linux-kernel - Re: [PATCH v5 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP

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Date:   Wed, 11 Nov 2020 19:04:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Yun Hsiang <hsiang023167@...il.com>, linux-kernel@...r.kernel.org,
        qais.yousef@....com, patrick.bellasi@...bug.net,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v5 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET
 flag to reset uclamp

On Wed, Nov 11, 2020 at 06:41:07PM +0100, Dietmar Eggemann wrote:
> diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
> index c852153ddb0d..b9165f17dddc 100644
> --- a/include/uapi/linux/sched/types.h
> +++ b/include/uapi/linux/sched/types.h
> @@ -115,8 +115,8 @@ struct sched_attr {
>  	__u64 sched_period;
>  
>  	/* Utilization hints */
> -	__u32 sched_util_min;
> -	__u32 sched_util_max;
> +	__s32 sched_util_min;
> +	__s32 sched_util_max;

So that's UAPI, not sure we can change the type here.

>  };
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3dc415f58bd7..caaa2a8434b9 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 || util_min > SCHED_CAPACITY_SCALE)
> +			return -EINVAL;
> +	}
> +
> +	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> +		util_max = attr->sched_util_max;
> +
> +		if (util_max < -1 || util_max > SCHED_CAPACITY_SCALE)
> +			return -EINVAL;
> +	}

Luckily we can write that range as a single branch like:

	if (util_{min,max} + 1 > SCHED_CAPACITY_SCALE+1)

which assumes u32 :-)

> +
> +	if (util_min != -1 && util_max != -1 && util_min > util_max)
>  		return -EINVAL;

I think that will compile as is, otherwise write it like ~0u, which is
the same bit pattern.