linux-kernel - Re: [PATCH] refperf: work around 64-bit division

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Date:   Fri, 29 May 2020 20:52:30 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] refperf: work around 64-bit division

On Fri, May 29, 2020 at 10:15:51PM +0200, Arnd Bergmann wrote:
> A 64-bit division was introduced in refperf, breaking compilation
> on all 32-bit architectures:
> 
> kernel/rcu/refperf.o: in function `main_func':
> refperf.c:(.text+0x57c): undefined reference to `__aeabi_uldivmod'
> 
> Work it by using div_u64 to mark the expensive operation.
> 
> Fixes: bd5b16d6c88d ("refperf: Allow decimal nanoseconds")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/rcu/refperf.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
> index 47df72c492b3..c2366648981d 100644
> --- a/kernel/rcu/refperf.c
> +++ b/kernel/rcu/refperf.c
> @@ -386,7 +386,7 @@ static int main_func(void *arg)
>  		if (torture_must_stop())
>  			goto end;
>  
> -		result_avg[exp] = 1000 * process_durations(nreaders) / (nreaders * loops);
> +		result_avg[exp] = div_u64(1000 * process_durations(nreaders), nreaders * loops);
>  	}
>  
>  	// Print the average of all experiments
> @@ -397,9 +397,14 @@ static int main_func(void *arg)
>  	strcat(buf, "Threads\tTime(ns)\n");
>  
>  	for (exp = 0; exp < nruns; exp++) {
> +		u64 avg;
> +		u32 rem;
> +
>  		if (errexit)
>  			break;
> -		sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, result_avg[exp] / 1000, (int)(result_avg[exp] % 1000));
> +
> +		avg = div_s64_rem(result_avg[exp], 1000, &rem);

Shouldn't this be div_u64_rem? result_avg is u64.

> +		sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, avg, rem);

Would %03u be the better specifier since rem is u32?

>  		strcat(buf, buf1);
>  	}
>  
> -- 
> 2.26.2
> 

Cheers,
Nathan