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rcu/nocb: Simplify (de-)offloading state machine

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Now that the (de-)offloading process can only apply to offline CPUs,
there is no more concurrency between rcu_core and nocb kthreads. Also
the mutation now happens on empty queues.

Therefore the state machine can be reduced to a single bit called
SEGCBLIST_OFFLOADED. Simplify the transition as follows:

* Upon offloading: queue the rdp to be added to the rcuog list and
  wait for the rcuog kthread to set the SEGCBLIST_OFFLOADED bit. Unpark
  rcuo kthread.

* Upon de-offloading: Park rcuo kthread. Queue the rdp to be removed
  from the rcuog list and wait for the rcuog kthread to clear the
  SEGCBLIST_OFFLOADED bit.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit is contained in:
Frederic Weisbecker 2024-07-04 00:56:40 +02:00 committed by Neeraj Upadhyay
parent 91e43b9044
commit 1fcb932c8b
4 changed files with 68 additions and 87 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/linux/rcu_segcblist.h
View File

@ -185,9 +185,7 @@ struct rcu_cblist {
* ----------------------------------------------------------------------------
*/
#define SEGCBLIST_ENABLED BIT(0)
#define SEGCBLIST_LOCKING BIT(1)
#define SEGCBLIST_KTHREAD_GP BIT(2)
#define SEGCBLIST_OFFLOADED BIT(3)
#define SEGCBLIST_OFFLOADED BIT(1)
struct rcu_segcblist {
struct rcu_head *head;

11
kernel/rcu/rcu_segcblist.c
View File

@ -260,17 +260,6 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
rcu_segcblist_clear_flags(rsclp, SEGCBLIST_ENABLED);
}
/*
* Mark the specified rcu_segcblist structure as offloaded (or not)
*/
void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
{
if (offload)
rcu_segcblist_set_flags(rsclp, SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED);
else
rcu_segcblist_clear_flags(rsclp, SEGCBLIST_OFFLOADED);
}
/*
* Does the specified rcu_segcblist structure contain callbacks that
* are ready to be invoked?

2
kernel/rcu/rcu_segcblist.h
View File

@ -89,7 +89,7 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
{
if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
rcu_segcblist_test_flags(rsclp, SEGCBLIST_LOCKING))
rcu_segcblist_test_flags(rsclp, SEGCBLIST_OFFLOADED))
return true;
return false;

138
kernel/rcu/tree_nocb.h
View File

@ -604,37 +604,33 @@ static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head,
}
}
static int nocb_gp_toggle_rdp(struct rcu_data *rdp)
static void nocb_gp_toggle_rdp(struct rcu_data *rdp_gp, struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int ret;
rcu_nocb_lock_irqsave(rdp, flags);
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
/*
* Locking orders future de-offloaded callbacks enqueue against previous
* handling of this rdp. Ie: Make sure rcuog is done with this rdp before
* deoffloaded callbacks can be enqueued.
*/
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
/*
* Offloading. Set our flag and notify the offload worker.
* We will handle this rdp until it ever gets de-offloaded.
*/
rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_GP);
ret = 1;
} else if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
rcu_segcblist_set_flags(cblist, SEGCBLIST_OFFLOADED);
} else {
/*
* De-offloading. Clear our flag and notify the de-offload worker.
* We will ignore this rdp until it ever gets re-offloaded.
*/
rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_GP);
ret = 0;
} else {
WARN_ON_ONCE(1);
ret = -1;
list_del(&rdp->nocb_entry_rdp);
rcu_segcblist_clear_flags(cblist, SEGCBLIST_OFFLOADED);
}
rcu_nocb_unlock_irqrestore(rdp, flags);
return ret;
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
}
static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
@ -841,14 +837,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
}
if (rdp_toggling) {
int ret;
ret = nocb_gp_toggle_rdp(rdp_toggling);
if (ret == 1)
list_add_tail(&rdp_toggling->nocb_entry_rdp, &my_rdp->nocb_head_rdp);
else if (ret == 0)
list_del(&rdp_toggling->nocb_entry_rdp);
nocb_gp_toggle_rdp(my_rdp, rdp_toggling);
swake_up_one(&rdp_toggling->nocb_state_wq);
}
@ -1018,16 +1007,11 @@ void rcu_nocb_flush_deferred_wakeup(void)
}
EXPORT_SYMBOL_GPL(rcu_nocb_flush_deferred_wakeup);
static int rdp_offload_toggle(struct rcu_data *rdp,
bool offload, unsigned long flags)
__releases(rdp->nocb_lock)
static int rcu_nocb_queue_toggle_rdp(struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
bool wake_gp = false;
rcu_segcblist_offload(cblist, offload);
rcu_nocb_unlock_irqrestore(rdp, flags);
unsigned long flags;
raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
// Queue this rdp for add/del to/from the list to iterate on rcuog
@ -1041,9 +1025,25 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
return wake_gp;
}
static bool rcu_nocb_rdp_deoffload_wait_cond(struct rcu_data *rdp)
{
unsigned long flags;
bool ret;
/*
* Locking makes sure rcuog is done handling this rdp before deoffloaded
* enqueue can happen. Also it keeps the SEGCBLIST_OFFLOADED flag stable
* while the ->nocb_lock is held.
*/
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
ret = !rcu_segcblist_test_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
return ret;
}
static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int wake_gp;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
@ -1056,51 +1056,42 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
/* Flush all callbacks from segcblist and bypass */
rcu_barrier();
/*
* Make sure the rcuoc kthread isn't in the middle of a nocb locked
* sequence while offloading is deactivated, along with nocb locking.
*/
if (rdp->nocb_cb_kthread)
kthread_park(rdp->nocb_cb_kthread);
rcu_nocb_lock_irqsave(rdp, flags);
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
rcu_nocb_unlock_irqrestore(rdp, flags);
wake_gp = rdp_offload_toggle(rdp, false, flags);
wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
if (rdp_gp->nocb_gp_kthread) {
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
swait_event_exclusive(rdp->nocb_state_wq,
!rcu_segcblist_test_flags(cblist,
SEGCBLIST_KTHREAD_GP));
if (rdp->nocb_cb_kthread)
kthread_park(rdp->nocb_cb_kthread);
rcu_nocb_rdp_deoffload_wait_cond(rdp));
} else {
/*
* No kthread to clear the flags for us or remove the rdp from the nocb list
* to iterate. Do it here instead. Locking doesn't look stricly necessary
* but we stick to paranoia in this rare path.
*/
rcu_nocb_lock_irqsave(rdp, flags);
rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
rcu_nocb_unlock_irqrestore(rdp, flags);
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
list_del(&rdp->nocb_entry_rdp);
}
mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
/*
* Lock one last time to acquire latest callback updates from kthreads
* so we can later handle callbacks locally without locking.
*/
rcu_nocb_lock_irqsave(rdp, flags);
/*
* Theoretically we could clear SEGCBLIST_LOCKING after the nocb
* lock is released but how about being paranoid for once?
*/
rcu_segcblist_clear_flags(cblist, SEGCBLIST_LOCKING);
/*
* Without SEGCBLIST_LOCKING, we can't use
* rcu_nocb_unlock_irqrestore() anymore.
*/
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
return 0;
}
@ -1129,10 +1120,20 @@ int rcu_nocb_cpu_deoffload(int cpu)
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
static bool rcu_nocb_rdp_offload_wait_cond(struct rcu_data *rdp)
{
unsigned long flags;
bool ret;
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
ret = rcu_segcblist_test_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
return ret;
}
static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
{
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int wake_gp;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
@ -1152,20 +1153,14 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
/*
* Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
* is set.
*/
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
wake_gp = rdp_offload_toggle(rdp, true, flags);
wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
if (wake_gp)
wake_up_process(rdp_gp->nocb_gp_kthread);
kthread_unpark(rdp->nocb_cb_kthread);
swait_event_exclusive(rdp->nocb_state_wq,
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
rcu_nocb_rdp_offload_wait_cond(rdp));
kthread_unpark(rdp->nocb_cb_kthread);
return 0;
}
@ -1340,8 +1335,7 @@ void __init rcu_init_nohz(void)
rdp = per_cpu_ptr(&rcu_data, cpu);
if (rcu_segcblist_empty(&rdp->cblist))
rcu_segcblist_init(&rdp->cblist);
rcu_segcblist_offload(&rdp->cblist, true);
rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_OFFLOADED);
}
rcu_organize_nocb_kthreads();
}