[PATCH -next v3 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 entire code path is dead:

- 30 overload conditions triggered with 300,000 callback flood
- 0 advancements actually occurred
- 100% of time blocked because current GP not done

The overload condition triggers when callbacks are coming in at a high
rate with GPs not completing as fast. But the advancement requires the
GP to be complete - a logical contradiction. Even if the GP did complete
in time, nocb_gp_wait() has to wake up anyway to do the advancement, so
it is pointless.

Since the advancement is dead code, the entire overload handling block
serves no purpose. Remove it entirely.

Suggested-by: Frederic Weisbecker <frederic@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