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rcu/nocb: Avoid ->nocb_lock capture by corresponding CPU

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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@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-05-28 07:18:08 -07:00
parent 7f36ef82e5
commit 81c0b3d724
2 changed files with 62 additions and 24 deletions
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18
kernel/rcu/tree.h
View File

@ -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);

68
kernel/rcu/tree_plugin.h
View File

@ -1498,14 +1498,36 @@ 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();
}
/*
@ -1575,19 +1597,19 @@ 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);
}
}
@ -1646,23 +1668,23 @@ 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;
}
@ -1692,7 +1714,7 @@ 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. */
@ -1712,7 +1734,7 @@ 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;
@ -1741,9 +1763,9 @@ 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));
}
@ -1784,12 +1806,12 @@ 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;
@ -1797,7 +1819,7 @@ 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,
@ -1839,9 +1861,9 @@ 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);