Re: rcu_preempt caused oom

[Paul E. McKenney]

On Wed, Dec 12, 2018 at 01:37:40AM +0000, He, Bo wrote:
> We reproduced the issue panic in hung_task with the patch "Improve diagnostics for failed RCU grace-period start", but unfortunately maybe it's due to the loglevel, the show_rcu_gp_kthreads doesn't print any logs, we will improve the build and run the test to double check.

Well, at least the diagnostics didn't prevent the problem from happening.  ;-)

							Thanx, Paul

> -----Original Message-----
> From: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> 
> Sent: Tuesday, December 11, 2018 12:47 PM
> To: He, Bo <bo.he@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; josh@xxxxxxxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx; jiangshanlai@xxxxxxxxx; Zhang, Jun <jun.zhang@xxxxxxxxx>; Xiao, Jin <jin.xiao@xxxxxxxxx>; Zhang, Yanmin <yanmin.zhang@xxxxxxxxx>; Bai, Jie A <jie.a.bai@xxxxxxxxx>
> Subject: Re: rcu_preempt caused oom
> 
> On Mon, Dec 10, 2018 at 04:38:38PM -0800, Paul E. McKenney wrote:
> > On Mon, Dec 10, 2018 at 06:56:18AM +0000, He, Bo wrote:
> > > Hi, 
> > >    We have start the test with the CONFIG_PROVE_RCU=y, and also add one 2s to detect the preempt rcu hang, hope we can get more useful logs tomorrow.
> > >    I also enclosed the config and the debug patches for you review.
> > 
> > I instead suggest the (lightly tested) debug patch shown below, which 
> > tracks wakeups of RCU's grace-period kthreads and dumps them out if a 
> > given requested grace period fails to start.  Again, it is necessary 
> > to build with CONFIG_PROVE_RCU=y, that is, with CONFIG_PROVE_LOCKING=y.
> 
> Right.  This time without commenting out the wakeup as a test of the diagnostic.  :-/
> 
> Please use the patch below instead of the one that I sent in my previous email.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit adfc7dff659495a3433d5084256be59eee0ac6df
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> Date:   Mon Dec 10 16:33:59 2018 -0800
> 
>     rcu: Improve diagnostics for failed RCU grace-period start
>     
>     Backported from v4.21/v5.0
>     
>     If a grace period fails to start (for example, because you commented
>     out the last two lines of rcu_accelerate_cbs_unlocked()), rcu_core()
>     will invoke rcu_check_gp_start_stall(), which will notice and complain.
>     However, this complaint is lacking crucial debugging information such
>     as when the last wakeup executed and what the value of ->gp_seq was at
>     that time.  This commit therefore removes the current pr_alert() from
>     rcu_check_gp_start_stall(), instead invoking show_rcu_gp_kthreads(),
>     which has been updated to print the needed information, which is collected
>     by rcu_gp_kthread_wake().
>     
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0b760c1369f7..4bcd8753e293 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -626,25 +626,57 @@ void rcu_sched_force_quiescent_state(void)
>  }
>  EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);
>  
> +/*
> + * Convert a ->gp_state value to a character string.
> + */
> +static const char *gp_state_getname(short gs) {
> +	if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
> +		return "???";
> +	return gp_state_names[gs];
> +}
> +
> +/*
> + * Return the root node of the specified rcu_state structure.
> + */
> +static struct rcu_node *rcu_get_root(struct rcu_state *rsp) {
> +	return &rsp->node[0];
> +}
> +
>  /*
>   * Show the state of the grace-period kthreads.
>   */
>  void show_rcu_gp_kthreads(void)
>  {
>  	int cpu;
> +	unsigned long j;
> +	unsigned long ja;
> +	unsigned long jr;
> +	unsigned long jw;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp;
>  	struct rcu_state *rsp;
>  
> +	j = jiffies;
>  	for_each_rcu_flavor(rsp) {
> -		pr_info("%s: wait state: %d ->state: %#lx\n",
> -			rsp->name, rsp->gp_state, rsp->gp_kthread->state);
> +		ja = j - READ_ONCE(rsp->gp_activity);
> +		jr = j - READ_ONCE(rsp->gp_req_activity);
> +		jw = j - READ_ONCE(rsp->gp_wake_time);
> +		pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
> +			rsp->name, gp_state_getname(rsp->gp_state),
> +			rsp->gp_state,
> +			rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL,
> +			ja, jr, jw, (long)READ_ONCE(rsp->gp_wake_seq),
> +			(long)READ_ONCE(rsp->gp_seq),
> +			(long)READ_ONCE(rcu_get_root(rsp)->gp_seq_needed),
> +			READ_ONCE(rsp->gp_flags));
>  		rcu_for_each_node_breadth_first(rsp, rnp) {
>  			if (ULONG_CMP_GE(rsp->gp_seq, rnp->gp_seq_needed))
>  				continue;
> -			pr_info("\trcu_node %d:%d ->gp_seq %lu ->gp_seq_needed %lu\n",
> -				rnp->grplo, rnp->grphi, rnp->gp_seq,
> -				rnp->gp_seq_needed);
> +			pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
> +				rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
> +				(long)rnp->gp_seq_needed);
>  			if (!rcu_is_leaf_node(rnp))
>  				continue;
>  			for_each_leaf_node_possible_cpu(rnp, cpu) { @@ -653,8 +685,8 @@ void show_rcu_gp_kthreads(void)
>  				    ULONG_CMP_GE(rsp->gp_seq,
>  						 rdp->gp_seq_needed))
>  					continue;
> -				pr_info("\tcpu %d ->gp_seq_needed %lu\n",
> -					cpu, rdp->gp_seq_needed);
> +				pr_info("\tcpu %d ->gp_seq_needed %ld\n",
> +					cpu, (long)rdp->gp_seq_needed);
>  			}
>  		}
>  		/* sched_show_task(rsp->gp_kthread); */ @@ -690,14 +722,6 @@ void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,  }  EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
>  
> -/*
> - * Return the root node of the specified rcu_state structure.
> - */
> -static struct rcu_node *rcu_get_root(struct rcu_state *rsp) -{
> -	return &rsp->node[0];
> -}
> -
>  /*
>   * Enter an RCU extended quiescent state, which can be either the
>   * idle loop or adaptive-tickless usermode execution.
> @@ -1285,16 +1309,6 @@ static void record_gp_stall_check_time(struct rcu_state *rsp)
>  	rsp->n_force_qs_gpstart = READ_ONCE(rsp->n_force_qs);  }
>  
> -/*
> - * Convert a ->gp_state value to a character string.
> - */
> -static const char *gp_state_getname(short gs) -{
> -	if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
> -		return "???";
> -	return gp_state_names[gs];
> -}
> -
>  /*
>   * Complain about starvation of grace-period kthread.
>   */
> @@ -1693,7 +1707,8 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
>   * Don't do a self-awaken, and don't bother awakening when there is
>   * nothing for the grace-period kthread to do (as in several CPUs
>   * raced to awaken, and we lost), and finally don't try to awaken
> - * a kthread that has not yet been created.
> + * a kthread that has not yet been created.  If all those checks are
> + * passed, track some debug information and awaken.
>   */
>  static void rcu_gp_kthread_wake(struct rcu_state *rsp)  { @@ -1701,6 +1716,8 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp)
>  	    !READ_ONCE(rsp->gp_flags) ||
>  	    !rsp->gp_kthread)
>  		return;
> +	WRITE_ONCE(rsp->gp_wake_time, jiffies);
> +	WRITE_ONCE(rsp->gp_wake_seq, READ_ONCE(rsp->gp_seq));
>  	swake_up_one(&rsp->gp_wq);
>  }
>  
> @@ -2802,16 +2819,11 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  		return;
>  	}
> -	pr_alert("%s: g%ld->%ld gar:%lu ga:%lu f%#x gs:%d %s->state:%#lx\n",
> -		 __func__, (long)READ_ONCE(rsp->gp_seq),
> -		 (long)READ_ONCE(rnp_root->gp_seq_needed),
> -		 j - rsp->gp_req_activity, j - rsp->gp_activity,
> -		 rsp->gp_flags, rsp->gp_state, rsp->name,
> -		 rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
>  	WARN_ON(1);
>  	if (rnp_root != rnp)
>  		raw_spin_unlock_rcu_node(rnp_root);
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +	show_rcu_gp_kthreads();
>  }
>  
>  /*
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 4e74df768c57..0e051d9b5f1a 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -327,6 +327,8 @@ struct rcu_state {
>  	struct swait_queue_head gp_wq;		/* Where GP task waits. */
>  	short gp_flags;				/* Commands for GP task. */
>  	short gp_state;				/* GP kthread sleep state. */
> +	unsigned long gp_wake_time;		/* Last GP kthread wake. */
> +	unsigned long gp_wake_seq;		/* ->gp_seq at ^^^. */
>  
>  	/* End of fields guarded by root rcu_node's lock. */
>  
>