[PATCH] eal: support lcore usage ratio

[Morten Brørup]

> From: Chengwen Feng [mailto:fengchengwen at huawei.com]
> Sent: Monday, 23 October 2023 06.08
> 
> Current, the lcore usage only display two key fields: busy_cycles and
> total_cycles, which is inconvenient to obtain the usage ratio
> immediately. So adds lcore usage ratio field.

Usage ratio in percentage is only useful if it doesn't vary much over time. Which use cases don't have a varying traffic pattern with busy hours and off-peak hours?

> 
> Signed-off-by: Chengwen Feng <fengchengwen at huawei.com>
> ---

 [...]

> +static float
> +calc_usage_ratio(const struct rte_lcore_usage *usage)
> +{
> +	return (usage->busy_cycles * 100.0) / (usage->total_cycles == 0 ? 1 :
> usage->total_cycles);
> +}

This correctly prevents division by zero. If total_cycles by some freak accident isn't updated, the result will be a very big number. You might consider this alternative:

return usage->total_cycles != 0 ? (usage->busy_cycles * 100.0) / usage->total_cycles : (float)0;

> +
>  static int
>  lcore_dump_cb(unsigned int lcore_id, void *arg)
>  {
> @@ -462,8 +468,9 @@ lcore_dump_cb(unsigned int lcore_id, void *arg)
>  	/* Guard against concurrent modification of lcore_usage_cb. */
>  	usage_cb = lcore_usage_cb;
>  	if (usage_cb != NULL && usage_cb(lcore_id, &usage) == 0) {
> -		if (asprintf(&usage_str, ", busy cycles %"PRIu64"/%"PRIu64,
> -				usage.busy_cycles, usage.total_cycles) < 0) {
> +		if (asprintf(&usage_str, ", busy cycles %"PRIu64"/%"PRIu64"
> (ratio %.3f%%)",

Is "%.3f%%" the community preference for human readable CPU usage percentages?

I prefer "%.02f%%", but don't object to the suggested format.

NB: The format also applies to format_usage_ratio() below.

> +				usage.busy_cycles, usage.total_cycles,
> +				calc_usage_ratio(&usage)) < 0) {
>  			return -ENOMEM;
>  		}
>  	}
> @@ -511,11 +518,19 @@ struct lcore_telemetry_info {
>  	struct rte_tel_data *d;
>  };
> 
> +static void
> +format_usage_ratio(char *buf, uint16_t size, const struct rte_lcore_usage
> *usage)
> +{
> +	float ratio = calc_usage_ratio(usage);
> +	snprintf(buf, size, "%.3f%%", ratio);
> +}