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

[Geert Uytterhoeven]

Hi Paul,

On Thu, Jun 30, 2016 at 3:24 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jun 30, 2016 at 09:58:51AM +0200, Geert Uytterhoeven wrote:
>> 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
>
> This is in fact the initial state for RCU grace periods.  In other words,
> all the earlier calls to synchronize_sched() likely happened while there
> was only one CPU online.
>
>>   devtmpfs: initialized
>>   VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1
>
> Could you please add the call_rcu() and timed delay as described in my
> earlier email?  That would hopefully help me see the state of the stalled
> grace period.

I already did, cfr. "it still hangs when calling call_rcu() or
synchronize_sched()".

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