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

From: Frederic Weisbecker
Date: Sun Oct 02 2011 - 18:50:34 EST

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?

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