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

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

On Tue, Nov 28, 2017 at 05:47:35PM +0100, Thomas Gleixner wrote:
> 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.

Like this, on top of the previous patch?

							Thanx, Paul

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

commit 1085cc784fe1e9d4b5e3bb2f849c0a7fa47b7cb7
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Tue Nov 28 09:05:39 2017 -0800

    EXP: timer: Dump more info if timer misses by huge amount
    
    "Please let it print base->clk, jiffies, base->next_expiry, timer->flags,
    and timer->expires."
    
    Suggested-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index db774b0f217e..a3321bb565db 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1803,7 +1803,7 @@ signed long __sched schedule_timeout(signed long timeout)
 		idx = timer_get_idx(&timer.timer);
 		idx_now = calc_wheel_index(j, base->clk);
 		raw_spin_unlock_irqrestore(&base->lock, flags);
-		pr_info("%s: Waylayed timer idx: %u idx_now: %u\n", __func__, idx, idx_now);
+		pr_info("%s: Waylayed timer base->clk: %#lx jiffies: %#lx base->next_expiry: %#lx timer->flags: %#x timer->expires %#lx idx: %u idx_now: %u\n", __func__, base->clk, j, base->next_expiry, timer.timer.flags, timer.timer.expires, idx, idx_now);
 	}
 	del_singleshot_timer_sync(&timer.timer);