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rcu/nocb: Prepare state machine for a new step

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Currently SEGCBLIST_SOFTIRQ_ONLY is a bit of an exception among the
segcblist flags because it is an exclusive state that doesn't mix up
with the other flags. Remove it in favour of:

_ A flag specifying that rcu_core() needs to perform callbacks execution
  and acceleration

and

_ A flag specifying we want the nocb lock to be held in any needed
  circumstances

This clarifies the code and is more flexible: It allows to have a state
where rcu_core() runs with locking while offloading hasn't started yet.
This is a necessary step to prepare for triggering rcu_core() at the
very beginning of the de-offloading process so that rcu_core() won't
dismiss work while being preempted by the de-offloading process, at
least not without a pending subsequent rcu_core() that will quickly
catch up.

Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-10-19 02:08:07 +02:00 committed by Paul E. McKenney
parent 118e0d4a1b
commit 213d56bf33
5 changed files with 50 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

37
include/linux/rcu_segcblist.h
View File

@ -69,7 +69,7 @@ struct rcu_cblist {
*
*
* ----------------------------------------------------------------------------
* | SEGCBLIST_SOFTIRQ_ONLY |
* | SEGCBLIST_RCU_CORE |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, without holding nocb_lock. |
@ -77,7 +77,7 @@ struct rcu_cblist {
* |
* v
* ----------------------------------------------------------------------------
* | SEGCBLIST_OFFLOADED |
* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads, |
@ -89,7 +89,9 @@ struct rcu_cblist {
* | |
* v v
* --------------------------------------- ----------------------------------|
* | SEGCBLIST_OFFLOADED | | | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_RCU_CORE | | | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | | | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | | | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | | SEGCBLIST_KTHREAD_GP |
* | | | |
* | | | |
@ -104,9 +106,10 @@ struct rcu_cblist {
* |
* v
* |--------------------------------------------------------------------------|
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_GP | |
* | SEGCBLIST_KTHREAD_CB |
* | |
* | Kthreads handle callbacks holding nocb_lock, local rcu_core() stops |
* | handling callbacks. Enable bypass queueing. |
@ -120,7 +123,8 @@ struct rcu_cblist {
*
*
* |--------------------------------------------------------------------------|
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_OFFLOADED | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | |
@ -130,6 +134,8 @@ struct rcu_cblist {
* |
* v
* |--------------------------------------------------------------------------|
* | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | |
* | SEGCBLIST_KTHREAD_CB | |
* | SEGCBLIST_KTHREAD_GP |
* | |
@ -143,7 +149,9 @@ struct rcu_cblist {
* | |
* v v
* ---------------------------------------------------------------------------|
* | |
* | | |
* | SEGCBLIST_RCU_CORE | | SEGCBLIST_RCU_CORE | |
* | SEGCBLIST_LOCKING | | SEGCBLIST_LOCKING | |
* | SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP |
* | | |
* | GP kthread woke up and | CB kthread woke up and |
@ -159,7 +167,7 @@ struct rcu_cblist {
* |
* v
* ----------------------------------------------------------------------------
* | 0 |
* | SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, while holding nocb_lock. Forbid nocb_timer to be armed. |
@ -168,17 +176,18 @@ struct rcu_cblist {
* |
* v
* ----------------------------------------------------------------------------
* | SEGCBLIST_SOFTIRQ_ONLY |
* | SEGCBLIST_RCU_CORE |
* | |
* | Callbacks processed by rcu_core() from softirqs or local |
* | rcuc kthread, without holding nocb_lock. |
* ----------------------------------------------------------------------------
*/
#define SEGCBLIST_ENABLED BIT(0)
#define SEGCBLIST_SOFTIRQ_ONLY BIT(1)
#define SEGCBLIST_KTHREAD_CB BIT(2)
#define SEGCBLIST_KTHREAD_GP BIT(3)
#define SEGCBLIST_OFFLOADED BIT(4)
#define SEGCBLIST_RCU_CORE BIT(1)
#define SEGCBLIST_LOCKING BIT(2)
#define SEGCBLIST_KTHREAD_CB BIT(3)
#define SEGCBLIST_KTHREAD_GP BIT(4)
#define SEGCBLIST_OFFLOADED BIT(5)
struct rcu_segcblist {
struct rcu_head *head;

6
kernel/rcu/rcu_segcblist.c
View File

@ -261,14 +261,14 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
}
/*
* Mark the specified rcu_segcblist structure as offloaded.
* Mark the specified rcu_segcblist structure as offloaded (or not)
*/
void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
{
if (offload) {
rcu_segcblist_clear_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY);
rcu_segcblist_set_flags(rsclp, SEGCBLIST_OFFLOADED);
rcu_segcblist_set_flags(rsclp, SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED);
} else {
rcu_segcblist_set_flags(rsclp, SEGCBLIST_RCU_CORE);
rcu_segcblist_clear_flags(rsclp, SEGCBLIST_OFFLOADED);
}
}

12
kernel/rcu/rcu_segcblist.h
View File

@ -80,11 +80,14 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
return rcu_segcblist_test_flags(rsclp, SEGCBLIST_ENABLED);
}
/* Is the specified rcu_segcblist offloaded, or is SEGCBLIST_SOFTIRQ_ONLY set? */
/*
* Is the specified rcu_segcblist NOCB offloaded (or in the middle of the
* [de]offloading process)?
*/
static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
{
if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
!rcu_segcblist_test_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY))
rcu_segcblist_test_flags(rsclp, SEGCBLIST_LOCKING))
return true;
return false;
@ -92,9 +95,8 @@ static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
static inline bool rcu_segcblist_completely_offloaded(struct rcu_segcblist *rsclp)
{
int flags = SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP | SEGCBLIST_OFFLOADED;
if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) && (rsclp->flags & flags) == flags)
if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
!rcu_segcblist_test_flags(rsclp, SEGCBLIST_RCU_CORE))
return true;
return false;

2
kernel/rcu/tree.c
View File

@ -79,7 +79,7 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
.dynticks = ATOMIC_INIT(1),
#ifdef CONFIG_RCU_NOCB_CPU
.cblist.flags = SEGCBLIST_SOFTIRQ_ONLY,
.cblist.flags = SEGCBLIST_RCU_CORE,
#endif
};
static struct rcu_state rcu_state = {

24
kernel/rcu/tree_nocb.h
View File

@ -1000,12 +1000,12 @@ static long rcu_nocb_rdp_deoffload(void *arg)
*/
rcu_nocb_lock_irqsave(rdp, flags);
/*
* Theoretically we could set SEGCBLIST_SOFTIRQ_ONLY after the nocb
* Theoretically we could clear SEGCBLIST_LOCKING after the nocb
* lock is released but how about being paranoid for once?
*/
rcu_segcblist_set_flags(cblist, SEGCBLIST_SOFTIRQ_ONLY);
rcu_segcblist_clear_flags(cblist, SEGCBLIST_LOCKING);
/*
* With SEGCBLIST_SOFTIRQ_ONLY, we can't use
* Without SEGCBLIST_LOCKING, we can't use
* rcu_nocb_unlock_irqrestore() anymore.
*/
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
@ -1058,14 +1058,14 @@ static long rcu_nocb_rdp_offload(void *arg)
pr_info("Offloading %d\n", rdp->cpu);
/*
* Can't use rcu_nocb_lock_irqsave() while we are in
* SEGCBLIST_SOFTIRQ_ONLY mode.
* Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
* is set.
*/
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.
* rdp->cblist with SEGCBLIST_LOCKING cleared (pure softirq/rcuc 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.
@ -1084,6 +1084,14 @@ static long rcu_nocb_rdp_offload(void *arg)
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
/*
* All kthreads are ready to work, we can finally relieve rcu_core() and
* enable nocb bypass.
*/
rcu_nocb_lock_irqsave(rdp, flags);
rcu_segcblist_clear_flags(cblist, SEGCBLIST_RCU_CORE);
rcu_nocb_unlock_irqrestore(rdp, flags);
return ret;
}
@ -1154,8 +1162,8 @@ void __init rcu_init_nohz(void)
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_CB);
rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_RCU_CORE);
}
rcu_organize_nocb_kthreads();
}