Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

[Oleg Nesterov <oleg@redhat.com>]

On 08/21, Frederic Weisbecker wrote:
>
> On Tue, Aug 20, 2013 at 06:33:12PM +0200, Oleg Nesterov wrote:
> >
> > --- x/kernel/sched/core.c
> > +++ x/kernel/sched/core.c
> > @@ -2435,6 +2435,9 @@ need_resched:
> >  		rq->curr = next;
> >  		++*switch_count;
> >
> > +		if (unlikely(prev->in_iowait))
> > +			rq->nr_iowait++;
> > +
> >  		context_switch(rq, prev, next); /* unlocks the rq */
> >  		/*
> >  		 * The context switch have flipped the stack from under us
> > @@ -2442,6 +2445,12 @@ need_resched:
> >  		 * this task called schedule() in the past. prev == current
> >  		 * is still correct, but it can be moved to another cpu/rq.
> >  		 */
> > +		if (unlikely(prev->in_iowait)) {
> > +			raw_spin_lock_irq(&rq->lock);
> > +			rq->nr_iowait--;
> > +			raw_spin_unlock_irq(&rq->lock);
> > +		}
> > +
>
> It seems that with this solution rq->nr_iowait is only ever modified locally.
> Can't we just disable irqs for rq->nr_iowait-- ?

Not really. rq holds the old value, which was used when this task
called context_switch() in the past.

IOW, if a task T does io_schedule() on CPU_0, then cpu_of(rq) is still 0
after context_switch(), even if T runs on another cpu.

> Also if this is only updated locally,

Unfortunately, I don't see how we can do this.

Oleg.