Re: [PATCH v2 0/5] sched: Add on_rq states and remove several double rq locks

From: Oleg Nesterov
Date: Sat Jul 26 2014 - 15:41:32 EST

Hi Kirill,

I'll try to read this series later, just one silly question for now.

On 07/26, Kirill Tkhai wrote:
> Patch [2/5] is main in the series. It introduces new state: ONRQ_MIGRATING
> and teaches scheduler to understand it (we need a little changes predominantly
> in try_to_wake_up()). This will be used in the following way:
> (we are changing task's rq)
> raw_spin_lock(&src_rq->lock);
> dequeue_task(src_rq, p, 0);
> p->on_rq = ONRQ_MIGRATING;
> set_task_cpu(p, dst_cpu);
> raw_spin_unlock(&src_rq->lock);
> raw_spin_lock(&dst_rq->lock);
> p->on_rq = ONRQ_QUEUED;
> enqueue_task(dst_rq, p, 0);
> raw_spin_unlock(&dst_rq->lock);

Hmm. And what if the code above doesn't hold p->pi_lock (4/5) and, say,
__sched_setscheduler() does fair_sched_class->rt_sched_class transition
in between?

ONRQ_MIGRATING helps to avoid the wrong dequeue + enqueue, but I am not
sure about check_class_changed().

Say, switched_from_fair() will use dst_rq even if p was never queued on
this rq... This only affects the .decay_count logic, perhaps this is fine,
I simply do not know what this code does.

What about switched_to_rt() ? we lose the push_rt_task() logic... Hmm,
which I can't understand too ;)

And we also lose ENQUEUE_HEAD in this case, but this looks fine.

In short: could you confirm there are no problems here?


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