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Date: Thu, 1 Aug 2019 16:16:30 -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 05/10] rcu/nocb: Avoid ->nocb_lock capture by corresponding CPU
A given rcu_data structure's ->nocb_lock can be acquired very frequently
by the corresponding CPU and occasionally by the corresponding no-CBs
grace-period and callbacks kthreads. In particular, these two kthreads
will have frequent gaps between ->nocb_lock acquisitions that are roughly
a grace period in duration. This means that any excessive ->nocb_lock
contention will be due to the CPU's acquisitions, and this in turn
enables a very naive contention-avoidance strategy to be quite effective.
This commit therefore modifies rcu_nocb_lock() to first
attempt a raw_spin_trylock(), and to atomically increment a
separate ->nocb_lock_contended across a raw_spin_lock(). This new
->nocb_lock_contended field is checked in __call_rcu_nocb_wake() when
interrupts are enabled, with a spin-wait for contending acquisitions
to complete, thus allowing the kthreads a chance to acquire the lock.
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
kernel/rcu/tree.h | 18 ++++++++++-
kernel/rcu/tree_plugin.h | 68 ++++++++++++++++++++++++++--------------
2 files changed, 62 insertions(+), 24 deletions(-)
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index c12e85c12310..7062f9d9c053 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -197,6 +197,7 @@ struct rcu_data {
struct swait_queue_head nocb_cb_wq; /* For nocb kthreads to sleep on. */
struct task_struct *nocb_gp_kthread;
raw_spinlock_t nocb_lock; /* Guard following pair of fields. */
+ atomic_t nocb_lock_contended; /* Contention experienced. */
int nocb_defer_wakeup; /* Defer wakeup of nocb_kthread. */
struct timer_list nocb_timer; /* Enforce finite deferral. */
@@ -430,7 +431,22 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
unsigned long flags);
#ifdef CONFIG_RCU_NOCB_CPU
static void __init rcu_organize_nocb_kthreads(void);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
+#define rcu_nocb_lock_irqsave(rdp, flags) \
+do { \
+ if (!rcu_segcblist_is_offloaded(&(rdp)->cblist)) { \
+ local_irq_save(flags); \
+ } else if (!raw_spin_trylock_irqsave(&(rdp)->nocb_lock, (flags))) {\
+ atomic_inc(&(rdp)->nocb_lock_contended); \
+ smp_mb__after_atomic(); /* atomic_inc() before lock. */ \
+ raw_spin_lock_irqsave(&(rdp)->nocb_lock, (flags)); \
+ smp_mb__before_atomic(); /* atomic_dec() after lock. */ \
+ atomic_dec(&(rdp)->nocb_lock_contended); \
+ } \
+} while (0)
+#else /* #ifdef CONFIG_RCU_NOCB_CPU */
+#define rcu_nocb_lock_irqsave(rdp, flags) local_irq_save(flags)
+#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
+
static void rcu_bind_gp_kthread(void);
static bool rcu_nohz_full_cpu(void);
static void rcu_dynticks_task_enter(void);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bda86098ca38..b6d9ed169edc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1496,14 +1496,36 @@ early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
/*
* Acquire the specified rcu_data structure's ->nocb_lock, but only
- * if it corresponds to a no-CBs CPU.
+ * if it corresponds to a no-CBs CPU. If the lock isn't immediately
+ * available, increment ->nocb_lock_contended to flag the contention.
*/
static void rcu_nocb_lock(struct rcu_data *rdp)
{
- if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
- lockdep_assert_irqs_disabled();
- raw_spin_lock(&rdp->nocb_lock);
- }
+ lockdep_assert_irqs_disabled();
+ if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||
+ raw_spin_trylock(&rdp->nocb_lock))
+ return;
+ atomic_inc(&rdp->nocb_lock_contended);
+ smp_mb__after_atomic(); /* atomic_inc() before lock. */
+ raw_spin_lock(&rdp->nocb_lock);
+ smp_mb__before_atomic(); /* atomic_dec() after lock. */
+ atomic_dec(&rdp->nocb_lock_contended);
+}
+
+/*
+ * Spinwait until the specified rcu_data structure's ->nocb_lock is
+ * not contended. Please note that this is extremely special-purpose,
+ * relying on the fact that at most two kthreads and one CPU contend for
+ * this lock, and also that the two kthreads are guaranteed to have frequent
+ * grace-period-duration time intervals between successive acquisitions
+ * of the lock. This allows us to use an extremely simple throttling
+ * mechanism, and further to apply it only to the CPU doing floods of
+ * call_rcu() invocations. Don't try this at home!
+ */
+static void rcu_nocb_wait_contended(struct rcu_data *rdp)
+{
+ while (atomic_read(&rdp->nocb_lock_contended))
+ cpu_relax();
}
/*
@@ -1573,19 +1595,19 @@ static void wake_nocb_gp(struct rcu_data *rdp, bool force,
lockdep_assert_held(&rdp->nocb_lock);
if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
return;
}
if (READ_ONCE(rdp_gp->nocb_gp_sleep) || force) {
del_timer(&rdp->nocb_timer);
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
smp_mb(); /* enqueue before ->nocb_gp_sleep. */
- raw_spin_lock_irqsave(&rdp_gp->nocb_lock, flags);
+ rcu_nocb_lock_irqsave(rdp_gp, flags);
WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
- raw_spin_unlock_irqrestore(&rdp_gp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp_gp, flags);
wake_up_process(rdp_gp->nocb_gp_kthread);
} else {
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
}
}
@@ -1644,23 +1666,23 @@ 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.
- rcu_nocb_unlock_irqrestore(rdp, flags);
} else if (rcu_segcblist_pend_cbs(&rdp->cblist) &&
raw_spin_trylock_rcu_node(rdp->mynode)) {
rcu_advance_cbs_nowake(rdp->mynode, rdp);
raw_spin_unlock_rcu_node(rdp->mynode);
- rcu_nocb_unlock_irqrestore(rdp, flags);
} else {
wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
TPS("WakeOvfIsDeferred"));
- rcu_nocb_unlock_irqrestore(rdp, flags);
}
+ rcu_nocb_unlock_irqrestore(rdp, flags);
} else {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
rcu_nocb_unlock_irqrestore(rdp, flags);
}
- if (!irqs_disabled_flags(flags))
+ if (!irqs_disabled_flags(flags)) {
lockdep_assert_irqs_enabled();
+ rcu_nocb_wait_contended(rdp);
+ }
return;
}
@@ -1690,7 +1712,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
if (rcu_segcblist_empty(&rdp->cblist))
continue; /* No callbacks here, try next. */
rnp = rdp->mynode;
- raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+ rcu_nocb_lock_irqsave(rdp, flags);
WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
del_timer(&my_rdp->nocb_timer);
raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
@@ -1710,7 +1732,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
} else {
needwake = false;
}
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
if (needwake) {
swake_up_one(&rdp->nocb_cb_wq);
gotcbs = true;
@@ -1739,9 +1761,9 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("EndWait"));
}
if (!rcu_nocb_poll) {
- raw_spin_lock_irqsave(&my_rdp->nocb_lock, flags);
+ rcu_nocb_lock_irqsave(my_rdp, flags);
WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
- raw_spin_unlock_irqrestore(&my_rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(my_rdp, flags);
}
WARN_ON(signal_pending(current));
}
@@ -1782,12 +1804,12 @@ static void nocb_cb_wait(struct rcu_data *rdp)
rcu_do_batch(rdp);
local_bh_enable();
lockdep_assert_irqs_enabled();
- raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+ rcu_nocb_lock_irqsave(rdp, flags);
raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
if (needwake_gp)
rcu_gp_kthread_wake();
return;
@@ -1795,7 +1817,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
WRITE_ONCE(rdp->nocb_cb_sleep, true);
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
if (needwake_gp)
rcu_gp_kthread_wake();
swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
@@ -1837,9 +1859,9 @@ static void do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
unsigned long flags;
int ndw;
- raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+ rcu_nocb_lock_irqsave(rdp, flags);
if (!rcu_nocb_need_deferred_wakeup(rdp)) {
- raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
return;
}
ndw = READ_ONCE(rdp->nocb_defer_wakeup);
--
2.17.1
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