[PATCH -next v4 2/3] rcu/nocb: Remove dead callback overload handling

[Joel Fernandes <joelagnelf@nvidia.com>]

During callback overload (exceeding qhimark), the NOCB code attempts
opportunistic advancement via rcu_advance_cbs_nowake(). Analysis shows
this code path is practically unreachable and serves no useful purpose.

Testing with 300,000 callback floods showed:
- 30 overload conditions triggered
- 0 advancements actually occurred

While a theoretical window exists where this code could execute (e.g.,
vCPU preemption between gp_seq update and rcu_nocb_gp_cleanup()), even
if it did, the advancement would be redundant. The rcuog kthread must
still run to wake the rcuoc callback thread - we would just be
duplicating work that rcuog will perform when it finally gets to run.

Since this path provides no meaningful benefit and extensive testing
confirms it is never useful, remove it entirely.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree_nocb.h | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index f525e4f7985b..64a8ff350f92 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -526,8 +526,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 				 __releases(rdp->nocb_lock)
 {
 	long bypass_len;
-	unsigned long cur_gp_seq;
-	unsigned long j;
 	long lazy_len;
 	long len;
 	struct task_struct *t;
@@ -562,16 +560,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 		}
 
 		return;
-	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
-		/* ... or if many callbacks queued. */
-		rdp->qlen_last_fqs_check = len;
-		j = jiffies;
-		if (j != rdp->nocb_gp_adv_time &&
-		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
-		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
-			rcu_advance_cbs_nowake(rdp->mynode, rdp);
-			rdp->nocb_gp_adv_time = j;
-		}
 	}
 
 	rcu_nocb_unlock(rdp);
-- 
2.34.1