Messages in this thread | | | Date | Mon, 3 Oct 2011 09:22:21 -0700 | From | "Paul E. McKenney" <> | Subject | Re: linux-next-20110923: warning kernel/rcutree.c:1833 |
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On Mon, Oct 03, 2011 at 02:59:03PM +0200, Frederic Weisbecker wrote: > 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.
Indeed, and it might be worth leaving in for that.
> > 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.
Sigh! This isn't the first time this year that I have forgotten that, is it?
> Perhaps you need something like kernel/sched.c:resched_cpu() > This adds some rq->lock contention though.
This would happen infrequently, and could be made to be event more infrequent. But I wonder what happens when you do this to a CPU that is running the idle task? Seems like it should work normally, but...
Thanx, Paul
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