From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E . McKenney" <paulmck@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Boqun Feng <boqun.feng@gmail.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Joel Fernandes <joel@joelfernandes.org>,
Josh Triplett <josh@joshtriplett.org>,
rcu@vger.kernel.org
Subject: [PATCH 02/19] rcu/nocb: Provide basic callback offloading state machine bits
Date: Fri, 13 Nov 2020 13:13:17 +0100 [thread overview]
Message-ID: <20201113121334.166723-3-frederic@kernel.org> (raw)
In-Reply-To: <20201113121334.166723-1-frederic@kernel.org>
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 <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
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>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
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 4714b0263c76..4ca08b089faf 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 e374f9c3ec2c..d7cb9dc044ea 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -254,6 +254,7 @@ 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 3d1195dbf030..70e108c0e7c9 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 413831b48648..48e8e63cdeb2 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
next prev parent reply other threads:[~2020-11-13 12:15 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-13 12:13 [PATCH 00/19] rcu/nocb: De-offload and re-offload support v4 Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 01/19] rcu/nocb: Turn enabled/offload states into a common flag Frederic Weisbecker
2020-11-13 12:13 ` Frederic Weisbecker [this message]
2020-11-13 12:13 ` [PATCH 03/19] rcu/nocb: Always init segcblist on CPU up Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 04/19] rcu/nocb: De-offloading CB kthread Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 05/19] rcu/nocb: Don't deoffload an offline CPU with pending work Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 06/19] rcu/nocb: De-offloading GP kthread Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 07/19] rcu/nocb: Re-offload support Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 08/19] rcu/nocb: Shutdown nocb timer on de-offloading Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 09/19] rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 10/19] rcu/nocb: Set SEGCBLIST_SOFTIRQ_ONLY at the very last stage of de-offloading Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 11/19] rcu/nocb: Only cond_resched() from actual offloaded batch processing Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 12/19] rcu/nocb: Process batch locally as long as offloading isn't complete Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 13/19] rcu/nocb: Locally accelerate callbacks " Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 14/19] tools/rcutorture: Support nocb toggle in TREE01 Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 15/19] rcutorture: Remove weak nocb declarations Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 16/19] rcutorture: Export nocb (de)offloading functions Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 17/19] cpu/hotplug: Add lockdep_is_cpus_held() Frederic Weisbecker
2020-12-15 23:49 ` Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 18/19] timer: Add timer_curr_running() Frederic Weisbecker
2020-11-13 12:13 ` [PATCH 19/19] rcu/nocb: Detect unsafe checks for offloaded rdp Frederic Weisbecker
2020-12-08 2:41 ` [PATCH 00/19] rcu/nocb: De-offload and re-offload support v4 Boqun Feng
2020-12-08 12:51 ` Frederic Weisbecker
2020-12-10 1:21 ` Paul E. McKenney
2020-12-10 3:08 ` Boqun Feng
2020-12-16 16:59 ` Paul E. McKenney
2020-12-29 14:21 ` Frederic Weisbecker
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