Re: RCU lockup issues when CONFIG_SOFTLOCKUP_DETECTOR=n - any one else seeing this?

[Nicholas Piggin <npiggin@gmail.com>]

On Wed, 26 Jul 2017 18:42:14 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Wed, Jul 26, 2017 at 04:22:00PM -0700, David Miller wrote:

> > Indeed, that really wouldn't explain how we end up with a RCU stall
> > dump listing almost all of the cpus as having missed a grace period.  
> 
> I have seen stranger things, but admittedly not often.

So the backtraces show the RCU gp thread in schedule_timeout.

Are you sure that it's timeout has expired and it's not being scheduled,
or could it be a bad (large) timeout (looks unlikely) or that it's being
scheduled but not correctly noting gps on other CPUs?

It's not in R state, so if it's not being scheduled at all, then it's
because the timer has not fired:

[ 1984.628602] rcu_preempt kthread starved for 5663 jiffies! g1566 c1565 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
[ 1984.638153] rcu_preempt     S    0     9      2 0x00000000
[ 1984.643626] Call trace:
[ 1984.646059] [<ffff000008084fb0>] __switch_to+0x90/0xa8
[ 1984.651189] [<ffff000008962274>] __schedule+0x19c/0x5d8
[ 1984.656400] [<ffff0000089626e8>] schedule+0x38/0xa0
[ 1984.661266] [<ffff000008965844>] schedule_timeout+0x124/0x218
[ 1984.667002] [<ffff000008121424>] rcu_gp_kthread+0x4fc/0x748
[ 1984.672564] [<ffff0000080df0b4>] kthread+0xfc/0x128
[ 1984.677429] [<ffff000008082ec0>] ret_from_fork+0x10/0x50