linux-kernel - Re: [PATCH 1/2] tracing: Move enabling tracepoints to just after rcu

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Message-ID: <54AF53BD.5070303@huawei.com>
Date:	Fri, 9 Jan 2015 12:06:21 +0800
From:	Wang Nan <wangnan0@...wei.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<mingo@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	<paulmck@...ux.vnet.ibm.com>, <mathieu.desnoyers@...icios.com>,
	xiakaixu 00238161 <xiakaixu@...wei.com>
Subject: Re: [PATCH 1/2] tracing: Move enabling tracepoints to just after
 rcu_init()

Hi Steven Rostedt,

During studying your code we find a problem, please see below.

> 
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> 
> Enabling tracepoints at boot up can be very useful. The tracepoint
> can be initialized right after RCU has been. There's no need to
> wait for the early_initcall() to be called. That's too late for some
> things that can use tracepoints for debugging. Move the logic to
> enable tracepoints out of the initcalls and into init/main.c to
> right after rcu_init().
> 
> This also allows trace_printk() to be used early too.
> 
> Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1412121539300.16494@nanos
> Link: http://lkml.kernel.org/r/20141214164104.307127356@goodmis.org
> 
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Tested-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

[...]

> +void __init trace_init(void)
> +{
> +	tracer_alloc_buffers();
> +	init_ftrace_syscalls();
> +	trace_event_init();	
> +}
> +

[...]

> +
> +void __init trace_event_init(void)
> +{
> +	event_trace_memsetup();
> +	init_ftrace_syscalls();
> +	event_trace_enable();
> +}
> +

init_ftrace_syscalls() get called twice by trace_init() and trace_event_init(), some resources are wasted.
At lease one of them can be removed.

In addition, could you please have a look at my early kprobe patch series?

http://lists.infradead.org/pipermail/linux-arm-kernel/2015-January/313835.html

Which enables kprobe very early, even before memory initialized. I think it is possible to combine these
early tracing facilities together.

Thank you!


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