linux-kernel - Re: [PATCH] refscale: Optimize process

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Date:   Sat, 28 Oct 2023 17:30:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH] refscale: Optimize process_durations()

On Sat, 28 Oct 2023 19:04:44 +0200
Christophe JAILLET <christophe.jaillet@...adoo.fr> wrote:

> process_durations() is not a hot path, but there is no good reason to
> iterate over and over the data already in 'buf'.
> 
> Using a seq_buf saves some useless strcat() and the need of a temp buffer.
> Data is written directly at the correct place.
> 

Agreed.

> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>  kernel/rcu/refscale.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
> index 2c2648a3ad30..861485d865ec 100644
> --- a/kernel/rcu/refscale.c
> +++ b/kernel/rcu/refscale.c
> @@ -28,6 +28,7 @@
>  #include <linux/rcupdate_trace.h>
>  #include <linux/reboot.h>
>  #include <linux/sched.h>
> +#include <linux/seq_buf.h>
>  #include <linux/spinlock.h>
>  #include <linux/smp.h>
>  #include <linux/stat.h>
> @@ -890,31 +891,36 @@ static u64 process_durations(int n)
>  {
>  	int i;
>  	struct reader_task *rt;
> -	char buf1[64];
> +	struct seq_buf s;
>  	char *buf;
>  	u64 sum = 0;
>  
>  	buf = kmalloc(800 + 64, GFP_KERNEL);
>  	if (!buf)
>  		return 0;
> -	buf[0] = 0;
> +
> +	seq_buf_init(&s, buf, 800 + 64);
> +
>  	sprintf(buf, "Experiment #%d (Format: <THREAD-NUM>:<Total loop time in ns>)",
>  		exp_idx);
>  
>  	for (i = 0; i < n && !torture_must_stop(); i++) {
>  		rt = &(reader_tasks[i]);
> -		sprintf(buf1, "%d: %llu\t", i, rt->last_duration_ns);
>  
>  		if (i % 5 == 0)
> -			strcat(buf, "\n");
> -		if (strlen(buf) >= 800) {
> +			seq_buf_putc(&s, '\n');

I was confused here thinking it was originally adding a '\n' to buf1 on
i % 5, but it's adding it to buf!

Yeah, using seq_buf is much less confusing and then less error prone.

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve


> +
> +		if (seq_buf_used(&s) >= 800) {
> +			seq_buf_terminate(&s);
>  			pr_alert("%s", buf);
> -			buf[0] = 0;
> +			seq_buf_clear(&s);
>  		}
> -		strcat(buf, buf1);
> +
> +		seq_buf_printf(&s, "%d: %llu\t", i, rt->last_duration_ns);
>  
>  		sum += rt->last_duration_ns;
>  	}
> +	seq_buf_terminate(&s);
>  	pr_alert("%s\n", buf);
>  
>  	kfree(buf);