We'll need to be able to runtime offload and de-offload the processing
of callback for a given CPU. In order to support a smooth transition
from unlocked local processing (softirq/rcuc) to locked offloaded
processing (rcuop/rcuog) and the reverse, provide the necessary bits and
documentation for the state machine that will carry up all the steps to
enforce correctness while serving callbacks processing all along.
Inspired-by: Paul E. McKenney <paul...@kernel.org>
Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
Cc: Paul E. McKenney <paul...@kernel.org>
Cc: Josh Triplett <j...@joshtriplett.org>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Cc: Lai Jiangshan <jiangshan...@gmail.com>
Cc: Joel Fernandes <j...@joelfernandes.org>
Cc: Neeraj Upadhyay <neer...@codeaurora.org>
---
include/linux/rcu_segcblist.h | 115 +++++++++++++++++++++++++++++++++-
kernel/rcu/rcu_segcblist.c | 1 +
kernel/rcu/rcu_segcblist.h | 12 +++-
kernel/rcu/tree.c | 3 +
4 files changed, 128 insertions(+), 3 deletions(-)
diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index dca2f39ee67f..8a0d3a211e7c 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -63,8 +63,121 @@ struct rcu_cblist {
#define RCU_NEXT_TAIL 3
#define RCU_CBLIST_NSEGS 4
+
+/*
+ * ==NOCB Offloading state machine==
+ *
+ *
+ *
----------------------------------------------------------------------------
+ * | SEGCBLIST_SOFTIRQ_ONLY
|
+ * |
|
+ * | Callbacks processed by rcu_core() from softirqs or local
|
+ * | rcuc kthread, without holding nocb_lock.
|
+ *
----------------------------------------------------------------------------
+ * |
+ * v
+ *
----------------------------------------------------------------------------
+ * | SEGCBLIST_OFFLOADED
|
+ * |
|
+ * | Callbacks processed by rcu_core() from softirqs or local
|
+ * | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads,
|
+ * | allowing nocb_timer to be armed. |
+ *
----------------------------------------------------------------------------
+ * |
+ * v
+ * -----------------------------------
+ * | |
+ * v v
+ * ---------------------------------------
----------------------------------|
+ * | SEGCBLIST_OFFLOADED | | | SEGCBLIST_OFFLOADED |
|
+ * | SEGCBLIST_KTHREAD_CB | | SEGCBLIST_KTHREAD_GP
|
+ * | | |
|
+ * | | |
|
+ * | CB kthread woke up and | | GP kthread woke up and
|
+ * | acknowledged SEGCBLIST_OFFLOADED. | | acknowledged
SEGCBLIST_OFFLOADED|
+ * | Processes callbacks concurrently | |
|
+ * | with rcu_core(), holding | |
|
+ * | nocb_lock. | |
|
+ * ---------------------------------------
-----------------------------------
+ * | |
+ * -----------------------------------
+ * |
+ * v
+ *
|--------------------------------------------------------------------------|
+ * | SEGCBLIST_OFFLOADED |
|
+ * | SEGCBLIST_KTHREAD_CB |
|
+ * | SEGCBLIST_KTHREAD_GP
|
+ * |
|
+ * | Kthreads handle callbacks holding nocb_lock, local rcu_core() stops
|
+ * | handling callbacks.
|
+ *
----------------------------------------------------------------------------
+ */
+
+
+
+/*
+ * ==NOCB De-Offloading state machine==
+ *
+ *
+ *
|--------------------------------------------------------------------------|
+ * | SEGCBLIST_OFFLOADED |
|
+ * | SEGCBLIST_KTHREAD_CB |
|
+ * | SEGCBLIST_KTHREAD_GP
|
+ * |
|
+ * | CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core()
|
+ * | ignores callbacks.
|
+ *
----------------------------------------------------------------------------
+ * |
+ * v
+ *
|--------------------------------------------------------------------------|
+ * | SEGCBLIST_KTHREAD_CB |
|
+ * | SEGCBLIST_KTHREAD_GP
|
+ * |
|
+ * | CB/GP kthreads and local rcu_core() handle callbacks concurrently
|
+ * | holding nocb_lock. Wake up CB and GP kthreads if necessary.
|
+ *
----------------------------------------------------------------------------
+ * |
+ * v
+ * -----------------------------------
+ * | |
+ * v v
+ *
---------------------------------------------------------------------------|
+ * |
|
+ * | SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP
|
+ * | |
|
+ * | GP kthread woke up and | CB kthread woke up and
|
+ * | acknowledged the fact that | acknowledged the fact that
|
+ * | SEGCBLIST_OFFLOADED got cleared. | SEGCBLIST_OFFLOADED got cleared.
|
+ * | | The CB kthread goes to sleep
|
+ * | The callbacks from the target CPU | until it ever gets re-offloaded.
|
+ * | will be ignored from the GP kthread |
|
+ * | loop. |
|
+ *
----------------------------------------------------------------------------
+ * | |
+ * -----------------------------------
+ * |
+ * v
+ *
----------------------------------------------------------------------------
+ * | 0
|
+ * |
|
+ * | Callbacks processed by rcu_core() from softirqs or local
|
+ * | rcuc kthread, while holding nocb_lock. Forbid nocb_timer to be armed.
|
+ * | Flush pending nocb_timer. Flush nocb bypass callbacks.
|
+ *
----------------------------------------------------------------------------
+ * |
+ * v
+ *
----------------------------------------------------------------------------
+ * | SEGCBLIST_SOFTIRQ_ONLY
|
+ * |
|
+ * | Callbacks processed by rcu_core() from softirqs or local
|
+ * | rcuc kthread, without holding nocb_lock.
|
+ *
----------------------------------------------------------------------------
+ */
#define SEGCBLIST_ENABLED BIT(0)
-#define SEGCBLIST_OFFLOADED BIT(1)
+#define SEGCBLIST_SOFTIRQ_ONLY BIT(1)
+#define SEGCBLIST_KTHREAD_CB BIT(2)
+#define SEGCBLIST_KTHREAD_GP BIT(3)
+#define SEGCBLIST_OFFLOADED BIT(4)
struct rcu_segcblist {
struct rcu_head *head;
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index e6522fa54311..a96511b7cc98 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -172,6 +172,7 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
*/
void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
{
+ rcu_segcblist_clear_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY);
rcu_segcblist_set_flags(rsclp, SEGCBLIST_OFFLOADED);
}
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index fc98761e3ee9..575896a2518b 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -80,8 +80,16 @@ static inline bool rcu_segcblist_is_enabled(struct
rcu_segcblist *rsclp)
/* Is the specified rcu_segcblist offloaded? */
static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
{
- return IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
- rcu_segcblist_test_flags(rsclp, SEGCBLIST_OFFLOADED);
+ if (IS_ENABLED(CONFIG_RCU_NOCB_CPU)) {
+ /*
+ * Complete de-offloading happens only when
SEGCBLIST_SOFTIRQ_ONLY
+ * is set.
+ */
+ if (!rcu_segcblist_test_flags(rsclp, SEGCBLIST_SOFTIRQ_ONLY))
+ return true;
+ }
+
+ return false;
}
/*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dc1e578644df..3b7adc9cc068 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -83,6 +83,9 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data,
rcu_data) = {
.dynticks_nesting = 1,
.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
+#ifdef CONFIG_RCU_NOCB_CPU
+ .cblist.flags = SEGCBLIST_SOFTIRQ_ONLY,
+#endif
};
static struct rcu_state rcu_state = {
.level = { &rcu_state.node[0] },
--
2.25.1
