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

[Valentin Schneider]

On 30/04/20 15:06, Dietmar Eggemann wrote:
>>>> +		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
> ...


Good catch; I suppose we'll want to special case SCHED_IDLE (IIRC should
map to nice 20).

As you pointed out, right now the newprio computation for CFS tasks is
kinda bonkers, so it seems we'll almost always clear DEQUEUE_MOVE from
queue_flags for them.

For CFS, not having DEQUEUE_MOVE here would lead to not calling
update_min_vruntime() on the dequeue. I'm not sure how much it matters in
this one case - I don't expect sched_setscheduler() calls to be *too*
frequent, and that oughta be fixed by the next entity_tick()) - but that is
an actual change.