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Date: Tue, 16 Dec 2014 10:42:51 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Miroslav Benes <mbenes@...e.cz> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: rcu_sched stall detected, but no state dump On Mon, Dec 15, 2014 at 02:26:30PM +0100, Miroslav Benes wrote: > On Fri, 12 Dec 2014, Paul E. McKenney wrote: > > > On Fri, Dec 12, 2014 at 03:06:20PM +0100, Miroslav Benes wrote: > > > On Thu, 11 Dec 2014, Paul E. McKenney wrote: > > > > > > > On Thu, Dec 11, 2014 at 08:50:10AM -0800, Paul E. McKenney wrote: > > > > > On Thu, Dec 11, 2014 at 10:35:15AM +0100, Miroslav Benes wrote: > > > > > > On Wed, 10 Dec 2014, Paul E. McKenney wrote: > > > > > > > > > > > > > On Wed, Dec 10, 2014 at 01:52:02PM +0100, Miroslav Benes wrote: > > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > today I came across RCU stall which was correctly detected, but there is > > > > > > > > no state dump. This is a bit suspicious, I think. > > > > > > > > > > > > > > > > This is the output in serial console: > > > > > > > > > > > > > > > > [ 105.727003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 105.727003] (detected by 0, t=21002 jiffies, g=3269, c=3268, q=138) > > > > > > > > [ 105.727003] INFO: Stall ended before state dump start > > > > > > > > [ 168.732006] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 168.732006] (detected by 0, t=84007 jiffies, g=3269, c=3268, q=270) > > > > > > > > [ 168.732006] INFO: Stall ended before state dump start > > > > > > > > [ 231.737003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 231.737003] (detected by 0, t=147012 jiffies, g=3269, c=3268, q=388) > > > > > > > > [ 231.737003] INFO: Stall ended before state dump start > > > > > > > > [ 294.742003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 294.742003] (detected by 0, t=210017 jiffies, g=3269, c=3268, q=539) > > > > > > > > [ 294.742003] INFO: Stall ended before state dump start > > > > > > > > [ 357.747003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 357.747003] (detected by 0, t=273022 jiffies, g=3269, c=3268, q=693) > > > > > > > > [ 357.747003] INFO: Stall ended before state dump start > > > > > > > > [ 420.752003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > > > > [ 420.752003] (detected by 0, t=336027 jiffies, g=3269, c=3268, q=806) > > > > > > > > [ 420.752003] INFO: Stall ended before state dump start > > > > > > > > ... > > > > > > > > > > > > > > > > It can be reproduced by trivial code attached to this mail (infinite > > > > > > > > loop in kernel thread created in kernel module). I have CONFIG_PREEMPT=n. > > > > > > > > The kernel thread is scheduled on the same CPU which causes soft lockup > > > > > > > > (reliably detected when lockup detector is on). There is certainly RCU > > > > > > > > stall, but I would expect a state dump. Is this an expected behaviour? > > > > > > > > Maybe I overlooked some config option, don't know. > > > > > > > > > > > > > > Definitely not expected behavior! Unless you have only one CPU, but in > > > > > > > that case you should be running tiny RCU, not tree RCU. > > > > > > > > > > > > So indeed I messed up my configs somehow and run the code on uniprocessor > > > > > > with SMP=y and tree RCU. With more processors RCU stall is detected and > > > > > > correct state is dumped. On uniprocessor with SMP=n and tiny RCU > > > > > > softlockup is detected, but no RCU stall in the log (is this correct?). So > > > > > > I'm really sorry for the noise. > > > > > > > > > > > > Anyway I still think that running SMP kernel with tree RCU on > > > > > > uniprocessor is possible option (albeit suboptimal and maybe improbable). > > > > > > Should I proceed with your patch below and bisection or am I mistaken > > > > > > completely and we can leave it because there is no problem? > > > > > > > > > > Not a problem, there have been some interesting RCU CPU stall warnings > > > > > recently, and your data did add some insight. > > > > > > > > > > So the combination SMP=n PREEMPT=y can happen straightforwardly via > > > > > kbuild. The combination SMP=n PREEMPT=n can happen (somewhat less) > > > > > straightforwardly by running an SMP=y PREEMPT=n kernel on a single-CPU > > > > > system. In both cases, what can happen is that RCU's grace-period > > > > > kthreads are starved, which can result in those reports. > > > > > > > > > > And these reports are confusing. I am considering attempting to improve > > > > > the diagnostics. If I do, would you be willing to test the resulting > > > > > patches? > > > > > > > > Like this one, for example. ;-) > > > > > > Ok, with next-20141212, where this patch is included, and SMP=y PREEMPT=n > > > I get the following > > > > > > ... > > > [ 206.949003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 206.949003] (detected by 0, t=21002 jiffies, g=3384, c=3383, q=247) > > > [ 206.949003] All QSes seen, last rcu_sched kthread activity 4294853243/4294874245, jiffies_till_next_fqs=3 > > > [ 269.954004] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 269.954004] (detected by 0, t=84007 jiffies, g=3384, c=3383, q=449) > > > [ 269.954004] All QSes seen, last rcu_sched kthread activity 4294853243/4294937250, jiffies_till_next_fqs=3 > > > [ 332.959004] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 332.959004] (detected by 0, t=147012 jiffies, g=3384, c=3383, q=651) > > > [ 332.959004] All QSes seen, last rcu_sched kthread activity 4294853243/4295000255, jiffies_till_next_fqs=3 > > > [ 395.964003] INFO: rcu_sched detected stalls on CPUs/tasks: > > > [ 395.964003] (detected by 0, t=210017 jiffies, g=3384, c=3383, q=858) > > > [ 395.964003] All QSes seen, last rcu_sched kthread activity 4294853243/4295063260, jiffies_till_next_fqs=3 > > > ... > > > > > > So it seems to work and it is not confusing anymore. Would it be possible > > > to dump the stack also in this case? The stall is there, the condition is > > > correctly detected, so I guess it could be possible. It would give the > > > information what caused the stall. > > > > Ah, good point! In this case, the stall is self-detected, but RCU > > incorrectly includes that it is some other CPU's fault because RCU is > > not waiting on the current CPU. Please see below for updated patch. > > > > > In SMP=n PREEMPT=y case there is no stall with my test. I guess it is > > > because module inclusion does not disable preemption. However it is > > > possible in other cases as you wrote above. > > > > OK, that matches expected behavior. > > > > > One last thing. Is there a reason that there is not any similar INFO in > > > the console for tiny implementation? I enabled RCU_TRACE and still got > > > nothing. I have only shallow knowledge of RCU, though. > > > > It is supposed to work given CONFIG_RCU_TRACE=y, and it looks like > > it should, at least as long as scheduling-clock interrupts are being > > delivered. One thing to try for diagnosis is to have every (say) 1000th > > call to check_cpu_stall() (in kernel/rcu/tiny_plugin.h) do a printk() > > and same for rcu_check_callbacks() (in kernel/rcu/tiny.c). > > Unfortunately nothing is there. I'll look into it when I have more time > later this week and let you know. > > > > > > I'll test more patches if you have some... > > > > Please see below for update that should print current CPU's stack for > > the "All QSes seen" case. This replaces the earlier patch. > > This gives > > ... > [ 149.320003] INFO: rcu_sched detected stalls on CPUs/tasks: > [ 149.320003] (detected by 0, t=21002 jiffies, g=2823, c=2822, q=209) > [ 149.320003] All QSes seen, last rcu_sched kthread activity 21002 (4294816616-4294795614), jiffies_till_next_fqs=3 > [ 149.320003] test_thread R running task 14832 4015 2 0x00000008 > [ 149.320003] 00000000000039f0 ffff88013fc03da8 ffffffff8107874c ffffffff810786b2 > [ 149.320003] ffffffff81e391c0 000000000000520a ffff88013fdcc4a0 ffffffff81e38cc0 > [ 149.320003] ffffffff81e38cc0 ffff88013fc03e28 ffffffff810b1049 0000000000000002 > [ 149.320003] Call Trace: > [ 149.320003] <IRQ> [<ffffffff8107874c>] sched_show_task+0x11c/0x190 > [ 149.320003] [<ffffffff810786b2>] ? sched_show_task+0x82/0x190 > [ 149.320003] [<ffffffff810b1049>] rcu_check_callbacks+0x839/0x850 > [ 149.320003] [<ffffffff810b627b>] update_process_times+0x4b/0x80 > [ 149.320003] [<ffffffff810c6865>] ? tick_sched_do_timer+0x35/0x40 > [ 149.320003] [<ffffffff810c60d6>] tick_sched_handle.isra.19+0x36/0x50 > [ 149.320003] [<ffffffff810c68b5>] tick_sched_timer+0x45/0x80 > [ 149.320003] [<ffffffff810b7022>] __run_hrtimer+0xb2/0x250 > [ 149.320003] [<ffffffff810b73cc>] ? hrtimer_interrupt+0x7c/0x250 > [ 149.320003] [<ffffffff810c6870>] ? tick_sched_do_timer+0x40/0x40 > [ 149.320003] [<ffffffff810b7447>] hrtimer_interrupt+0xf7/0x250 > [ 149.320003] [<ffffffffa0000000>] ? 0xffffffffa0000000 > [ 149.320003] [<ffffffff810356eb>] local_apic_timer_interrupt+0x3b/0x70 > [ 149.320003] [<ffffffff818a9845>] smp_apic_timer_interrupt+0x45/0x60 > [ 149.320003] [<ffffffff818a7c2f>] apic_timer_interrupt+0x6f/0x80 > [ 149.320003] <EOI> [<ffffffff818a7960>] ? retint_restore_args+0xe/0xe > [ 149.320003] [<ffffffff8106d004>] ? kthread_should_stop+0x24/0x30 > [ 149.320003] [<ffffffffa0000015>] ? test_thread_fn+0x15/0x20 [kthread_mod] > [ 149.320003] [<ffffffff8106d460>] kthread+0xf0/0x110 > [ 149.320003] [<ffffffff8106d370>] ? kthread_create_on_node+0x240/0x240 > [ 149.320003] [<ffffffff818a6cec>] ret_from_fork+0x7c/0xb0 > [ 149.320003] [<ffffffff8106d370>] ? kthread_create_on_node+0x240/0x240 > ... > > So it works as expected which is great. You can add my Tested-by if you > feel it is worth it. Got it, thank you! 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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