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

Frederic Weisbecker
2011-10-03T12:59:17Z

On Sun, Oct 02, 2011 at 05:28:32PM -0700, Paul E. McKenney wrote:
> 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 thought it was there for !PREEMPT cases where the task has TIF_RESCHED
but takes too much time to find an opportunity to go to sleep.

> 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!

Note you can't call smp_call_function_*() while irqs are disabled.

Perhaps you need something like kernel/sched.c:resched_cpu()
This adds some rq->lock contention though.