On Thu, Jun 30, 2016 at 03:31:57PM +0200, Geert Uytterhoeven wrote:
> Hi Paul,
>
> On Thu, Jun 30, 2016 at 3:24 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jun 30, 2016 at 09:58:51AM +0200, Geert Uytterhoeven wrote:
> >> On Thu, Jun 30, 2016 at 9:47 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> >> > 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:
> >> >> > 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)
> >> > {
> >>
> >> Thanks for your patch!
> >>
> >> While this does move jiffies clocksource initialization before secondary CPU
> >> bringup, it still hangs when calling call_rcu() or synchronize_sched():
> >>
> >> 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
> >> clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff,
> >> max_idle_ns: 19112604462750000 ns
> >> 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: rcu_sched GP kthread: c784e1c0 state: 1 flags: 0x0 g:-300 c:-300
> >> jiffies: 0xffff8ad0 GP start: 0x0 Last GP activity: 0x0
> >> rcu_node tree layout dump
> >> 0:1/0x0/0x3 ^0
> >
> > This is in fact the initial state for RCU grace periods. In other words,
> > all the earlier calls to synchronize_sched() likely happened while there
> > was only one CPU online.
> >
> >> devtmpfs: initialized
> >> VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1
> >
> > Could you please add the call_rcu() and timed delay as described in my
> > earlier email? That would hopefully help me see the state of the stalled
> > grace period.
>
> I already did, cfr. "it still hangs when calling call_rcu() or
> synchronize_sched()".
Ah, sorry for my inattention.
I am a bit surprised that it could hang when calling call_rcu(), given
that call_rcu() is callable from atomic contexts. Could you please show
me the current test code you have?
If the hang is in call_rcu(), could you please try disabling irqs across
the call to call_rcu()?
Thanx, Paul
