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rcu/nocb: Add bypass callback queueing 

Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs
takes advantage of unrelated grace periods, thus reducing the memory
footprint in the face of floods of call_rcu() invocations.  However,
the ->cblist field is a more-complex rcu_segcblist structure which must
be protected via locking.  Even though there are only three entities
which can acquire this lock (the CPU invoking call_rcu(), the no-CBs
grace-period kthread, and the no-CBs callbacks kthread), the contention
on this lock is excessive under heavy stress.

This commit therefore greatly reduces contention by provisioning
an rcu_cblist structure field named ->nocb_bypass within the
rcu_data structure.  Each no-CBs CPU is permitted only a limited
number of enqueues onto the ->cblist per jiffy, controlled by a new
nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to
about 16 enqueues per millisecond (16 * 1000 / HZ).  When that limit is
exceeded, the CPU instead enqueues onto the new ->nocb_bypass.

The ->nocb_bypass is flushed into the ->cblist every jiffy or when
the number of callbacks on ->nocb_bypass exceeds qhimark, whichever
happens first.  During call_rcu() floods, this flushing is carried out
by the CPU during the course of its call_rcu() invocations.  However,
a CPU could simply stop invoking call_rcu() at any time.  The no-CBs
grace-period kthread therefore carries out less-aggressive flushing
(every few jiffies or when the number of callbacks on ->nocb_bypass
exceeds (2 * qhimark), whichever comes first).  This means that the
no-CBs grace-period kthread cannot be permitted to do unbounded waits
while there are callbacks on ->nocb_bypass.  A ->nocb_bypass_timer is
used to provide the needed wakeups.

[ paulmck: Apply Coverity feedback reported by Colin Ian King. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-07-02 16:03:33 -07:00
parent eda669a6a2
commit d1b222c6be
5 changed files with 396 additions and 42 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
kernel/rcu/rcu_segcblist.c
View File

@ -36,6 +36,36 @@ void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
WRITE_ONCE(rclp->len, rclp->len + 1);
}
/*
* Flush the second rcu_cblist structure onto the first one, obliterating
* any contents of the first. If rhp is non-NULL, enqueue it as the sole
* element of the second rcu_cblist structure, but ensuring that the second
* rcu_cblist structure, if initially non-empty, always appears non-empty
* throughout the process. If rdp is NULL, the second rcu_cblist structure
* is instead initialized to empty.
*/
void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
struct rcu_cblist *srclp,
struct rcu_head *rhp)
{
drclp->head = srclp->head;
if (drclp->head)
drclp->tail = srclp->tail;
else
drclp->tail = &drclp->head;
drclp->len = srclp->len;
drclp->len_lazy = srclp->len_lazy;
if (!rhp) {
rcu_cblist_init(srclp);
} else {
rhp->next = NULL;
srclp->head = rhp;
srclp->tail = &rhp->next;
WRITE_ONCE(srclp->len, 1);
srclp->len_lazy = 0;
}
}
/*
* Dequeue the oldest rcu_head structure from the specified callback
* list. This function assumes that the callback is non-lazy, but

5
kernel/rcu/rcu_segcblist.h
View File

@ -25,6 +25,10 @@ static inline void rcu_cblist_dequeued_lazy(struct rcu_cblist *rclp)
}
void rcu_cblist_init(struct rcu_cblist *rclp);
void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
struct rcu_cblist *srclp,
struct rcu_head *rhp);
struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);
/*
@ -92,6 +96,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
return !READ_ONCE(*READ_ONCE(rsclp->tails[seg]));
}
void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
void rcu_segcblist_init(struct rcu_segcblist *rsclp);
void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
void rcu_segcblist_offload(struct rcu_segcblist *rsclp);

16
kernel/rcu/tree.c
View File

@ -1251,6 +1251,7 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
unsigned long gp_seq_req;
bool ret = false;
rcu_lockdep_assert_cblist_protected(rdp);
raw_lockdep_assert_held_rcu_node(rnp);
/* If no pending (not yet ready to invoke) callbacks, nothing to do. */
@ -1292,7 +1293,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
unsigned long c;
bool needwake;
lockdep_assert_irqs_disabled();
rcu_lockdep_assert_cblist_protected(rdp);
c = rcu_seq_snap(&rcu_state.gp_seq);
if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
/* Old request still live, so mark recent callbacks. */
@ -1318,6 +1319,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
*/
static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
{
rcu_lockdep_assert_cblist_protected(rdp);
raw_lockdep_assert_held_rcu_node(rnp);
/* If no pending (not yet ready to invoke) callbacks, nothing to do. */
@ -1341,6 +1343,7 @@ 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)
{
rcu_lockdep_assert_cblist_protected(rdp);
if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||
!raw_spin_trylock_rcu_node(rnp))
return;
@ -2187,7 +2190,9 @@ static void rcu_do_batch(struct rcu_data *rdp)
* The following usually indicates a double call_rcu(). To track
* this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
*/
WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));
WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));
WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
count != 0 && rcu_segcblist_empty(&rdp->cblist));
rcu_nocb_unlock_irqrestore(rdp, flags);
@ -2564,8 +2569,9 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, bool lazy)
if (rcu_segcblist_empty(&rdp->cblist))
rcu_segcblist_init(&rdp->cblist);
}
rcu_nocb_lock(rdp);
was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
return; // Enqueued onto ->nocb_bypass, so just leave.
/* If we get here, rcu_nocb_try_bypass() acquired ->nocb_lock. */
rcu_segcblist_enqueue(&rdp->cblist, head, lazy);
if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rcu_state.name, head,
@ -2839,6 +2845,7 @@ static void rcu_barrier_func(void *unused)
rdp->barrier_head.func = rcu_barrier_callback;
debug_rcu_head_queue(&rdp->barrier_head);
rcu_nocb_lock(rdp);
WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) {
atomic_inc(&rcu_state.barrier_cpu_count);
} else {
@ -3192,6 +3199,7 @@ void rcutree_migrate_callbacks(int cpu)
my_rdp = this_cpu_ptr(&rcu_data);
my_rnp = my_rdp->mynode;
rcu_nocb_lock(my_rdp); /* irqs already disabled. */
WARN_ON_ONCE(!rcu_nocb_flush_bypass(my_rdp, NULL, jiffies));
raw_spin_lock_rcu_node(my_rnp); /* irqs already disabled. */
/* Leverage recent GPs and set GP for new callbacks. */
needwake = rcu_advance_cbs(my_rnp, rdp) ||

28
kernel/rcu/tree.h
View File

@ -200,18 +200,26 @@ struct rcu_data {
atomic_t nocb_lock_contended; /* Contention experienced. */
int nocb_defer_wakeup; /* Defer wakeup of nocb_kthread. */
struct timer_list nocb_timer; /* Enforce finite deferral. */
unsigned long nocb_gp_adv_time; /* Last call_rcu() CB adv (jiffies). */
/* The following fields are used by call_rcu, hence own cacheline. */
raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;
struct rcu_cblist nocb_bypass; /* Lock-contention-bypass CB list. */
unsigned long nocb_bypass_first; /* Time (jiffies) of first enqueue. */
unsigned long nocb_nobypass_last; /* Last ->cblist enqueue (jiffies). */
int nocb_nobypass_count; /* # ->cblist enqueues at ^^^ time. */
/* The following fields are used by GP kthread, hence own cacheline. */
raw_spinlock_t nocb_gp_lock ____cacheline_internodealigned_in_smp;
bool nocb_gp_sleep;
/* Is the nocb GP thread asleep? */
struct timer_list nocb_bypass_timer; /* Force nocb_bypass flush. */
bool nocb_gp_sleep; /* Is the nocb GP thread asleep? */
struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
bool nocb_cb_sleep; /* Is the nocb CB thread asleep? */
struct task_struct *nocb_cb_kthread;
struct rcu_data *nocb_next_cb_rdp;
/* Next rcu_data in wakeup chain. */
/* The following fields are used by CB kthread, hence new cachline. */
/* The following fields are used by CB kthread, hence new cacheline. */
struct rcu_data *nocb_gp_rdp ____cacheline_internodealigned_in_smp;
/* GP rdp takes GP-end wakeups. */
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
@ -419,6 +427,10 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
static void rcu_init_one_nocb(struct rcu_node *rnp);
static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j);
static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
bool *was_alldone, unsigned long flags);
static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
unsigned long flags);
static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
@ -430,19 +442,15 @@ static void rcu_nocb_lock(struct rcu_data *rdp);
static void rcu_nocb_unlock(struct rcu_data *rdp);
static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
unsigned long flags);
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp);
#ifdef CONFIG_RCU_NOCB_CPU
static void __init rcu_organize_nocb_kthreads(void);
#define rcu_nocb_lock_irqsave(rdp, flags) \
do { \
if (!rcu_segcblist_is_offloaded(&(rdp)->cblist)) { \
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. */ \
else \
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)

359
kernel/rcu/tree_plugin.h
View File

@ -1497,19 +1497,26 @@ static int __init parse_rcu_nocb_poll(char *arg)
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 the lock isn't immediately
* available, increment ->nocb_lock_contended to flag the contention.
* Don't bother bypassing ->cblist if the call_rcu() rate is low.
* After all, the main point of bypassing is to avoid lock contention
* on ->nocb_lock, which only can happen at high call_rcu() rates.
*/
static void rcu_nocb_lock(struct rcu_data *rdp)
int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
module_param(nocb_nobypass_lim_per_jiffy, int, 0);
/*
* Acquire the specified rcu_data structure's ->nocb_bypass_lock. If the
* lock isn't immediately available, increment ->nocb_lock_contended to
* flag the contention.
*/
static void rcu_nocb_bypass_lock(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||
raw_spin_trylock(&rdp->nocb_lock))
if (raw_spin_trylock(&rdp->nocb_bypass_lock))
return;
atomic_inc(&rdp->nocb_lock_contended);
smp_mb__after_atomic(); /* atomic_inc() before lock. */
raw_spin_lock(&rdp->nocb_lock);
raw_spin_lock(&rdp->nocb_bypass_lock);
smp_mb__before_atomic(); /* atomic_dec() after lock. */
atomic_dec(&rdp->nocb_lock_contended);
}
@ -1530,6 +1537,37 @@ static void rcu_nocb_wait_contended(struct rcu_data *rdp)
cpu_relax();
}
/*
* Conditionally acquire the specified rcu_data structure's
* ->nocb_bypass_lock.
*/
static bool rcu_nocb_bypass_trylock(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
return raw_spin_trylock(&rdp->nocb_bypass_lock);
}
/*
* Release the specified rcu_data structure's ->nocb_bypass_lock.
*/
static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
raw_spin_unlock(&rdp->nocb_bypass_lock);
}
/*
* Acquire the specified rcu_data structure's ->nocb_lock, but only
* if it corresponds to a no-CBs CPU.
*/
static void rcu_nocb_lock(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
if (!rcu_segcblist_is_offloaded(&rdp->cblist))
return;
raw_spin_lock(&rdp->nocb_lock);
}
/*
* Release the specified rcu_data structure's ->nocb_lock, but only
* if it corresponds to a no-CBs CPU.
@ -1557,6 +1595,15 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
}
}
/* Lockdep check that ->cblist may be safely accessed. */
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
if (rcu_segcblist_is_offloaded(&rdp->cblist) &&
cpu_online(rdp->cpu))
lockdep_assert_held(&rdp->nocb_lock);
}
/*
* Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
* grace period.
@ -1593,24 +1640,27 @@ static void wake_nocb_gp(struct rcu_data *rdp, bool force,
unsigned long flags)
__releases(rdp->nocb_lock)
{
bool needwake = false;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
lockdep_assert_held(&rdp->nocb_lock);
if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("AlreadyAwake"));
rcu_nocb_unlock_irqrestore(rdp, flags);
return;
}
if (READ_ONCE(rdp_gp->nocb_gp_sleep) || force) {
del_timer(&rdp->nocb_timer);
rcu_nocb_unlock_irqrestore(rdp, flags);
smp_mb(); /* enqueue before ->nocb_gp_sleep. */
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
del_timer(&rdp->nocb_timer);
rcu_nocb_unlock_irqrestore(rdp, flags);
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
wake_up_process(rdp_gp->nocb_gp_kthread);
} else {
rcu_nocb_unlock_irqrestore(rdp, flags);
needwake = true;
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
}
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
if (needwake)
wake_up_process(rdp_gp->nocb_gp_kthread);
}
/*
@ -1627,6 +1677,189 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
}
/*
* Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
* However, if there is a callback to be enqueued and if ->nocb_bypass
* proves to be initially empty, just return false because the no-CB GP
* kthread may need to be awakened in this case.
*
* Note that this function always returns true if rhp is NULL.
*/
static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j)
{
struct rcu_cblist rcl;
WARN_ON_ONCE(!rcu_segcblist_is_offloaded(&rdp->cblist));
rcu_lockdep_assert_cblist_protected(rdp);
lockdep_assert_held(&rdp->nocb_bypass_lock);
if (rhp && !rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
raw_spin_unlock(&rdp->nocb_bypass_lock);
return false;
}
/* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
if (rhp)
rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
WRITE_ONCE(rdp->nocb_bypass_first, j);
rcu_nocb_bypass_unlock(rdp);
return true;
}
/*
* Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
* However, if there is a callback to be enqueued and if ->nocb_bypass
* proves to be initially empty, just return false because the no-CB GP
* kthread may need to be awakened in this case.
*
* Note that this function always returns true if rhp is NULL.
*/
static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j)
{
if (!rcu_segcblist_is_offloaded(&rdp->cblist))
return true;
rcu_lockdep_assert_cblist_protected(rdp);
rcu_nocb_bypass_lock(rdp);
return rcu_nocb_do_flush_bypass(rdp, rhp, j);
}
/*
* If the ->nocb_bypass_lock is immediately available, flush the
* ->nocb_bypass queue into ->cblist.
*/
static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
{
rcu_lockdep_assert_cblist_protected(rdp);
if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||
!rcu_nocb_bypass_trylock(rdp))
return;
WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
}
/*
* See whether it is appropriate to use the ->nocb_bypass list in order
* to control contention on ->nocb_lock. A limited number of direct
* enqueues are permitted into ->cblist per jiffy. If ->nocb_bypass
* is non-empty, further callbacks must be placed into ->nocb_bypass,
* otherwise rcu_barrier() breaks. Use rcu_nocb_flush_bypass() to switch
* back to direct use of ->cblist. However, ->nocb_bypass should not be
* used if ->cblist is empty, because otherwise callbacks can be stranded
* on ->nocb_bypass because we cannot count on the current CPU ever again
* invoking call_rcu(). The general rule is that if ->nocb_bypass is
* non-empty, the corresponding no-CBs grace-period kthread must not be
* in an indefinite sleep state.
*
* Finally, it is not permitted to use the bypass during early boot,
* as doing so would confuse the auto-initialization code. Besides
* which, there is no point in worrying about lock contention while
* there is only one CPU in operation.
*/
static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
bool *was_alldone, unsigned long flags)
{
unsigned long c;
unsigned long cur_gp_seq;
unsigned long j = jiffies;
long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
return false; /* Not offloaded, no bypassing. */
}
lockdep_assert_irqs_disabled();
// Don't use ->nocb_bypass during early boot.
if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING) {
rcu_nocb_lock(rdp);
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
return false;
}
// If we have advanced to a new jiffy, reset counts to allow
// moving back from ->nocb_bypass to ->cblist.
if (j == rdp->nocb_nobypass_last) {
c = rdp->nocb_nobypass_count + 1;
} else {
WRITE_ONCE(rdp->nocb_nobypass_last, j);
c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
if (ULONG_CMP_LT(rdp->nocb_nobypass_count,
nocb_nobypass_lim_per_jiffy))
c = 0;
else if (c > nocb_nobypass_lim_per_jiffy)
c = nocb_nobypass_lim_per_jiffy;
}
WRITE_ONCE(rdp->nocb_nobypass_count, c);
// If there hasn't yet been all that many ->cblist enqueues
// this jiffy, tell the caller to enqueue onto ->cblist. But flush
// ->nocb_bypass first.
if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
rcu_nocb_lock(rdp);
*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
if (*was_alldone)
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("FirstQ"));
WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
return false; // Caller must enqueue the callback.
}
// If ->nocb_bypass has been used too long or is too full,
// flush ->nocb_bypass to ->cblist.
if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
ncbs >= qhimark) {
rcu_nocb_lock(rdp);
if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
if (*was_alldone)
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("FirstQ"));
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
return false; // Caller must enqueue the callback.
}
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_irqrestore(rdp, flags);
return true; // Callback already enqueued.
}
// We need to use the bypass.
rcu_nocb_wait_contended(rdp);
rcu_nocb_bypass_lock(rdp);
ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
if (!ncbs) {
WRITE_ONCE(rdp->nocb_bypass_first, j);
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
}
rcu_nocb_bypass_unlock(rdp);
smp_mb(); /* Order enqueue before wake. */
if (ncbs) {
local_irq_restore(flags);
} else {
// No-CBs GP kthread might be indefinitely asleep, if so, wake.
rcu_nocb_lock(rdp); // Rare during call_rcu() flood.
if (!rcu_segcblist_pend_cbs(&rdp->cblist)) {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("FirstBQwake"));
__call_rcu_nocb_wake(rdp, true, flags);
} else {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("FirstBQnoWake"));
rcu_nocb_unlock_irqrestore(rdp, flags);
}
}
return true; // Callback already enqueued.
}
/*
* Awaken the no-CBs grace-period kthead if needed, either due to it
* legitimately being asleep or due to overload conditions.
@ -1685,23 +1918,33 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
rcu_nocb_unlock_irqrestore(rdp, flags);
}
if (!irqs_disabled_flags(flags)) {
lockdep_assert_irqs_enabled();
rcu_nocb_wait_contended(rdp);
}
return;
}
/* Wake up the no-CBs GP kthread to flush ->nocb_bypass. */
static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
{
unsigned long flags;
struct rcu_data *rdp = from_timer(rdp, t, nocb_bypass_timer);
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
rcu_nocb_lock_irqsave(rdp, flags);
__call_rcu_nocb_wake(rdp, true, flags);
}
/*
* No-CBs GP kthreads come here to wait for additional callbacks to show up
* or for grace periods to end.
*/
static void nocb_gp_wait(struct rcu_data *my_rdp)
{
bool bypass = false;
long bypass_ncbs;
int __maybe_unused cpu = my_rdp->cpu;
unsigned long cur_gp_seq;
unsigned long flags;
bool gotcbs;
unsigned long j = jiffies;
bool needwait_gp = false; // This prevents actual uninitialized use.
bool needwake;
bool needwake_gp;
@ -1715,21 +1958,50 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
* and the global grace-period kthread are awakened if needed.
*/
for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
if (rcu_segcblist_empty(&rdp->cblist))
continue; /* No callbacks here, try next. */
rnp = rdp->mynode;
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
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. */
needwake_gp = rcu_advance_cbs(rnp, rdp);
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
if (bypass_ncbs &&
(time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
bypass_ncbs > 2 * qhimark)) {
// Bypass full or old, so flush it.
(void)rcu_nocb_try_flush_bypass(rdp, j);
bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
rcu_nocb_unlock_irqrestore(rdp, flags);
continue; /* No callbacks here, try next. */
}
if (bypass_ncbs) {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("Bypass"));
bypass = true;
}
rnp = rdp->mynode;
if (bypass) { // Avoid race with first bypass CB.
WRITE_ONCE(my_rdp->nocb_defer_wakeup,
RCU_NOCB_WAKE_NOT);
del_timer(&my_rdp->nocb_timer);
}
// Advance callbacks if helpful and low contention.
needwake_gp = false;
if (!rcu_segcblist_restempty(&rdp->cblist,
RCU_NEXT_READY_TAIL) ||
(rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
needwake_gp = rcu_advance_cbs(rnp, rdp);
raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
}
// Need to wait on some grace period?
WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
RCU_NEXT_READY_TAIL));
if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
if (!needwait_gp ||
ULONG_CMP_LT(cur_gp_seq, wait_gp_seq))
wait_gp_seq = cur_gp_seq;
needwait_gp = true;
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("NeedWaitGP"));
}
if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
needwake = rdp->nocb_cb_sleep;
@ -1747,6 +2019,13 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
rcu_gp_kthread_wake();
}
if (bypass && !rcu_nocb_poll) {
// At least one child with non-empty ->nocb_bypass, so set
// timer in order to avoid stranding its callbacks.
raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
}
if (rcu_nocb_poll) {
/* Polling, so trace if first poll in the series. */
if (gotcbs)
@ -1757,6 +2036,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
!READ_ONCE(my_rdp->nocb_gp_sleep));
trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
} else {
rnp = my_rdp->mynode;
trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
@ -1768,6 +2048,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
}
if (!rcu_nocb_poll) {
raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
if (bypass)
del_timer(&my_rdp->nocb_bypass_timer);
WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
}
@ -1949,8 +2231,11 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
init_swait_queue_head(&rdp->nocb_cb_wq);
init_swait_queue_head(&rdp->nocb_gp_wq);
raw_spin_lock_init(&rdp->nocb_lock);
raw_spin_lock_init(&rdp->nocb_bypass_lock);
raw_spin_lock_init(&rdp->nocb_gp_lock);
timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
timer_setup(&rdp->nocb_bypass_timer, do_nocb_bypass_wakeup_timer, 0);
rcu_cblist_init(&rdp->nocb_bypass);
}
/*
@ -2094,6 +2379,12 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
local_irq_restore(flags);
}
/* Lockdep check that ->cblist may be safely accessed. */
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
}
static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
}
@ -2107,6 +2398,18 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
{
}
static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j)
{
return true;
}
static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
bool *was_alldone, unsigned long flags)
{
return false;
}
static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
unsigned long flags)
{