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
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