Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Quentin Perret <qperret@google.com>]

Hi Qais,

On Friday 03 Apr 2020 at 13:30:19 (+0100), Qais Yousef wrote:
<snip>
> +	/*
> +	 * The new value will be applied to all RT tasks the next time they
> +	 * wakeup, assuming the task is using the system default and not a user
> +	 * specified value. In the latter we shall leave the value as the user
> +	 * requested.
> +	 */
>  	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max ||
>  	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE) {
>  		result = -EINVAL;
>  		goto undo;
>  	}
>  
> +	if (sysctl_sched_rt_default_uclamp_util_min > SCHED_CAPACITY_SCALE) {
> +		result = -EINVAL;
> +		goto undo;
> +	}

Hmm, checking:

	if (sysctl_sched_rt_default_uclamp_util_min > sysctl_sched_uclamp_util_min)

would probably make sense too, but then that would make writing in
sysctl_sched_uclamp_util_min cumbersome for sysadmins as they'd need to
lower the rt default first. Is that the reason for checking against
SCHED_CAPACITY_SCALE? That might deserve a comment or something.

<snip>
> @@ -1241,9 +1293,13 @@ static void uclamp_fork(struct task_struct *p)
>  	for_each_clamp_id(clamp_id) {
>  		unsigned int clamp_value = uclamp_none(clamp_id);
>  
> -		/* By default, RT tasks always get 100% boost */
> +		/*
> +		 * By default, RT tasks always get 100% boost, which the admins
> +		 * are allowed to change via
> +		 * sysctl_sched_rt_default_uclamp_util_min knob.
> +		 */
>  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			clamp_value = uclamp_none(UCLAMP_MAX);
> +			clamp_value = sysctl_sched_rt_default_uclamp_util_min;
>  
>  		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
>  	}

And that, as per 20200414161320.251897-1-qperret@google.com, should not
be there :)

Otherwise the patch pretty looks good to me!

Thanks,
Quentin