Subject: Boot failure on emev2/kzm9d (was: Re: [PATCH v2 11/11] mm/slab: lockless decision to grow cache)

From: [email-masked] (Paul E. McKenney)
Date: Thu, 30 Jun 2016 09:52:48 -0700

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-masked]> 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-masked]> 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-masked]> 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-masked]> 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