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SubjectRe: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases
On Mon, 20 Apr 2015 19:20:48 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > > + */
> > > + if (preempt_count() & PREEMPT_ACTIVE)
> > > + enqueue_pushable_task_preempted(rq, p);
> > > + else
> > > + enqueue_pushable_task(rq, p);
> > > + }
> > > }
>
> This looks wrong, what do you want to find? _any_ preemption? In that
> case PREEMPT_ACTIVE is wrong. What you need to check is if the task is
> still on the RQ or not.
>
> If the task was put to sleep it got dequeued, if it was not dequeued, it
> got preempted.
>
> PREEMPT_ACTIVE is only ever set for forced kernel preemption, which is a
> special sub case only ever triggered with CONFIG_PREEMPT=y.

Ah, you're right. I was thinking of just forced preemption, but, I
wasn't thinking about voluntary preemption (preemption points). We want
this behavior for that too (for kernel).

And yes, if we preempt in user space, this isn't enough either.

Actually, I think we only care if the state of the task is
TASK_RUNNING, if it is anything else, the task is probably going to
sleep anyway and we don't care about FIFO order then.

-- Steve



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