Re: [PATCH v2] refscale: Optimize process_durations()

[Paul E. McKenney]

On Sun, Jul 21, 2024 at 09:23:46AM +0200, Christophe JAILLET 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.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> ---
> This patch is compile tested only.
> I introduced a bug in v1 (see 1st item in the changes below), so it should
> definitively be tested by someone!
> 
> v1 was un-usable because seq_buf_putc() was not exported. It is now
> available since v6.7.
> 
> Because of the changes, I don't know if the previous R-b should kept.
> In case, it was:
> 	Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> 	Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>

It would be good for them to take another look.  In the meantime:

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> Changes in v2:
>   - Use seq_buf_printf() alseo for the first string, otherwise it would
>     get lost
>   - Use seq_buf_str() instead of seq_buf_terminate() because the API has
>     changed
> 
> v1: https://lore.kernel.org/all/bbbab32e3e104bdc2238724a6a4a85e539f49ddd.1698512661.git.christophe.jaillet@xxxxxxxxxx/
> ---
>  kernel/rcu/refscale.c | 25 ++++++++++++++-----------
>  1 file changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
> index f4ea5b1ec068..cfec0648e141 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>
> @@ -891,32 +892,34 @@ 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;
> -	sprintf(buf, "Experiment #%d (Format: <THREAD-NUM>:<Total loop time in ns>)",
> -		exp_idx);
> +	seq_buf_init(&s, buf, 800 + 64);
> +
> +	seq_buf_printf(&s, "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) {
> -			pr_alert("%s", buf);
> -			buf[0] = 0;
> +			seq_buf_putc(&s, '\n');
> +
> +		if (seq_buf_used(&s) >= 800) {
> +			pr_alert("%s", seq_buf_str(&s));
> +			seq_buf_clear(&s);
>  		}
> -		strcat(buf, buf1);
> +
> +		seq_buf_printf(&s, "%d: %llu\t", i, rt->last_duration_ns);
>  
>  		sum += rt->last_duration_ns;
>  	}
> -	pr_alert("%s\n", buf);
> +	pr_alert("%s\n", seq_buf_str(&s));
>  
>  	kfree(buf);
>  	return sum;
> -- 
> 2.45.2
> 
>