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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 23 Apr 2020 11:26:20 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> 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.

Heh, that was one of my first patches accepted in the mainline kernel! :-) 

And the reason we added it, was because there was a small time when the RT
patch (or my variation of it) had MAX_USER_RT_PRIO and MAX_RT_PRIO different
values, and would crash in that case here.

d46523ea32a79 ("fix MAX_USER_RT_PRIO and MAX_RT_PRIO")

I would say if we get rid of that check, get rid of the MAX_USER_RT_PRIO
with it, and make everything use MAX_RT_PRIO.

-- Steve