Re: [PATCH v2] sched/uclamp: Allow to reset a task uclamp constraint value

[Yun Hsiang]

Hi Dietmar,
On Fri, Nov 13, 2020 at 12:34:54PM +0100, Dietmar Eggemann wrote:
> In case the user wants to stop controlling a uclamp constraint value
> for a task, use the magic value -1 in sched_util_{min,max} with the
> appropriate sched_flags (SCHED_FLAG_UTIL_CLAMP_{MIN,MAX}) to indicate
> the reset.
> 
> The advantage over the 'additional flag' approach (i.e. introducing
> SCHED_FLAG_UTIL_CLAMP_RESET) is that no additional flag has to be
> exported via uapi. This avoids the need to document how this new flag
> has be used in conjunction with the existing uclamp related flags.
> 
> The following subtle issue is fixed as well. When a uclamp constraint
> value is set on a !user_defined uclamp_se it is currently first reset
> and then set.
> Fix this by AND'ing !user_defined with !SCHED_FLAG_UTIL_CLAMP which
> stands for the 'sched class change' case.
> The related condition 'if (uc_se->user_defined)' moved from
> __setscheduler_uclamp() into uclamp_reset().

I think this is great, thanks!
Reviewed-by: Yun Hsiang <hsiang023167@xxxxxxxxx>

> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> ---
> 
> v1 [1] -> v2:
> 
> 1) Removed struct sched_attr (UAPI) change.
> 
> 2) Use single branch range check in uclamp_validate().
> 
> [1] https://lkml.kernel.org/r/f3b59aad-3d5d-039b-205d-024308b609a1@xxxxxxx
> 
>  include/uapi/linux/sched/types.h |  2 +
>  kernel/sched/core.c              | 70 +++++++++++++++++++++++---------
>  2 files changed, 53 insertions(+), 19 deletions(-)
> 
> diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
> index c852153ddb0d..f2c4589d4dbf 100644
> --- a/include/uapi/linux/sched/types.h
> +++ b/include/uapi/linux/sched/types.h
> @@ -96,6 +96,8 @@ struct sched_param {
>   * on a CPU with a capacity big enough to fit the specified value.
>   * A task with a max utilization value smaller than 1024 is more likely
>   * scheduled on a CPU with no more capacity than the specified value.
> + *
> + * A task utilization boundary can be reset by setting the attribute to -1.
>   */
>  struct sched_attr {
>  	__u32 size;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3dc415f58bd7..a4805747b304 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1413,17 +1413,24 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  static int uclamp_validate(struct task_struct *p,
>  			   const struct sched_attr *attr)
>  {
> -	unsigned int lower_bound = p->uclamp_req[UCLAMP_MIN].value;
> -	unsigned int upper_bound = p->uclamp_req[UCLAMP_MAX].value;
> +	int util_min = p->uclamp_req[UCLAMP_MIN].value;
> +	int util_max = p->uclamp_req[UCLAMP_MAX].value;
>  
> -	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN)
> -		lower_bound = attr->sched_util_min;
> -	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX)
> -		upper_bound = attr->sched_util_max;
> +	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> +		util_min = attr->sched_util_min;
>  
> -	if (lower_bound > upper_bound)
> -		return -EINVAL;
> -	if (upper_bound > SCHED_CAPACITY_SCALE)
> +		if (util_min + 1 > SCHED_CAPACITY_SCALE + 1)
> +			return -EINVAL;
> +	}
> +
> +	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> +		util_max = attr->sched_util_max;
> +
> +		if (util_max + 1 > SCHED_CAPACITY_SCALE + 1)
> +			return -EINVAL;
> +	}
> +
> +	if (util_min != -1 && util_max != -1 && util_min > util_max)
>  		return -EINVAL;
>  
>  	/*
> @@ -1438,20 +1445,41 @@ static int uclamp_validate(struct task_struct *p,
>  	return 0;
>  }
>  
> +static bool uclamp_reset(const struct sched_attr *attr,
> +			 enum uclamp_id clamp_id,
> +			 struct uclamp_se *uc_se)
> +{
> +	/* Reset on sched class change for a non user-defined clamp value. */
> +	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)) &&
> +	    !uc_se->user_defined)
> +		return true;
> +
> +	/* Reset on sched_util_{min,max} == -1. */
> +	if (clamp_id == UCLAMP_MIN &&
> +	    attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN &&
> +	    attr->sched_util_min == -1) {
> +		return true;
> +	}
> +
> +	if (clamp_id == UCLAMP_MAX &&
> +	    attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX &&
> +	    attr->sched_util_max == -1) {
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  static void __setscheduler_uclamp(struct task_struct *p,
>  				  const struct sched_attr *attr)
>  {
>  	enum uclamp_id clamp_id;
>  
> -	/*
> -	 * On scheduling class change, reset to default clamps for tasks
> -	 * without a task-specific value.
> -	 */
>  	for_each_clamp_id(clamp_id) {
>  		struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
> +		unsigned int value;
>  
> -		/* Keep using defined clamps across class changes */
> -		if (uc_se->user_defined)
> +		if (!uclamp_reset(attr, clamp_id, uc_se))
>  			continue;
>  
>  		/*
> @@ -1459,21 +1487,25 @@ static void __setscheduler_uclamp(struct task_struct *p,
>  		 * at runtime.
>  		 */
>  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			__uclamp_update_util_min_rt_default(p);
> +			value = sysctl_sched_uclamp_util_min_rt_default;
>  		else
> -			uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
> +			value = uclamp_none(clamp_id);
> +
> +		uclamp_se_set(uc_se, value, false);
>  
>  	}
>  
>  	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
>  		return;
>  
> -	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> +	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN &&
> +	    attr->sched_util_min != -1) {
>  		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
>  			      attr->sched_util_min, true);
>  	}
>  
> -	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> +	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX &&
> +	    attr->sched_util_max != -1) {
>  		uclamp_se_set(&p->uclamp_req[UCLAMP_MAX],
>  			      attr->sched_util_max, true);
>  	}
> -- 
> 2.17.1
> 

Best Regards,
Yun