From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
"Paul E. McKenney" <paulmck@linux.ibm.com>
Subject: [PATCH tip/core/rcu 07/10] rcu/nocb: Reduce contention at no-CBs registry-time CB advancement
Date: Thu, 1 Aug 2019 16:16:32 -0700 [thread overview]
Message-ID: <20190801231636.23115-7-paulmck@linux.ibm.com> (raw)
In-Reply-To: <20190801231619.GA22610@linux.ibm.com>
Currently, __call_rcu_nocb_wake() conditionally acquires the leaf rcu_node
structure's ->lock, and only afterwards does rcu_advance_cbs_nowake()
check to see if it is possible to advance callbacks without potentially
needing to awaken the grace-period kthread. Given that the no-awaken
check can be done locklessly, this commit reverses the order, so that
rcu_advance_cbs_nowake() is invoked without holding the leaf rcu_node
structure's ->lock and rcu_advance_cbs_nowake() checks the grace-period
state before conditionally acquiring that lock, thus reducing the number
of needless acquistions of the leaf rcu_node structure's ->lock.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
kernel/rcu/tree.c | 5 +++--
kernel/rcu/tree_plugin.h | 4 +---
2 files changed, 4 insertions(+), 5 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a6ddfae6978d..ec320658aeef 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1341,10 +1341,11 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
struct rcu_data *rdp)
{
- raw_lockdep_assert_held_rcu_node(rnp);
- if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
+ if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||
+ !raw_spin_trylock_rcu_node(rnp))
return;
WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
+ raw_spin_unlock_rcu_node(rnp);
}
/*
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 06b4fe275b3a..a1a2fc9df6d8 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1666,10 +1666,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
if (!rdp->nocb_cb_sleep &&
rcu_segcblist_ready_cbs(&rdp->cblist)) {
// Already going full tilt, so don't try to rewake.
- } else if (rcu_segcblist_pend_cbs(&rdp->cblist) &&
- raw_spin_trylock_rcu_node(rdp->mynode)) {
+ } else if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
rcu_advance_cbs_nowake(rdp->mynode, rdp);
- raw_spin_unlock_rcu_node(rdp->mynode);
} else {
wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
TPS("WakeOvfIsDeferred"));
--
2.17.1
next prev parent reply other threads:[~2019-08-01 23:17 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 23:16 [PATCH tip/core/rcu 0/10] No-CBs contention-reduction updates for v5.3-rc2 Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 01/10] rcu/nocb: Enable re-awakening under high callback load Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 02/10] rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp_defer() Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 03/10] rcu/nocb: Make __call_rcu_nocb_wake() safe for many callbacks Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 04/10] rcu/nocb: Avoid needless wakeups of no-CBs grace-period kthread Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 05/10] rcu/nocb: Avoid ->nocb_lock capture by corresponding CPU Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 06/10] rcu/nocb: Round down for number of no-CBs grace-period kthreads Paul E. McKenney
2019-08-01 23:16 ` Paul E. McKenney [this message]
2019-08-01 23:16 ` [PATCH tip/core/rcu 08/10] rcu/nocb: Reduce contention at no-CBs invocation-done time Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 09/10] rcu/nocb: Reduce ->nocb_lock contention with separate ->nocb_gp_lock Paul E. McKenney
2019-08-01 23:16 ` [PATCH tip/core/rcu 10/10] rcu/nocb: Unconditionally advance and wake for excessive CBs Paul E. McKenney
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