Re: [lkp-robot] [torture] b151f93a71: INFO:rcu_preempt_detected_stalls_on_CPUs/tasks

[Thomas Gleixner]

On Tue, 28 Nov 2017, Paul E. McKenney wrote:

> On Tue, Nov 28, 2017 at 03:16:30PM +0100, Thomas Gleixner wrote:
> > On Mon, 27 Nov 2017, Paul E. McKenney wrote:
> > > On Sun, Nov 26, 2017 at 04:42:03PM +0800, kernel test robot wrote:
> > > > [  116.353432] rcu_preempt kthread starved for 9974 jiffies! g4294967208 c4294967207 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x402 ->cpu=0
> > > 
> > > So the immediate reason for the stall warning is that the RCU grace-period
> > > kthread isn't being allowed to run.
> > > 
> > > > [  116.355517] rcu_preempt     I 7464     8      2 0x80000000
> > > > [  116.356543] Call Trace:
> > > > [  116.357008]  __schedule+0x493/0x620
> > > > [  116.357682]  schedule+0x24/0x40
> > > > [  116.358291]  schedule_timeout+0x330/0x3b0
> > > 
> > > And the reason that it isn't being allowed to run is that its few-jiffy
> > > schedule_timeout has extended for more than nine thousand jiffies.
> > > 
> > > There was an odd combination of kernel parameters that Thomas Gleixner
> > > came across that could cause writer-thread stalls (since fixed in -rcu
> > > by the exact patch you call out here), but I don't see how this could
> > > cause an RCU CPU stall warning.
> > 
> > <Trimmed a lot of useless crap.....>
> > 
> > The only reasonable explanation is that either a wakeup is missed or the
> > timer is not expired. Hard to tell from that back trace, but it would be
> > interesting to figure that out. Let me think about how that can be done.
> 
> Please see below for my latest patch in service of futility.  It leverages
> the fact that the stall-warning code takes the desperation step of sending
> an unsolicited wakeup to the grace-period kthread.  This allows the patch
> to use a stupid but effective heuristic to detect that the timer (or the
> wakeup) has gone AWOL.
> 
> I get the following "Waylayed timer" line from it:
> 
> [ 8015.327554] rcu_sched kthread starved for 21096 jiffies! g12440 c12439 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x402 ->cpu=3
> [ 8015.327554] rcu_sched       I14992     9      2 0x80000000
> [ 8015.327554] Call Trace:
> [ 8015.327554]  ? __schedule+0x33c/0x6f0
> [ 8015.327554]  ? preempt_count_add+0x51/0x90
> [ 8015.327554]  schedule+0x37/0x90
> [ 8015.327554]  schedule_timeout+0x159/0x410
> [ 8015.327554]  ? __next_timer_interrupt+0xc0/0xc0
> [ 8015.327554]  rcu_gp_kthread+0xa42/0x1080
> [ 8015.327554]  kthread+0xf0/0x130
> [ 8015.327554]  ? rcu_barrier+0x10/0x10
> [ 8015.327554]  ? kthread_destroy_worker+0x40/0x40
> [ 8015.327554]  ret_from_fork+0x1f/0x30
> [ 8015.421044] schedule_timeout: Waylayed timer idx: 56 idx_now: 222
> 
> Is there something more useful that I could make this thing print?

Please let it print base->clk, jiffies, base->next_expiry, timer->flags,
and timer->expires.

Thanks,