linux-kernel - Re: [PATCH] sched: make p->prio independent of p->mm

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Date:   Thu, 23 Apr 2020 11:26:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Mike Galbraith <efault@....de>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: make p->prio independent of p->mm

On Thu, Apr 23, 2020 at 12:01:28PM +0800, Hillf Danton wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4796,13 +4796,19 @@ recheck:
>  		return -EINVAL;
>  
>  	/*
> -	 * Valid priorities for SCHED_FIFO and SCHED_RR are
> -	 * 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL,
> -	 * SCHED_BATCH and SCHED_IDLE is 0.
> +	 * The MAX_USER_RT_PRIO value allows the actual maximum
> +	 * RT priority to be separate from the value exported to
> +	 * user-space.  This allows kernel threads to set their
> +	 * priority to a value higher than any user task.
>  	 */
> -	if ((p->mm && attr->sched_priority > MAX_USER_RT_PRIO-1) ||
> -	    (!p->mm && attr->sched_priority > MAX_RT_PRIO-1))
> -		return -EINVAL;
> +	if (p->flags & PF_KTHREAD) {
> +		if (attr->sched_priority > MAX_RT_PRIO - 1)
> +			return -EINVAL;
> +	} else {
> +		if (attr->sched_priority > MAX_USER_RT_PRIO - 1)
> +			return -EINVAL;
> +	}
> +

Arguably we can do away with the check entirely, MAX_RT_PRIO ==
MAX_USER_RT_PRIO.