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Commit fd897573 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Improve diagnostics for failed RCU grace-period start 



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: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
parent a9fefdb2

kernel/rcu/tree.c

+32 −23
Original line number Diff line number Diff line
@@ -512,6 +512,14 @@ void rcu_force_quiescent_state(void) 
}

EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);



/*

 * Return the root node of the rcu_state structure.

 */

static struct rcu_node *rcu_get_root(void)

{

	return &rcu_state.node[0];

}



/*

 * Convert a ->gp_state value to a character string.

 */
@@ -529,19 +537,30 @@ 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;



	j = jiffies - READ_ONCE(rcu_state.gp_activity);

	pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %ld\n",

	j = jiffies;

	ja = j - READ_ONCE(rcu_state.gp_activity);

	jr = j - READ_ONCE(rcu_state.gp_req_activity);

	jw = j - READ_ONCE(rcu_state.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",

		rcu_state.name, gp_state_getname(rcu_state.gp_state),

		rcu_state.gp_state, rcu_state.gp_kthread->state, j);

		rcu_state.gp_state,

		rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,

		ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),

		(long)READ_ONCE(rcu_state.gp_seq),

		(long)READ_ONCE(rcu_get_root()->gp_seq_needed),

		READ_ONCE(rcu_state.gp_flags));

	rcu_for_each_node_breadth_first(rnp) {

		if (ULONG_CMP_GE(rcu_state.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) {
@@ -550,8 +569,8 @@ void show_rcu_gp_kthreads(void) 
			    ULONG_CMP_GE(rcu_state.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(rcu_state.gp_kthread); */
@@ -577,14 +596,6 @@ 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 rcu_state structure.

 */

static struct rcu_node *rcu_get_root(void)

{

	return &rcu_state.node[0];

}



/*

 * Enter an RCU extended quiescent state, which can be either the

 * idle loop or adaptive-tickless usermode execution.
@@ -1560,7 +1571,8 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp) 
 * Awaken the grace-period kthread.  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.

 * don't try to awaken 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(void)

{
@@ -1568,6 +1580,8 @@ static void rcu_gp_kthread_wake(void) 
	    !READ_ONCE(rcu_state.gp_flags) ||

	    !rcu_state.gp_kthread)

		return;

	WRITE_ONCE(rcu_state.gp_wake_time, jiffies);

	WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));

	swake_up_one(&rcu_state.gp_wq);

}
@@ -2657,16 +2671,11 @@ rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp, 
		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(rcu_state.gp_seq),

		 (long)READ_ONCE(rnp_root->gp_seq_needed),

		 j - rcu_state.gp_req_activity, j - rcu_state.gp_activity,

		 rcu_state.gp_flags, rcu_state.gp_state, rcu_state.name,

		 rcu_state.gp_kthread ? rcu_state.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();

}



/*

kernel/rcu/tree.h

+2 −0
Original line number Diff line number Diff line
@@ -309,6 +309,8 @@ 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. */