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Date: Sun, 10 Feb 2008 09:43:55 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Heiko Carstens <heiko.carstens@...ibm.com> Cc: Gautham R Shenoy <ego@...ibm.com>, Dipankar Sarma <dipankar@...ibm.com>, Steven Rostedt <srostedt@...hat.com>, Ingo Molnar <mingo@...e.hu>, Martin Schwidefsky <schwidefsky@...ibm.com>, linux-kernel@...r.kernel.org Subject: Re: preempt rcu bug on s390 On Sun, Feb 10, 2008 at 02:01:50PM +0100, Heiko Carstens wrote: > > > > > Using CONFIG_PREEMPT_RCU and CONFIG_NO_IDLE_HZ on s390 my system always > > > > > gets stuck when running with more than one cpu. > > > > > When booting with four cpus I get all four cpus caught withing cpu_idle > > > > > and not advancing anymore. However there is the init process which is > > > > > waitung for synchronize_rcu() to complete (lcrash output): > > > > > > > > > > If I change the code so that timer ticks won't be disabled everything > > > > > runs fine. So my guess is that rcu_needs_cpu() doesn't do the right > > > > > thing for the rcu preemptible case. > > > > > > > > > > Kernel version is git head of today. > > > > > > > > > > Any ideas? > > > > > > > > Does this tree have http://lkml.org/lkml/2008/1/29/208 applied? > > > > > > > > If not, could you please check it out? > > > > > > It's not applied, however it doesn't change anything. Also the patch > > > is tied to the dynticks implementation which is differently from > > > s390's nohz implementation. > > > I had to add the patch below so it would make at least some sense. > > > But it doesn't fix the problem. > > > > OK, I was afraid of that. ;-) > > > > Does s390 start out in nohz mode? The reason I ask is that it feels like > > an off-by-one error for the dynticks_progress_counter. > > Actually I forgot to add a few ifdefs to make the code do something :) > That just reveals that we have a conflict with the dynticks implementation > and s390's nohz that shows up in what rcu_irq_enter/exit assume. > I didn't patch s390 and common code so it will work, but I think the > patch you mentionened will fix the problem I reported. > So I guess we should either convert s390 to use the generic dynticks > implementation or disable preemptible rcu on s390 until we converted > our code. Sounds good to me!!! (Especially converting s390 to generic algorithm.) I believe that the generic implementation will do what you need, but I am sure you will let me know of any problems that arise. > Thanks for helping debugging this! Thank you for tracking it down! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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