Re: [PATCH 2/4] sched: set new prio after checking schedule policy

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 30/04/2020 14:13, Hillf Danton wrote:
> 
> On Tue, 28 Apr 2020 17:32:45 Valentin Schneider wrote:
>>
>>> +	else if (fair_policy(policy)) {
>>> +		if (attr->sched_nice < MIN_NICE ||
>>> +		    attr->sched_nice > MAX_NICE)
>>> +			return -EINVAL;
>>
>> We can't hit this with the syscall route, since we (silently) clamp those
>> values in sched_copy_attr(). setpriority() does the same. There's this
>> comment in sched_copy_attr() that asks whether we should clamp or return an
>> error; seems like the current consensus is on clamping, but then we might
>> want to get rid of that comment :)
>>
> Yes it's quite likely for me to miss the cases covered by that clamp;
> otherwise what is added does not break that consensus.
> 
>>> +		newprio = NICE_TO_PRIO(attr->sched_nice);
>>
>> This is new, however AFAICT it doesn't change anything for CFS (or about to
>> be) tasks since what matters is calling check_class_changed() further down.
> 
> Yes it's only used by rt_effective_prio(). 
> 

Looks like changing a SCHED_NORMAL to a SCHED_BATCH task will create a different
queue_flags value.

# chrt -p $$
pid 2803's current scheduling policy: SCHED_OTHER
pid 2803's current scheduling priority: 0

# chrt -b -p 0 $$

...
[bash 2803] policy=3 oldprio=120 newprio=[99->120] new_effective_prio=[99->120] queue_flags=[0xe->0xa]
[bash 2803] queued=0 running=0
...

But since in this example 'queued=0' it has no further effect here.

Why is SCHED_NORMAL/SCHED_BATCH (fair_policy()) now treated differently than SCHED_IDLE?

# chrt -i -p 0 $$

...
[bash 2803] policy=5 newprio=99 oldprio=120 new_effective_prio=99 queue_flags=0xe
[bash 2803] queued=0 running=0
...