Re: [PATCH] minor sched-E1 tweaks and questions

[Ingo Molnar]

On Thu, 10 Jan 2002, Rusty Russell wrote:

> > > Q. How can this happen in expire_task():
> > > 	if (p->array != rq->active) {
> > > 		p->need_resched = 1;
> > > 		return;
> > > 	}
> >
> > if a task gets delayed by some really heavy IRQ load and the timer
> > interrupt hits the task twice.
>
> Hmm... still don't see it.  update_process_times() surely doesn't
> re-enter?  And another CPU cannot load_balance() p (== current) away
> from us.

the issue is when a task has not rescheduled yet even though the previous
timer tick has has told it to do so. Then we'll have the p->array !=
rq->active condition. If some other event (like a heavy SCSI irq or
something else) delays the task from getting into the scheduler for more
than a jiffy, we can get an expire_task() call again - and hit the
condition.

this situation is especially likely to happen with HZ=1024 or higher.

> Another question:
>
> 		context_switch(prev, next);
> 		/*
> 		 * The runqueue pointer might be from another CPU
> 		 * if the new task was last running on a different
> 		 * CPU - thus re-load it.
> 		 */
> 		barrier();
> 		rq = this_rq();
> 	}
> 	spin_unlock_irq(&rq->lock);
>
> I do not understand this comment.  How can rq (ie. smp_processor_id())
> change?  Nothing sleeps here, and if it DID change, the
> spin_unlock_irq() would be wrong...

the 'rq' variable is not 'constant' across context-switch if you look at
what happens on the CPU - we switch away from a task into some other task.
That other task might have a much older 'rq' variable on its kernel stack,
which might be invalid, if the (now executing) task was load-balanced over
to this CPU.

>  } runqueues [NR_CPUS] __cacheline_aligned;
>
> You want each entry in the array to be aligned, not the whole array!

hm, right you are. Will be in my next patch.

	Ingo