Re: Boot failure on emev2/kzm9d (was: Re: [PATCH v2 11/11] mm/slab: lockless decision to grow cache)

[Geert Uytterhoeven]

Hi Joonsoo,

On Thu, Jun 30, 2016 at 9:47 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> On Wed, Jun 29, 2016 at 11:12:08AM -0700, Paul E. McKenney wrote:
>> On Wed, Jun 29, 2016 at 07:52:06PM +0200, Geert Uytterhoeven wrote:
>> > On Wed, Jun 29, 2016 at 6:44 PM, Paul E. McKenney
>> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > > On Wed, Jun 29, 2016 at 04:54:44PM +0200, Geert Uytterhoeven wrote:
>> > >> On Thu, Jun 23, 2016 at 4:53 AM, Paul E. McKenney
>> > >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > >> > On Wed, Jun 22, 2016 at 07:47:42PM -0700, Paul E. McKenney wrote:
>> > >
>> > > [ . . . ]
>> > >
>> > >> > @@ -4720,11 +4720,18 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp)
>> > >> >                         pr_info(" ");
>> > >> >                         level = rnp->level;
>> > >> >                 }
>> > >> > -               pr_cont("%d:%d ^%d  ", rnp->grplo, rnp->grphi, rnp->grpnum);
>> > >> > +               pr_cont("%d:%d/%#lx/%#lx ^%d  ", rnp->grplo, rnp->grphi,
>> > >> > +                       rnp->qsmask,
>> > >> > +                       rnp->qsmaskinit | rnp->qsmaskinitnext, rnp->grpnum);
>> > >> >         }
>> > >> >         pr_cont("\n");
>> > >> >  }
>> > >>
>> > >> For me it always crashes during the 37th call of synchronize_sched() in
>> > >> setup_kmem_cache_node(), which is the first call after secondary CPU bring up.
>> > >> With your and my debug code, I get:
>> > >>
>> > >>   CPU: Testing write buffer coherency: ok
>> > >>   CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
>> > >>   Setting up static identity map for 0x40100000 - 0x40100058
>> > >>   cnt = 36, sync
>> > >>   CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
>> > >>   Brought up 2 CPUs
>> > >>   SMP: Total of 2 processors activated (2132.00 BogoMIPS).
>> > >>   CPU: All CPU(s) started in SVC mode.
>> > >>   rcu_node tree layout dump
>> > >>    0:1/0x0/0x3 ^0
>> > >
>> > > Thank you for running this!
>> > >
>> > > OK, so RCU knows about both CPUs (the "0x3"), and the previous
>> > > grace period has seen quiescent states from both of them (the "0x0").
>> > > That would indicate that your synchronize_sched() showed up when RCU was
>> > > idle, so it had to start a new grace period.  It also rules out failure
>> > > modes where RCU thinks that there are more CPUs than really exist.
>> > > (Don't laugh, such things have really happened.)
>> > >
>> > >>   devtmpfs: initialized
>> > >>   VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1
>> > >>   clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff,
>> > >> max_idle_ns: 19112604462750000 ns
>> > >>
>> > >> I hope it helps. Thanks!
>> > >
>> > > I am going to guess that this was the first grace period since the second
>> > > CPU came online.  When there only on CPU online, synchronize_sched()
>> > > is a no-op.
>> > >
>> > > OK, this showed some things that aren't a problem.  What might the
>> > > problem be?
>> > >
>> > > o       The grace-period kthread has not yet started.  It -should- start
>> > >         at early_initcall() time, but who knows?  Adding code to print
>> > >         out that kthread's task_struct address.
>> > >
>> > > o       The grace-period kthread might not be responding to wakeups.
>> > >         Checking this requires that a grace period be in progress,
>> > >         so please put a call_rcu_sched() just before the call to
>> > >         rcu_dump_rcu_node_tree().  (Sample code below.)  Adding code
>> > >         to my patch to print out more GP-kthread state as well.
>> > >
>> > > o       One of the CPUs might not be responding to RCU.  That -should-
>> > >         result in an RCU CPU stall warning, so I will ignore this
>> > >         possibility for the moment.
>> > >
>> > >         That said, do you have some way to determine whether scheduling
>> > >         clock interrupts are really happening?  Without these interrupts,
>> > >         no RCU CPU stall warnings.
>> >
>> > I believe there are no clocksources yet. The jiffies clocksource is the first
>> > clocksource found, and that happens after the first call to
>> > synchronize_sched(), cfr. my dmesg snippet above.
>> >
>> > In a working boot:
>> > # cat /sys/bus/clocksource/devices/clocksource0/available_clocksource
>> > e0180000.timer jiffies
>> > # cat /sys/bus/clocksource/devices/clocksource0/current_clocksource
>> > e0180000.timer
>>
>> Ah!  But if there is no jiffies clocksource, then schedule_timeout()
>> and friends will never return, correct?  If so, I guarantee you that
>> synchronize_sched() will unconditionally hang.
>>
>> So if I understand correctly, the fix is to get the jiffies clocksource
>> running before the first call to synchronize_sched().
>
> If so, following change would be sufficient.
>
> Thanks.
>
> ------>8-------
> diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
> index 555e21f..4f6471f 100644
> --- a/kernel/time/jiffies.c
> +++ b/kernel/time/jiffies.c
> @@ -98,7 +98,7 @@ static int __init init_jiffies_clocksource(void)
>         return __clocksource_register(&clocksource_jiffies);
>  }
>
> -core_initcall(init_jiffies_clocksource);
> +early_initcall(init_jiffies_clocksource);
>
>  struct clocksource * __init __weak clocksource_default_clock(void)
>  {

Thanks for your patch!

While this does move jiffies clocksource initialization before secondary CPU
bringup, it still hangs when calling call_rcu() or synchronize_sched():

  CPU: Testing write buffer coherency: ok
  CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
  Setting up static identity map for 0x40100000 - 0x40100058
  cnt = 36, sync
  clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff,
max_idle_ns: 19112604462750000 ns
  CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
  Brought up 2 CPUs
  SMP: Total of 2 processors activated (2132.00 BogoMIPS).
  CPU: All CPU(s) started in SVC mode.
  RCU: rcu_sched GP kthread: c784e1c0 state: 1 flags: 0x0 g:-300 c:-300
       jiffies: 0xffff8ad0  GP start: 0x0 Last GP activity: 0x0
  rcu_node tree layout dump
   0:1/0x0/0x3 ^0
  devtmpfs: initialized
  VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds