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Commit 9261dd0d authored by Paul E. McKenney's avatar Paul E. McKenney
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rcu: Add tracing for rcuo no-CBs CPU wakeup handshake 



Lost wakeups from call_rcu() to the rcuo kthreads can result in hangs
that are difficult to diagnose.  This commit therefore adds tracing to
help pin down the cause of these hangs.

Reported-by: default avatarClark Williams <williams@redhat.com>
Reported-by: default avatarCarsten Emde <C.Emde@osadl.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add const per kbuild test robot's advice. ]
parent bb311ecc

include/trace/events/rcu.h

+37 −0
Original line number Diff line number Diff line
@@ -171,6 +171,42 @@ TRACE_EVENT(rcu_grace_period_init, 
		  __entry->grplo, __entry->grphi, __entry->qsmask)

);



/*

 * Tracepoint for RCU no-CBs CPU callback handoffs.  This event is intended

 * to assist debugging of these handoffs.

 *

 * The first argument is the name of the RCU flavor, and the second is

 * the number of the offloaded CPU are extracted.  The third and final

 * argument is a string as follows:

 *

 *	"WakeEmpty": Wake rcuo kthread, first CB to empty list.

 *	"WakeOvf": Wake rcuo kthread, CB list is huge.

 *	"WakeNot": Don't wake rcuo kthread.

 *	"WakeNotPoll": Don't wake rcuo kthread because it is polling.

 *	"WokeEmpty": rcuo kthread woke to find empty list.

 *	"WokeNonEmpty": rcuo kthread woke to find non-empty list.

 */

TRACE_EVENT(rcu_nocb_wake,



	TP_PROTO(const char *rcuname, int cpu, const char *reason),



	TP_ARGS(rcuname, cpu, reason),



	TP_STRUCT__entry(

		__field(const char *, rcuname)

		__field(int, cpu)

		__field(const char *, reason)

	),



	TP_fast_assign(

		__entry->rcuname = rcuname;

		__entry->cpu = cpu;

		__entry->reason = reason;

	),



	TP_printk("%s %d %s", __entry->rcuname, __entry->cpu, __entry->reason)

);



/*

 * Tracepoint for tasks blocking within preemptible-RCU read-side

 * critical sections.  Track the type of RCU (which one day might
@@ -667,6 +703,7 @@ TRACE_EVENT(rcu_barrier, 
#define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \

				      level, grplo, grphi, event) \

				      do { } while (0)

#define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)

#define trace_rcu_preempt_task(rcuname, pid, gpnum) do { } while (0)

#define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)

#define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, \

kernel/rcutree_plugin.h

+13 −1
Original line number Diff line number Diff line
@@ -2108,15 +2108,22 @@ static void __call_rcu_nocb_enqueue(struct rcu_data *rdp, 


	/* If we are not being polled and there is a kthread, awaken it ... */

	t = ACCESS_ONCE(rdp->nocb_kthread);

	if (rcu_nocb_poll | !t)

	if (rcu_nocb_poll | !t) {

		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,

				    TPS("WakeNotPoll"));

		return;

	}

	len = atomic_long_read(&rdp->nocb_q_count);

	if (old_rhpp == &rdp->nocb_head) {

		wake_up(&rdp->nocb_wq); /* ... only if queue was empty ... */

		rdp->qlen_last_fqs_check = 0;

		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeEmpty"));

	} else if (len > rdp->qlen_last_fqs_check + qhimark) {

		wake_up_process(t); /* ... or if many callbacks queued. */

		rdp->qlen_last_fqs_check = LONG_MAX / 2;

		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeOvf"));

	} else {

		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeNot"));

	}

	return;

}
@@ -2233,10 +2240,15 @@ static int rcu_nocb_kthread(void *arg) 
			wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);

		list = ACCESS_ONCE(rdp->nocb_head);

		if (!list) {

			if (!rcu_nocb_poll)

				trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,

						    TPS("WokeEmpty"));

			schedule_timeout_interruptible(1);

			flush_signals(current);

			continue;

		}

		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,

				    TPS("WokeNonEmpty"));



		/*

		 * Extract queued callbacks, update counts, and wait