On Thu, 30 Jun 2016, Paul E. McKenney wrote:
> On Thu, Jun 30, 2016 at 03:31:57PM +0200, Geert Uytterhoeven wrote:
> > 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()?
These are my local changes:
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c7f1bc4f817c4a34..50bea263e510006f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4707,11 +4707,16 @@ static void __init rcu_init_geometry(void)
* Dump out the structure of the rcu_node combining tree associated
* with the rcu_state structure referenced by rsp.
*/
-static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp)
+static void rcu_dump_rcu_node_tree(struct rcu_state *rsp)
{
int level = 0;
struct rcu_node *rnp;
+ pr_info("RCU: %s GP kthread: %p state: %d flags: %#x g:%ld c:%ld\n",
+ rsp->name, rsp->gp_kthread, rsp->gp_state, rsp->gp_flags,
+ (long)rsp->gpnum, (long)rsp->completed);
+ pr_info(" jiffies: %#lx GP start: %#lx Last GP activity: %#lx\n",
+ jiffies, rsp->gp_start, rsp->gp_activity);
pr_info("rcu_node tree layout dump\n");
pr_info(" ");
rcu_for_each_node_breadth_first(rsp, rnp) {
@@ -4720,11 +4725,32 @@ 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");
}
+static void do_nothing_cb(struct rcu_head *rcu_head)
+{
+}
+
+void rcu_dump_rcu_sched_tree(void)
+{
+ struct rcu_head rh;
+ unsigned long flags;
+
+ rcu_dump_rcu_node_tree(&rcu_sched_state); /* Initial state. */
+ local_irq_save(flags);
+ // call_rcu(&rh, do_nothing_cb);
+ local_irq_restore(flags);
+ // 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. */
+}
+
void __init rcu_init(void)
{
int cpu;
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index 555e21f7b966c789..4f6471f54f69a6fe 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)
{
diff --git a/mm/slab.c b/mm/slab.c
index cc8bbc1e6bc9b6fe..f9b2f50adc705173 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -909,6 +909,8 @@ static int init_cache_node_node(int node)
return 0;
}
+extern void rcu_dump_rcu_sched_tree(void);
+
static int setup_kmem_cache_node(struct kmem_cache *cachep,
int node, gfp_t gfp, bool force_change)
{
@@ -964,8 +966,19 @@ static int setup_kmem_cache_node(struct kmem_cache *cachep,
* guaranteed to be valid until irq is re-enabled, because it will be
* freed after synchronize_sched().
*/
- if (force_change)
- synchronize_sched();
+ if (force_change) {
+ static int cnt;
+
+ if (++cnt < 37) {
+printk("cnt = %d, sync\n", cnt);
+ synchronize_sched();
+ } else if (cnt == 37) {
+printk("cnt = %d, dump\n", cnt);
+ rcu_dump_rcu_sched_tree();
+ } else {
+printk("cnt = %d\n", cnt);
+ }
+ }
fail:
kfree(old_shared);
With this it boots fine:
...
cnt = 35, sync
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.
cnt = 37, dump
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
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
devtmpfs: initialized
VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 1
cnt = 38
cnt = 39
...
When enabling any of the 4 commented-out lines in rcu_dump_rcu_sched_tree(),
it will lock up.
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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