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Date: Tue, 1 Apr 2014 18:16:16 -0400
From: Dave Jones <davej@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: rcu_prempt stalls / lockup
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On Tue, Apr 01, 2014 at 11:32:45AM -0700, Paul E. McKenney wrote:
 
 > >  > Given that you have CONFIG_RCU_TRACE=y, could you please enable the
 > >  > following trace events and dump the trace before things hang?
 > >  > 
 > >  > 	trace_event=rcu:rcu_grace_period,rcu:rcu_grace_period_init
 > >  > 
 > >  > If it is not feasible to dump the trace before things hang, let me
 > >  > know, and I will work out some other diagnostic regime.
 > > 
 > > I'll give that a shot when I get back in a few hours.

ok, this is tricky, and I'm not sure how helpful the below is.
because I don't know when the hang is going to happen, in parallel to my usual workload
I did this..
while [ 1 ];
do
  cat trace
  echo > trace
done

and got this...



# entries-in-buffer/entries-written: 7/7   #P:4
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
          <idle>-0     [000] dNs3   851.748475: rcu_grace_period: rcu_preempt 21342 cpuend
          <idle>-0     [000] dNs3   851.748477: rcu_grace_period: rcu_preempt 21343 cpustart
          <idle>-0     [000] dNs3   851.748477: rcu_grace_period: rcu_sched 20080 cpuend
          <idle>-0     [000] dN.2   851.748480: rcu_grace_period: rcu_preempt 21343 cpuqs
     rcu_preempt-9     [000] ...1   851.748485: rcu_grace_period: rcu_preempt 21343 fqsstart
     rcu_preempt-9     [000] ...1   851.748487: rcu_grace_period: rcu_preempt 21343 fqsend
     rcu_preempt-9     [000] ...1   851.748487: rcu_grace_period: rcu_preempt 21343 fqswait
# tracer: nop
#
# entries-in-buffer/entries-written: 13/13   #P:4
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
     rcu_preempt-9     [003] d..2   851.878457: rcu_grace_period: rcu_preempt 21349 end
     rcu_preempt-9     [003] d..2   851.878459: rcu_grace_period: rcu_preempt 21349 newreq
     rcu_preempt-9     [003] ...1   851.878459: rcu_grace_period: rcu_preempt 21349 reqwait
     rcu_preempt-9     [003] d..2   851.878460: rcu_grace_period: rcu_preempt 21350 start
     rcu_preempt-9     [003] d..2   851.878461: rcu_grace_period: rcu_preempt 21350 cpustart
     rcu_preempt-9     [003] d..2   851.878462: rcu_grace_period_init: rcu_preempt 21350 0 0 7 f
     rcu_preempt-9     [003] ...1   851.878463: rcu_grace_period: rcu_preempt 21350 fqswait
     rcu_preempt-9     [003] d..2   851.878464: rcu_grace_period: rcu_preempt 21350 cpuqs
          <idle>-0     [000] dNs3   851.878482: rcu_grace_period: rcu_preempt 21349 cpuend
          <idle>-0     [000] dNs3   851.878483: rcu_grace_period: rcu_preempt 21350 cpustart
          <idle>-0     [000] dNs3   851.878484: rcu_grace_period: rcu_sched 20086 cpuend
          <idle>-0     [000] .N.2   851.878486: rcu_grace_period: rcu_sched 20086 cpuqs
          <idle>-0     [000] dN.2   851.878487: rcu_grace_period: rcu_preempt 21350 cpuqs


followed by a half dozen 'empty' traces before it totally locked up.

	Dave

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