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().
Thanx, Paul
> > 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
>
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