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
> OK, that should be enough for the next phase, please see the end for the
> patch. This patch applies on top of my previous one.
>
> Could you please set this up as follows?
>
> struct rcu_head rh;
>
> rcu_dump_rcu_node_tree(&rcu_sched_state); /* Initial state. */
> call_rcu(&rh, do_nothing_cb);
I added an empty do_nothing_cb() for this:
static void do_nothing_cb(struct rcu_head *rcu_head)
{
}
According to the debugging technique "comment everything out until it boots",
it now hangs in call_rcu().
> schedule_timeout_uninterruptible(5 * HZ); /* Or whatever delay. */
> rcu_dump_rcu_node_tree(&rcu_sched_state); /* GP state. */
> synchronize_sched(); /* Probably hangs. */
> rcu_barrier(); /* Drop RCU's references to rh before return. */
Thanks!
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
