03-10 17:05       ` Daniel Thompson
2020-03-09 18:37 ` Mathieu Desnoyers
2020-03-09 18:44   ` Steven Rostedt
2020-03-09 18:52     ` Mathieu Desnoyers [this message]
2020-03-09 19:09       ` Steven Rostedt
2020-03-09 19:25         ` Mathieu Desnoyers
2020-03-09 19:52   ` Thomas Gleixner
2020-03-10 15:03     ` Mathieu Desnoyers
2020-03-10 16:48       ` Thomas Gleixner
2020-03-10 17:40         ` Mathieu Desnoyers
2020-03-10 18:31           ` Thomas Gleixner
2020-03-10 18:37             ` Mathieu Desnoyers
2020-03-10  1:40   ` Alexei Starovoitov
2020-03-10  8:02     ` Thomas Gleixner
2020-03-10 16:54     ` Paul E. McKenney
2020-03-17 17:56     ` Joel Fernandes
2020-03-09 20:18 ` Peter Zijlstra
2020-03-09 20:47 ` Paul E. McKenney
2020-03-09 20:58   ` Steven Rostedt
2020-03-09 21:25     ` Paul E. McKenney
2020-03-09 23:52   ` Frederic Weisbecker
2020-03-10  2:26     ` Paul E. McKenney
2020-03-10 15:13   ` Mathieu Desnoyers
2020-03-10 16:49     ` Paul E. McKenney
2020-03-10 17:22       ` Mathieu Desnoyers
2020-03-10 17:26         ` Paul E. McKenney

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