linux-kernel - [PATCH tip/core/rcu 01/10] rcu/nocb: Enable re-awakening under high callback load

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Date:   Thu,  1 Aug 2019 16:16:26 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 01/10] rcu/nocb: Enable re-awakening under high callback load

The __call_rcu_nocb_wake() function and its predecessors set
->qlen_last_fqs_check to zero for the first callback and to LONG_MAX / 2
for forced reawakenings.  The former can result in a too-quick reawakening
when there are many callbacks ready to invoke and the latter prevents a
second reawakening.  This commit therefore sets ->qlen_last_fqs_check
to the current number of callbacks in both cases.  While in the area,
this commit also moves both assignments under ->nocb_lock.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c1dfbac8cd39..297b38732e28 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1626,6 +1626,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	// Need to actually to a wakeup.
 	len = rcu_segcblist_n_cbs(&rdp->cblist);
 	if (was_alldone) {
+		rdp->qlen_last_fqs_check = len;
 		if (!irqs_disabled_flags(flags)) {
 			/* ... if queue was empty ... */
 			wake_nocb_gp(rdp, false, flags);
@@ -1636,9 +1637,9 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 					   TPS("WakeEmptyIsDeferred"));
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 		}
-		rdp->qlen_last_fqs_check = 0;
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
 		/* ... or if many callbacks queued. */
+		rdp->qlen_last_fqs_check = len;
 		if (!irqs_disabled_flags(flags)) {
 			wake_nocb_gp(rdp, true, flags);
 			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
@@ -1648,7 +1649,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 					   TPS("WakeOvfIsDeferred"));
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 		}
-		rdp->qlen_last_fqs_check = LONG_MAX / 2;
 	} else {
 		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
 		rcu_nocb_unlock_irqrestore(rdp, flags);
-- 
2.17.1