[PATCH -next 3/8] rcu/nocb: Remove unnecessary WakeOvfIsDeferred wake path

[Joel Fernandes <joelagnelf@nvidia.com>]

The WakeOvfIsDeferred code path in __call_rcu_nocb_wake() attempts to
wake rcuog when the callback count exceeds qhimark and callbacks aren't
done with their GP (newly queued or awaiting GP). However, a lot of
testing proves this wake is always redundant or useless.

In the flooding case, rcuog is always waiting for a GP to finish. So
waking up the rcuog thread is pointless. The timer wakeup adds overhead,
rcuog simply wakes up and goes back to sleep achieving nothing.

This path also adds a full memory barrier, and additional timer expiry
modifications unnecessarily.

The root cause is that WakeOvfIsDeferred fires when
!rcu_segcblist_ready_cbs() (GP not complete), but waking rcuog cannot
accelerate GP completion.

This commit therefore removes this path.

Tested with rcutorture scenarios: TREE01, TREE05, TREE08 (all NOCB
configurations) - all pass. Also stress tested using a kernel module
that floods call_rcu() to trigger the overload conditions and made the
observations confirming the findings.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree.h      |  1 -
 kernel/rcu/tree_nocb.h | 35 +++++++++--------------------------
 2 files changed, 9 insertions(+), 27 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 2265b9c2906e..653fb4ba5852 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -301,7 +301,6 @@ struct rcu_data {
 #define RCU_NOCB_WAKE_BYPASS	1
 #define RCU_NOCB_WAKE_LAZY	2
 #define RCU_NOCB_WAKE		3
-#define RCU_NOCB_WAKE_FORCE	4
 
 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
 					/* For jiffies_till_first_fqs and */
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e6cd56603cad..daff2756cd90 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -518,10 +518,8 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 }
 
 /*
- * Awaken the no-CBs grace-period kthread if needed, either due to it
- * legitimately being asleep or due to overload conditions.
- *
- * If warranted, also wake up the kthread servicing this CPUs queues.
+ * Awaken the no-CBs grace-period kthread if needed due to it legitimately
+ * being asleep.
  */
 static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 				 unsigned long flags)
@@ -533,7 +531,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	long lazy_len;
 	long len;
 	struct task_struct *t;
-	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
 
 	// If we are being polled or there is no kthread, just leave.
 	t = READ_ONCE(rdp->nocb_gp_kthread);
@@ -549,22 +546,22 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	lazy_len = READ_ONCE(rdp->lazy_len);
 	if (was_alldone) {
 		rdp->qlen_last_fqs_check = len;
+		rcu_nocb_unlock(rdp);
 		// Only lazy CBs in bypass list
 		if (lazy_len && bypass_len == lazy_len) {
-			rcu_nocb_unlock(rdp);
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
 					   TPS("WakeLazy"));
 		} else if (!irqs_disabled_flags(flags)) {
 			/* ... if queue was empty ... */
-			rcu_nocb_unlock(rdp);
 			wake_nocb_gp(rdp, false);
 			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
 					    TPS("WakeEmpty"));
 		} else {
-			rcu_nocb_unlock(rdp);
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
 					   TPS("WakeEmptyIsDeferred"));
 		}
+
+		return;
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
 		/* ... or if many callbacks queued. */
 		rdp->qlen_last_fqs_check = len;
@@ -575,21 +572,10 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 			rcu_advance_cbs_nowake(rdp->mynode, rdp);
 			rdp->nocb_gp_adv_time = j;
 		}
-		smp_mb(); /* Enqueue before timer_pending(). */
-		if ((rdp->nocb_cb_sleep ||
-		     !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
-		    !timer_pending(&rdp_gp->nocb_timer)) {
-			rcu_nocb_unlock(rdp);
-			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
-					   TPS("WakeOvfIsDeferred"));
-		} else {
-			rcu_nocb_unlock(rdp);
-			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
-		}
-	} else {
-		rcu_nocb_unlock(rdp);
-		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
 	}
+
+	rcu_nocb_unlock(rdp);
+	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
 }
 
 static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head,
@@ -966,7 +952,6 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp_gp,
 					   unsigned long flags)
 	__releases(rdp_gp->nocb_gp_lock)
 {
-	int ndw;
 	int ret;
 
 	if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) {
@@ -974,8 +959,7 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp_gp,
 		return false;
 	}
 
-	ndw = rdp_gp->nocb_defer_wakeup;
-	ret = __wake_nocb_gp(rdp_gp, rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
+	ret = __wake_nocb_gp(rdp_gp, rdp, false, flags);
 	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));
 
 	return ret;
@@ -991,7 +975,6 @@ static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
 	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
 
 	raw_spin_lock_irqsave(&rdp->nocb_gp_lock, flags);
-	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
 	do_nocb_deferred_wakeup_common(rdp, rdp, RCU_NOCB_WAKE_BYPASS, flags);
 }
 
-- 
2.34.1