linux-kernel - Re: [PATCH] ftrace: sample: avoid open-coded 64-bit division

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Message-ID: <Y9fBFKeJ1zMBtv4b@FVFF77S0Q05N>
Date:   Mon, 30 Jan 2023 13:07:32 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: sample: avoid open-coded 64-bit division

On Mon, Jan 30, 2023 at 02:02:37PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Calculating the average period requires a 64-bit division that leads
> to a link failure on 32-bit architectures:
> 
> x86_64-linux-ld: samples/ftrace/ftrace-ops.o: in function `ftrace_ops_sample_init':
> ftrace-ops.c:(.init.text+0x23b): undefined reference to `__udivdi3'

Whoops; sorry about that.

> Use the div_u64() helper to do this instead. Since this is an init function that
> is not called frequently, the runtime overhead is going to be acceptable.
> 
> Fixes: b56c68f705ca ("ftrace: Add sample with custom ops")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  samples/ftrace/ftrace-ops.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/samples/ftrace/ftrace-ops.c b/samples/ftrace/ftrace-ops.c
> index 24deb51c7261..0c8da87ff5c3 100644
> --- a/samples/ftrace/ftrace-ops.c
> +++ b/samples/ftrace/ftrace-ops.c
> @@ -223,7 +223,7 @@ static int __init ftrace_ops_sample_init(void)
>  
>  	pr_info("Attempted %u calls to %ps in %lluns (%lluns / call)\n",
>  		nr_function_calls, tracee_relevant,
> -		period, period / nr_function_calls);
> +		period, div_u64(period, nr_function_calls));
>  
>  	if (persist)
>  		return 0;
> -- 
> 2.39.0
>