Hi Paul,

On Wed, Jun 29, 2016 at 6:44 PM, Paul E. McKenney
[off-list ref] wrote:

On Wed, Jun 29, 2016 at 04:54:44PM +0200, Geert Uytterhoeven wrote:

quoted

On Thu, Jun 23, 2016 at 4:53 AM, Paul E. McKenney
[off-list ref] wrote:

quoted

On Wed, Jun 22, 2016 at 07:47:42PM -0700, Paul E. McKenney wrote:

[ . . . ]

quoted

quoted

@@ -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.)

quoted

  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 at linux-m68k.org

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