linux-kernel - Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace

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Date:   Sat, 19 Jun 2021 11:22:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument
 ftrace_disabled

On Sat, 19 Jun 2021 14:29:19 +0800
Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:

> If set CONFIG_FUNCTION_TRACER=y, we can use the following command to
> disable ftrace after boot up:
> echo 0 > /proc/sys/kernel/ftrace_enabled

I'd love to just remove that. It's original purpose was to stop function
tracing when the latency tracers were active. That's now done via a tracefs
option. The "ftrace_enabled" really has no use anymore.

> 
> ftrace_disabled is much stronger than ftrace_enabled, introduce a new
> cmdline argument ftrace_disabled for user to control whether to disable
> ftrace when boot up.

"ftrace_disabled" is triggered when an anomaly is detected, and for the
safety of the system, ftrace shuts down. It was never meant to be a user
triggered event.

You have no rationale for this change. What's the purpose of this?

-- Steve


> 
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  kernel/trace/ftrace.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 72ef4dc..a015699 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5517,6 +5517,14 @@ static char ftrace_filter_buf[FTRACE_FILTER_SIZE] __initdata;
>  /* Used by function selftest to not test if filter is set */
>  bool ftrace_filter_param __initdata;
>  
> +static int __init set_ftrace_disabled(char *str)
> +{
> +	pr_info("Set ftrace_disabled to disable ftrace\n");
> +	ftrace_disabled = 1;
> +	return 1;
> +}
> +__setup("ftrace_disabled", set_ftrace_disabled);
> +
>  static int __init set_ftrace_notrace(char *str)
>  {
>  	ftrace_filter_param = true;