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@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)
{
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