linux-kernel - Re: [PATCH] ftrace: Avoid needless updates of the ftrace function call

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Message-Id: <20221124003047.827b6ef942b0dc529face3b9@kernel.org>
Date:   Thu, 24 Nov 2022 00:30:47 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Song Shuai <suagrfillet@...il.com>
Subject: Re: [PATCH] ftrace: Avoid needless updates of the ftrace function
 call

On Tue, 22 Nov 2022 18:09:05 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Song Shuai reported:
> 
>     The list func (ftrace_ops_list_func) will be patched first
>     before the transition between old and new calls are set,
>     which fixed the race described in this commit `59338f75`.
> 
>     While ftrace_trace_function changes from the list func to a
>     ftrace_ops func, like unregistering the klp_ops to leave the only
>     global_ops in ftrace_ops_list, the ftrace_[regs]_call will be
>     replaced with the list func although it already exists. So there
>     should be a condition to avoid this.
> 
> And suggested using another variable to keep track of what the ftrace
> function is set to. But this could be simplified by using a helper
> function that does the same with a static variable.
> 
> Link: https://lore.kernel.org/lkml/20221026132039.2236233-1-suagrfillet@gmail.com/
> 

This looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> Reported-by: Song Shuai <suagrfillet@...il.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ftrace.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 65a5d36463e0..d04552c0c275 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2763,6 +2763,19 @@ void __weak ftrace_arch_code_modify_post_process(void)
>  {
>  }
>  
> +static int update_ftrace_func(ftrace_func_t func)
> +{
> +	static ftrace_func_t save_func;
> +
> +	/* Avoid updating if it hasn't changed */
> +	if (func == save_func)
> +		return 0;
> +
> +	save_func = func;
> +
> +	return ftrace_update_ftrace_func(func);
> +}
> +
>  void ftrace_modify_all_code(int command)
>  {
>  	int update = command & FTRACE_UPDATE_TRACE_FUNC;
> @@ -2783,7 +2796,7 @@ void ftrace_modify_all_code(int command)
>  	 * traced.
>  	 */
>  	if (update) {
> -		err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> +		err = update_ftrace_func(ftrace_ops_list_func);
>  		if (FTRACE_WARN_ON(err))
>  			return;
>  	}
> @@ -2799,7 +2812,7 @@ void ftrace_modify_all_code(int command)
>  		/* If irqs are disabled, we are in stop machine */
>  		if (!irqs_disabled())
>  			smp_call_function(ftrace_sync_ipi, NULL, 1);
> -		err = ftrace_update_ftrace_func(ftrace_trace_function);
> +		err = update_ftrace_func(ftrace_trace_function);
>  		if (FTRACE_WARN_ON(err))
>  			return;
>  	}
> -- 
> 2.35.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>