On 07/14, Mathieu Desnoyers wrote:
>
> * Oleg Nesterov ([EMAIL PROTECTED]) wrote:
> > On 07/14, Mathieu Desnoyers wrote:
> > >
> > > @@ -4891,10 +4948,42 @@ static int migration_thread(void *data)
> > > list_del_init(head->next);
> > >
> > > spin_unlock(&rq->lock);
> > > - __migrate_task(req->task, cpu, req->dest_cpu);
> > > + migrated = __migrate_task(req->task, cpu, req->dest_cpu);
> > > local_irq_enable();
> > > -
> > > - complete(&req->done);
> > > + if (!migrated) {
> > > + /*
> > > + * If the process has not been migrated, let it run
> > > + * until it reaches a migration_check() so it can
> > > + * wake us up.
> > > + */
> > > + spin_lock_irq(&rq->lock);
> > > + head = &rq->migration_queue;
> > > + list_add(&req->list, head);
> > > + if (req->task->se.on_rq
> > > + || !task_migrate_count(req->task)) {
> > > + /*
> > > + * The process is on the runqueue, it could
> > > + * exit its critical section at any moment,
> > > + * don't race with it and retry actively.
> > > + * Also, if the thread is not on the runqueue
> > > + * and has a zero migration count
> > > + * (__migrate_task failed because cpus allowed
> > > + * changed), just retry.
> > > + */
> > > + spin_unlock_irq(&rq->lock);
> > > + continue;
> >
> > Again, this can deadlock. migration_thread() is SCHED_FIFO, and it shares
> > the
> > same CPU with req->task. We are doing a busy-wait loop, req->task may have
> > no
> > chance to finish its critical section.
> >
>
> If we share the CPU with the other thread, it means that it won't be on
> the runqueue while we are holding the rq lock.
Why? The req->task could be runnable, but preempted by migration_thread().
In that case req->task->se.on_rq should be true.
I didn't read the new scheduler yet, but I belive on_rq == 0 only when
the task sleeps, it is like the current ->array = NULL. Please correct
me if I am wrong.
Oleg.
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