Re: linux-next-20110923: warning kernel/rcutree.c:1833

Paul E. McKenney
2011-10-03T00:28:46Z

On Mon, Oct 03, 2011 at 12:50:22AM +0200, Frederic Weisbecker wrote:
> On Fri, Sep 30, 2011 at 12:24:38PM -0700, Paul E. McKenney wrote:
> > @@ -328,11 +326,11 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
> > return 1;
> > }
> >
> > - /* If preemptible RCU, no point in sending reschedule IPI. */
> > - if (rdp->preemptible)
> > - return 0;
> > -
> > - /* The CPU is online, so send it a reschedule IPI. */
> > + /*
> > + * The CPU is online, so send it a reschedule IPI. This forces
> > + * it through the scheduler, and (inefficiently) also handles cases
> > + * where idle loops fail to inform RCU about the CPU being idle.
> > + */
>
> If the idle loop forgets to call rcu_idle_enter() before going to
> sleep, I don't know if it's a good idea to try to cure that situation
> by forcing a quiescent state remotely. It may make the thing worse
> because we actually won't notice the lack of call to rcu_idle_enter()
> that the rcu stall detector would otherwise report to us.
>
> Also I don't think that works. If the task doesn't have
> TIF_RESCHED, it won't go through the scheduler on irq exit.
> smp_send_reschedule() doesn't set the flag. And also scheduler_ipi()
> returns right away if no wake up is pending.
>
> So, other than resuming the idle loop to sleep again, nothing may happen.
>
> Or am I missing something?

Hmmm... Seems like the IPIs aren't helping in any case, then?

I suppose that I could do an smp_call_function_single(), which then
did a set_need_resched()...

But this is a separate issue that I need to deal with. That said, any
suggestions are welcome!

Thanx, Paul

> > if (rdp->cpu != smp_processor_id())
> > smp_send_reschedule(rdp->cpu);
> > else