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rcu/nocb: Re-offload support 

To re-offload the callback processing off of a CPU, it is necessary to
clear SEGCBLIST_SOFTIRQ_ONLY, set SEGCBLIST_OFFLOADED, and then notify
both the CB and GP kthreads so that they both set their own bit flag and
start processing the callbacks remotely.  The re-offloading worker is
then notified that it can stop the RCU_SOFTIRQ handler (or rcuc kthread,
as the case may be) from processing the callbacks locally.

Ordering must be carefully enforced so that the callbacks that used to be
processed locally without locking will have the same ordering properties
when they are invoked by the nocb CB and GP kthreads.

This commit makes this change.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[ paulmck: Export rcu_nocb_cpu_offload(). ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2020-11-13 13:13:22 +01:00 committed by Paul E. McKenney
parent 5bb39dc956
commit 254e11efde
2 changed files with 140 additions and 24 deletions
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2
include/linux/rcupdate.h
View File

@ -104,9 +104,11 @@ static inline void rcu_user_exit(void) { }
#ifdef CONFIG_RCU_NOCB_CPU
void rcu_init_nohz(void);
int rcu_nocb_cpu_offload(int cpu);
int rcu_nocb_cpu_deoffload(int cpu);
#else /* #ifdef CONFIG_RCU_NOCB_CPU */
static inline void rcu_init_nohz(void) { }
static inline int rcu_nocb_cpu_offload(int cpu) { return -EINVAL; }
static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

162
kernel/rcu/tree_plugin.h
View File

@ -1928,6 +1928,20 @@ static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
__call_rcu_nocb_wake(rdp, true, flags);
}
/*
* Check if we ignore this rdp.
*
* We check that without holding the nocb lock but
* we make sure not to miss a freshly offloaded rdp
* with the current ordering:
*
* rdp_offload_toggle() nocb_gp_enabled_cb()
* ------------------------- ----------------------------
* WRITE flags LOCK nocb_gp_lock
* LOCK nocb_gp_lock READ/WRITE nocb_gp_sleep
* READ/WRITE nocb_gp_sleep UNLOCK nocb_gp_lock
* UNLOCK nocb_gp_lock READ flags
*/
static inline bool nocb_gp_enabled_cb(struct rcu_data *rdp)
{
u8 flags = SEGCBLIST_OFFLOADED | SEGCBLIST_KTHREAD_GP;
@ -1940,6 +1954,11 @@ static inline bool nocb_gp_update_state(struct rcu_data *rdp, bool *needwake_sta
struct rcu_segcblist *cblist = &rdp->cblist;
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_GP);
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB))
*needwake_state = true;
}
return true;
} else {
/*
@ -2003,6 +2022,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
rcu_nocb_unlock_irqrestore(rdp, flags);
if (needwake_state)
swake_up_one(&rdp->nocb_state_wq);
continue; /* No callbacks here, try next. */
}
if (bypass_ncbs) {
@ -2054,6 +2075,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
}
if (needwake_gp)
rcu_gp_kthread_wake();
if (needwake_state)
swake_up_one(&rdp->nocb_state_wq);
}
my_rdp->nocb_gp_bypass = bypass;
@ -2159,6 +2182,11 @@ static void nocb_cb_wait(struct rcu_data *rdp)
WRITE_ONCE(rdp->nocb_cb_sleep, true);
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)) {
rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_CB);
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
needwake_state = true;
}
if (rcu_segcblist_ready_cbs(cblist))
WRITE_ONCE(rdp->nocb_cb_sleep, false);
} else {
@ -2254,12 +2282,44 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
do_nocb_deferred_wakeup_common(rdp);
}
static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
static int rdp_offload_toggle(struct rcu_data *rdp,
bool offload, unsigned long flags)
__releases(rdp->nocb_lock)
{
struct rcu_segcblist *cblist = &rdp->cblist;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
bool wake_cb = false, wake_gp = false;
bool wake_gp = false;
rcu_segcblist_offload(cblist, offload);
if (rdp->nocb_cb_sleep)
rdp->nocb_cb_sleep = false;
rcu_nocb_unlock_irqrestore(rdp, flags);
/*
* Ignore former value of nocb_cb_sleep and force wake up as it could
* have been spuriously set to false already.
*/
swake_up_one(&rdp->nocb_cb_wq);
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
if (rdp_gp->nocb_gp_sleep) {
rdp_gp->nocb_gp_sleep = false;
wake_gp = true;
}
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
return 0;
}
static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int ret;
printk("De-offloading %d\n", rdp->cpu);
@ -2273,31 +2333,11 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
return -EBUSY;
}
rcu_segcblist_offload(cblist, false);
if (rdp->nocb_cb_sleep) {
rdp->nocb_cb_sleep = false;
wake_cb = true;
}
rcu_nocb_unlock_irqrestore(rdp, flags);
if (wake_cb)
swake_up_one(&rdp->nocb_cb_wq);
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
if (rdp_gp->nocb_gp_sleep) {
rdp_gp->nocb_gp_sleep = false;
wake_gp = true;
}
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
ret = rdp_offload_toggle(rdp, false, flags);
swait_event_exclusive(rdp->nocb_state_wq,
!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
SEGCBLIST_KTHREAD_GP));
return 0;
return ret;
}
static long rcu_nocb_rdp_deoffload(void *arg)
@ -2335,6 +2375,80 @@ int rcu_nocb_cpu_deoffload(int cpu)
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
static int __rcu_nocb_rdp_offload(struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int ret;
/*
* For now we only support re-offload, ie: the rdp must have been
* offloaded on boot first.
*/
if (!rdp->nocb_gp_rdp)
return -EINVAL;
printk("Offloading %d\n", rdp->cpu);
/*
* Can't use rcu_nocb_lock_irqsave() while we are in
* SEGCBLIST_SOFTIRQ_ONLY mode.
*/
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
/*
* We didn't take the nocb lock while working on the
* rdp->cblist in SEGCBLIST_SOFTIRQ_ONLY mode.
* Every modifications that have been done previously on
* rdp->cblist must be visible remotely by the nocb kthreads
* upon wake up after reading the cblist flags.
*
* The layout against nocb_lock enforces that ordering:
*
* __rcu_nocb_rdp_offload() nocb_cb_wait()/nocb_gp_wait()
* ------------------------- ----------------------------
* WRITE callbacks rcu_nocb_lock()
* rcu_nocb_lock() READ flags
* WRITE flags READ callbacks
* rcu_nocb_unlock() rcu_nocb_unlock()
*/
ret = rdp_offload_toggle(rdp, true, flags);
swait_event_exclusive(rdp->nocb_state_wq,
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
return ret;
}
static long rcu_nocb_rdp_offload(void *arg)
{
struct rcu_data *rdp = arg;
WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
return __rcu_nocb_rdp_offload(rdp);
}
int rcu_nocb_cpu_offload(int cpu)
{
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
int ret = 0;
mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
if (cpu_online(cpu)) {
ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
} else {
ret = __rcu_nocb_rdp_offload(rdp);
}
if (!ret)
cpumask_set_cpu(cpu, rcu_nocb_mask);
}
cpus_read_unlock();
mutex_unlock(&rcu_state.barrier_mutex);
return ret;
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
void __init rcu_init_nohz(void)
{
int cpu;