On Thu, Jun 30, 2016 at 05:53:42PM +0200, Geert Uytterhoeven wrote:
> 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
> > > <[email protected]> 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 <[email protected]>
> > > >> 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
> > > >> >> > <[email protected]> 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
> > > >> >> > >> <[email protected]> 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.
OK, but that includes schedule_timeout_uninterruptible(5 * HZ), right?
If schedule_timeout_uninterruptible() hangs, RCU grace periods have no
chance of completing.
I am still surprised that call_rcu() hangs, but timed wait not working
will definitely cause RCU grace-period hangs. Perhaps fixing timed
waits will also prevent the call_rcu() hang.
Thanx, Paul
