Re: RCU stall and the system boot hang with nfsroot

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Dec 30, 2015 at 09:41:45AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 30, 2015 at 03:03:33PM +0800, Aaron Ma wrote:
> > On Wed, Dec 30, 2015 at 7:42 AM, Paul E. McKenney
> > <paulmck@linux.vnet.ibm.com> wrote:

[ . . . ]

> > cfg80211: Calling CRDA to update world regulatory domain
> > cfg80211: Calling CRDA to update world regulatory domain
> > cfg80211: Exceeded CRDA call max attempts. Not calling CRDA
> > INFO: rcu_preempt detected stalls on CPUs/tasks:
> >     71: (0 ticks this GP) idle=1ac/0/0 softirq=0/0 fqs=0
> >     (detected by 62, t=26002 jiffies, g=3735, c=3734, q=366014)
> > Task dump for CPU 71:
> > swapper/71      R  running task        0     0      1 0x00200000
> >  ffffffff81492587 ffff8804633cbe58 ffffffff814f21d7 0000000000000004
> >  0000000000000004 ffffe8fffb405310 ffffffff820dc5c0 ffff8804633cbea8
> >  ffffffff8181db85 0000000000000000 0000000000000000 0000000000000046
> > Call Trace:
> >  [<ffffffff81492587>] ? debug_smp_processor_id+0x17/0x20
> >  [<ffffffff814f21d7>] ? intel_idle+0x137/0x140
> >  [<ffffffff8181db85>] ? cpuidle_enter_state+0x65/0x3e0
> >  [<ffffffff8181df37>] ? cpuidle_enter+0x17/0x20
> >  [<ffffffff810a849d>] ? cpu_startup_entry+0x33d/0x630
> >  [<ffffffff8103ceae>] ? start_secondary+0x12e/0x140
> > rcu_preempt kthread starved for 26002 jiffies!
> > rcu_check_gp_kthread_starvation --->show task:
> > rcu_preempt     S ffff880456413c68     0     8      2 0x00000000
> >  ffff880456413c68 ffff8804564025d0 000000000000d7a0 ffff880456b18000
> >  ffff8804564025d0 ffff880456413c38 ffffffff81492587 ffff880456413c58
> >  ffff880456414000 ffff8804564025d0 ffff880456413cb8 ffff880869dce500
> > Call Trace:
> >  [<ffffffff81492587>] ? debug_smp_processor_id+0x17/0x20
> >  [<ffffffff81b5ce9f>] schedule+0x3f/0xd0
> >  [<ffffffff81b5ef19>] schedule_timeout+0x189/0x3f0
> >  [<ffffffff810a7904>] ? swait_prepare+0x24/0x90
> >  [<ffffffff810e8e60>] ? timer_cpu_notify+0x190/0x190
> >  [<ffffffff810a793b>] ? swait_prepare+0x5b/0x90
> >  [<ffffffff810de3f8>] rcu_gp_kthread+0x8a8/0x2190
> >  [<ffffffff810b275d>] ? trace_hardirqs_on+0xd/0x10
> >  [<ffffffff81b5c18f>] ? __schedule+0x4af/0x1180
> >  [<ffffffff810ddb50>] ? call_rcu_sched+0x20/0x20
> >  [<ffffffff8107f844>] kthread+0xe4/0x100
> >  [<ffffffff810b275d>] ? trace_hardirqs_on+0xd/0x10
> >  [<ffffffff8107f760>] ? kthread_create_on_node+0x240/0x240
> >  [<ffffffff81b61562>] ret_from_fork+0x42/0x70
> >  [<ffffffff8107f760>] ? kthread_create_on_node+0x240/0x240
> > rcu_check_gp_kthread_starvation --->end
> > 
> > It seems wait in rcu_gp_kthread. it should be no task blocked right?
> > If so, why the swait_event_interruptible_timeout is not awaken? the
> > timeout is CONFIG_HZ=1000.
> 
> Given that this happens at boot, perhaps ftrace is a good next step.
> The thought would be to enable ftrace via the kernel boot parameters
> for the timers.
> 
> And how often does this problem occur?

And does the following diagnostic patch help?  Its expected behavior
would be to turn a hard hang into something that recovered in a few
minutes, while giving a few stall-warning splats.

							Thanx, Paul

------------------------------------------------------------------------

commit 7798a5efb2acabfa3ca788dd9b5b118eb1bff443
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Thu Dec 31 08:48:36 2015 -0800

    rcu: Awaken grace-period kthread when stalled
    
    Recent kernels can fail to awaken the grace-period kthread for
    quiescent-state forcing.  This commit is a crude hack that does
    a wakeup any time a stall is detected.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4b3de6718f7c..51da7ef3561f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1225,8 +1225,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
 		       rsp->gp_flags,
 		       gp_state_getname(rsp->gp_state), rsp->gp_state,
 		       rsp->gp_kthread ? rsp->gp_kthread->state : ~0);
-		if (rsp->gp_kthread)
+		if (rsp->gp_kthread) {
 			sched_show_task(rsp->gp_kthread);
+			wake_up_process(rsp->gp_kthread);
+		}
 	}
 }