linux-kernel - Re: [PATCH 4/5] ftrace: function graph return for function entry

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Message-ID: <20081203081301.GF21006@elte.hu>
Date:	Wed, 3 Dec 2008 09:13:01 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/5] ftrace: function graph return for function entry


* Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <srostedt@...hat.com>
> 
> Impact: feature, let entry function decide to trace or not
> 
> This patch lets the graph tracer entry function decide if the tracing 
> should be done at the end as well. This requires all function graph 
> entry functions return 1 if it should trace, or 0 if the return should 
> not be traced.

> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 47aa5f0..cef05a2 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -119,6 +119,9 @@ ENTRY(mcount)
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	cmpq $ftrace_stub, ftrace_graph_return
>  	jnz ftrace_graph_caller
> +
> +	cmpq $ftrace_graph_entry_stub, ftrace_graph_entry
> +	jnz ftrace_graph_caller

hm, that's in the hotpath. What's the point of this? Do we want some sort 
of configuration vector that allows per function graphing versus 
entry-only traces?

	Ingo
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