Date:	Thu, 30 Jun 2016 16:47:10 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...nel.org>,
	Linux MM <linux-mm@...ck.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: Boot failure on emev2/kzm9d (was: Re: [PATCH v2 11/11] mm/slab:
 lockless decision to grow cache)

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:
> > Hi Paul,
> > 
> > On Wed, Jun 29, 2016 at 6:44 PM, Paul E. McKenney
> > <paulmck@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com> 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)
 {

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