On Thu, Nov 19, 2020 at 07:14AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 19, 2020 at 01:53:57PM +0100, Marco Elver wrote:
> > On Wed, Nov 18, 2020 at 03:38PM -0800, Paul E. McKenney wrote:
> > > On Wed, Nov 18, 2020 at 11:56:21PM +0100, Marco Elver wrote:
> > > > [...]
> > > > I think I figured out one piece of the puzzle. Bisection keeps pointing
> > > > me at some -rcu merge commit, which kept throwing me off. Nor did it
> > > > help that reproduction is a bit flaky. However, I think there are 2
> > > > independent problems, but the manifestation of 1 problem triggers the
> > > > 2nd problem:
> > > > 
> > > > 1. problem: slowed forward progress (workqueue lockup / RCU stall reports)
> > > > 
> > > > 2. problem: DEADLOCK which causes complete system lockup
> > > > 
> > > > 	| ...
> > > > 	|        CPU0
> > > > 	|        ----
> > > > 	|   lock(rcu_node_0);
> > > > 	|   <Interrupt>
> > > > 	|     lock(rcu_node_0);
> > > > 	| 
> > > > 	|  *** DEADLOCK ***
> > > > 	| 
> > > > 	| 1 lock held by event_benchmark/105:
> > > > 	|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: print_other_cpu_stall kernel/rcu/tree_stall.h:493 [inline]
> > > > 	|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: check_cpu_stall kernel/rcu/tree_stall.h:652 [inline]
> > > > 	|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_pending kernel/rcu/tree.c:3752 [inline]
> > > > 	|  #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x428/0xd40 kernel/rcu/tree.c:2581
> > > > 	| ...
> > > > 
> > > > Problem 2 can with reasonable confidence (5 trials) be fixed by reverting:
> > > > 
> > > > 	rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled
> > > > 
> > > > At which point the system always boots to user space -- albeit with a
> > > > bunch of warnings still (attached). The supposed "good" version doesn't
> > > > end up with all those warnings deterministically, so I couldn't say if
> > > > the warnings are expected due to recent changes or not (Arm64 QEMU
> > > > emulation, 1 CPU, and lots of debugging tools on).
> > > > 
> > > > Does any of that make sense?
> > > 
> > > Marco, it makes all too much sense!  :-/
> > > 
> > > Does the patch below help?
> > > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > commit 444ef3bbd0f243b912fdfd51f326704f8ee872bf
> > > Author: Peter Zijlstra <peterz@infradead.org>
> > > Date:   Sat Aug 29 10:22:24 2020 -0700
> > > 
> > >     sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled
> > 
> > My assumption is that this is a replacement for "rcu: Don't invoke
> > try_invoke_on_locked_down_task() with irqs disabled", right?
> 
> Hmmm...  It was actually intended to be in addition.
> 
> > That seems to have the same result (same test setup) as only reverting
> > "rcu: Don't invoke..." does: still results in a bunch of workqueue
> > lockup warnings and RCU stall warnings, but boots to user space. I
> > attached a log. If the warnings are expected (are they?), then it looks
> > fine to me.
> 
> No, they are not at all expected, but might be a different symptom
> of the original problem.  Please see below.
> 
> > (And just in case: with "rcu: Don't invoke..." and "sched/core:
> > Allow..." both applied I still get DEADLOCKs -- but that's probably
> > expected.)
> 
> As noted earlier, it is a surprise.  Could you please send me the
> console output?
 
I've attached the output of a run with both commits applied.

> > Testing all events: OK
> > hrtimer: interrupt took 17120368 ns
> > Running tests again, along with the function tracer
> > Running tests on all trace events:
> > Testing all events: 
> > BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 12s!
> > Showing busy workqueues and worker pools:
> > workqueue events: flags=0x0
> >   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> >     pending: vmstat_shepherd
> > BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 17s!
> > Showing busy workqueues and worker pools:
> > workqueue events: flags=0x0
> >   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> >     pending: vmstat_shepherd
> > workqueue events_power_efficient: flags=0x82
> >   pwq 2: cpus=0 flags=0x4 nice=0 active=1/256 refcnt=3
> >     pending: neigh_periodic_work
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 1 at kernel/rcu/tree_stall.h:758 rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline]
> > WARNING: CPU: 0 PID: 1 at kernel/rcu/tree_stall.h:758 rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711
> 
> I have different line numbering,

This is still using next-20201110. I'll rerun with latest -next as well.

> but the only warning that I see in this
> function is the one complaining that RCU has been ignoring a request to
> start a grace period for too long.  This usually happens because the RCU
> grace-period kthread (named "rcu_preempt" in your case, but can also be
> named "rcu_sched") is being prevented from running, but can be caused
> by other things as well.
> 
> > Modules linked in:
> > CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.0-rc3-next-20201110-00003-g920304642405-dirty #30
> > Hardware name: linux,dummy-virt (DT)
> > pstate: 20000085 (nzCv daIf -PAN -UAO -TCO BTYPE=--)
> > pc : rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline]
> > pc : rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711
> > lr : __xchg_mb arch/arm64/include/asm/cmpxchg.h:88 [inline]
> > lr : atomic_xchg include/asm-generic/atomic-instrumented.h:615 [inline]
> > lr : rcu_check_gp_start_stall kernel/rcu/tree_stall.h:751 [inline]
> > lr : rcu_check_gp_start_stall.isra.0+0x148/0x210 kernel/rcu/tree_stall.h:711
> 
> Two program counters and four link registers?  Awesome!  ;-)

Ah I'm using syzkaller's symbolizer, which duplicates lines if there was
an inline function (remove all the "[inline]" and it should make sense,
but the "[inline]" tell you the actual line). Obviously for things like
this it's a bit unintuitive. :-)
 
> > sp : ffff800010003d20
> > x29: ffff800010003d20 x28: ffff274ac3a10000 
> > x27: 0000000000000000 x26: ffff274b3dbe72d8 
> > x25: ffffbcb867722000 x24: 0000000000000000 
> > x23: 0000000000000000 x22: ffffbcb8681d1260 
> > x21: ffffbcb86735b000 x20: ffffbcb867404440 
> > x19: ffffbcb867404440 x18: 0000000000000123 
> > x17: ffffbcb865d400f0 x16: 0000000000000002 
> > x15: 0000000000000002 x14: 0000000000000000 
> > x13: 003d090000000000 x12: 00001e8480000000 
> > x11: ffffbcb867958980 x10: ffff800010003cf0 
> > x9 : ffffbcb864f4b7c8 x8 : 0000000000000080 
> > x7 : 0000000000000026 x6 : ffffbcb86774e4c0 
> > x5 : 0000000000000000 x4 : 00000000d4001f4b 
> > x3 : 0000000000000000 x2 : 0000000000000000 
> > x1 : 0000000000000001 x0 : 0000000000000000 
> > Call trace:
> >  rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline]
> >  rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711
> >  rcu_core+0x168/0x9e0 kernel/rcu/tree.c:2719
> >  rcu_core_si+0x18/0x28 kernel/rcu/tree.c:2737
> 
> The RCU_SOFTIRQ handler is causing this checking to occur, for whatever
> that is worth.
> 
> >  __do_softirq+0x188/0x6b4 kernel/softirq.c:298
> >  do_softirq_own_stack include/linux/interrupt.h:568 [inline]
> >  invoke_softirq kernel/softirq.c:393 [inline]
> >  __irq_exit_rcu kernel/softirq.c:423 [inline]
> >  irq_exit+0x1cc/0x1e0 kernel/softirq.c:447
> >  __handle_domain_irq+0xb4/0x130 kernel/irq/irqdesc.c:690
> >  handle_domain_irq include/linux/irqdesc.h:170 [inline]
> >  gic_handle_irq+0x70/0x108 drivers/irqchip/irq-gic.c:370
> >  el1_irq+0xc0/0x180 arch/arm64/kernel/entry.S:651
> >  arch_local_irq_restore+0x8/0x10 arch/arm64/include/asm/irqflags.h:124
> >  release_probes kernel/tracepoint.c:113 [inline]
> >  tracepoint_remove_func kernel/tracepoint.c:315 [inline]
> >  tracepoint_probe_unregister+0x220/0x378 kernel/tracepoint.c:382
> >  trace_event_reg+0x58/0x150 kernel/trace/trace_events.c:298
> >  __ftrace_event_enable_disable+0x424/0x608 kernel/trace/trace_events.c:412
> >  ftrace_event_enable_disable kernel/trace/trace_events.c:495 [inline]
> >  __ftrace_set_clr_event_nolock+0x120/0x180 kernel/trace/trace_events.c:811
> >  __ftrace_set_clr_event+0x60/0x90 kernel/trace/trace_events.c:833
> >  event_trace_self_tests+0xd4/0x114 kernel/trace/trace_events.c:3661
> >  event_trace_self_test_with_function kernel/trace/trace_events.c:3734 [inline]
> >  event_trace_self_tests_init+0x88/0xa8 kernel/trace/trace_events.c:3747
> >  do_one_initcall+0xa4/0x500 init/main.c:1212
> >  do_initcall_level init/main.c:1285 [inline]
> >  do_initcalls init/main.c:1301 [inline]
> >  do_basic_setup init/main.c:1321 [inline]
> >  kernel_init_freeable+0x344/0x3c4 init/main.c:1521
> >  kernel_init+0x20/0x16c init/main.c:1410
> >  ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961
> > irq event stamp: 3274113
> > hardirqs last  enabled at (3274112): [<ffffbcb864f8aee4>] rcu_core+0x974/0x9e0 kernel/rcu/tree.c:2716
> > hardirqs last disabled at (3274113): [<ffffbcb866233bf0>] __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:108 [inline]
> > hardirqs last disabled at (3274113): [<ffffbcb866233bf0>] _raw_spin_lock_irqsave+0xb8/0x14c kernel/locking/spinlock.c:159
> > softirqs last  enabled at (3272576): [<ffffbcb864e10b80>] __do_softirq+0x630/0x6b4 kernel/softirq.c:325
> > softirqs last disabled at (3274101): [<ffffbcb864ec6c54>] do_softirq_own_stack include/linux/interrupt.h:568 [inline]
> > softirqs last disabled at (3274101): [<ffffbcb864ec6c54>] invoke_softirq kernel/softirq.c:393 [inline]
> > softirqs last disabled at (3274101): [<ffffbcb864ec6c54>] __irq_exit_rcu kernel/softirq.c:423 [inline]
> > softirqs last disabled at (3274101): [<ffffbcb864ec6c54>] irq_exit+0x1cc/0x1e0 kernel/softirq.c:447
> > ---[ end trace 902768efebf5a607 ]---
> > rcu: rcu_preempt: wait state: RCU_GP_WAIT_GPS(1) ->state: 0x0 delta ->gp_activity 4452 ->gp_req_activity 3848 ->gp_wake_time 3848 ->gp_wake_seq 2696 ->gp_seq 2696 ->gp_seq_needed 2700 ->gp_flags 0x1
> 
> The last thing that RCU's grace-period kthread did was to go to sleep
> waiting for a grace-period request (RCU_GP_WAIT_GPS).
> 
> > rcu: 	rcu_node 0:0 ->gp_seq 2696 ->gp_seq_needed 2700
> > rcu: RCU callbacks invoked since boot: 2583
> > rcu_tasks: RTGS_WAIT_CBS(11) since 567120 g:1 i:0/0 k. 
> > rcu_tasks_rude: RTGS_WAIT_CBS(11) since 567155 g:1 i:0/1 k. 
> > rcu_tasks_trace: RTGS_INIT(0) since 4295464549 g:0 i:0/0 k. N0 h:0/0/0
> > rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > 	(detected by 0, t=3752 jiffies, g=2705, q=8)
> > rcu: All QSes seen, last rcu_preempt kthread activity 557 (4295471128-4295470571), jiffies_till_next_fqs=1, root ->qsmask 0x0
> > rcu: rcu_preempt kthread starved for 557 jiffies! g2705 f0x2 RCU_GP_CLEANUP(7) ->state=0x0 ->cpu=0
> 
> And here we see that RCU's grace-period kthread has in fact been starved.
> 
> This kthread is now in RCU_GP_CLEANUP, perhaps because of the wakeup that is
> sent in rcu_check_gp_kthread_starvation().
> 
> My current guess is that this is a consequence of the earlier failures,
> but who knows?
 
I can try bisection again, or reverting some commits that might be
suspicious? But we'd need some selection of suspicious commits.

Thanks,
-- Marco