[PATCH v4 tip/core/rcu 10/38] rcu-tasks: Add an RCU-tasks rude variant

paulmck
2020-04-15T18:20:28Z

This commit adds a "rude" variant of RCU-tasks that has as quiescent
states schedule(), cond_resched_tasks_rcu_qs(), userspace execution,
and (in theory, anyway) cond_resched(). In other words, RCU-tasks rude
readers are regions of code with preemption disabled, but excluding code
early in the CPU-online sequence and late in the CPU-offline sequence.
Updates make use of IPIs and force an IPI and a context switch on each
online CPU. This variant is useful in some situations in tracing.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
[ paulmck: Apply EXPORT_SYMBOL_GPL() feedback from Qiujun Huang. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ paulmck: Apply review feedback from Steve Rostedt. ]
---
include/linux/rcupdate.h | 3 ++
kernel/rcu/Kconfig | 11 +++++-
kernel/rcu/tasks.h | 98 ++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 111 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5523145..2be97a8 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -37,6 +37,7 @@
/* Exported common interfaces */
void call_rcu(struct rcu_head *head, rcu_callback_t func);
void rcu_barrier_tasks(void);
+void rcu_barrier_tasks_rude(void);
void synchronize_rcu(void);

#ifdef CONFIG_PREEMPT_RCU
@@ -138,6 +139,8 @@ static inline void rcu_init_nohz(void) { }
#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
void synchronize_rcu_tasks(void);
+void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
+void synchronize_rcu_tasks_rude(void);
void exit_tasks_rcu_start(void);
void exit_tasks_rcu_finish(void);
#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 38475d0..6ee6372 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -71,7 +71,7 @@ config TREE_SRCU
This option selects the full-fledged version of SRCU.

config TASKS_RCU_GENERIC
- def_bool TASKS_RCU
+ def_bool TASKS_RCU || TASKS_RUDE_RCU
select SRCU
help
This option enables generic infrastructure code supporting
@@ -84,6 +84,15 @@ config TASKS_RCU
only voluntary context switch (not preemption!), idle, and
user-mode execution as quiescent states. Not for manual selection.

+config TASKS_RUDE_RCU
+ def_bool 0
+ help
+ This option enables a task-based RCU implementation that uses
+ only context switch (including preemption) and user-mode
+ execution as quiescent states. It forces IPIs and context
+ switches on all online CPUs, including idle ones, so use
+ with caution.
+
config RCU_STALL_COMMON
def_bool TREE_RCU
help
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d77921e..7f9ed20 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -180,6 +180,9 @@ static void __init rcu_tasks_bootup_oddness(void)
else
pr_info("\tTasks RCU enabled.\n");
#endif /* #ifdef CONFIG_TASKS_RCU */
+#ifdef CONFIG_TASKS_RUDE_RCU
+ pr_info("\tRude variant of Tasks RCU enabled.\n");
+#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
}

#endif /* #ifndef CONFIG_TINY_RCU */
@@ -410,3 +413,98 @@ static int __init rcu_spawn_tasks_kthread(void)
core_initcall(rcu_spawn_tasks_kthread);

#endif /* #ifdef CONFIG_TASKS_RCU */
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+
+////////////////////////////////////////////////////////////////////////
+//
+// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
+// passing an empty function to schedule_on_each_cpu(). This approach
+// provides an asynchronous call_rcu_tasks_rude() API and batching
+// of concurrent calls to the synchronous synchronize_rcu_rude() API.
+// This sends IPIs far and wide and induces otherwise unnecessary context
+// switches on all online CPUs, whether idle or not.
+
+// Empty function to allow workqueues to force a context switch.
+static void rcu_tasks_be_rude(struct work_struct *work)
+{
+}
+
+// Wait for one rude RCU-tasks grace period.
+static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
+{
+ schedule_on_each_cpu(rcu_tasks_be_rude);
+}
+
+void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude);
+
+/**
+ * call_rcu_tasks_rude() - Queue a callback rude task-based grace period
+ * @rhp: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a full grace
+ * period elapses, in other words after all currently executing RCU
+ * read-side critical sections have completed. call_rcu_tasks_rude()
+ * assumes that the read-side critical sections end at context switch,
+ * cond_resched_rcu_qs(), or transition to usermode execution. As such,
+ * there are no read-side primitives analogous to rcu_read_lock() and
+ * rcu_read_unlock() because this primitive is intended to determine
+ * that all tasks have passed through a safe state, not so much for
+ * data-strcuture synchronization.
+ *
+ * See the description of call_rcu() for more detailed information on
+ * memory ordering guarantees.
+ */
+void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
+{
+ call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
+}
+EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
+
+/**
+ * synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
+ *
+ * Control will return to the caller some time after a rude rcu-tasks
+ * grace period has elapsed, in other words after all currently
+ * executing rcu-tasks read-side critical sections have elapsed. These
+ * read-side critical sections are delimited by calls to schedule(),
+ * cond_resched_tasks_rcu_qs(), userspace execution, and (in theory,
+ * anyway) cond_resched().
+ *
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function preambles
+ * and profiling hooks. The synchronize_rcu_tasks_rude() function is not
+ * (yet) intended for heavy use from multiple CPUs.
+ *
+ * See the description of synchronize_rcu() for more detailed information
+ * on memory ordering guarantees.
+ */
+void synchronize_rcu_tasks_rude(void)
+{
+ synchronize_rcu_tasks_generic(&rcu_tasks_rude);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
+
+/**
+ * rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
+ *
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ */
+void rcu_barrier_tasks_rude(void)
+{
+ /* There is only one callback queue, so this is easy. ;-) */
+ synchronize_rcu_tasks_rude();
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
+
+static int __init rcu_spawn_tasks_rude_kthread(void)
+{
+ rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
+ return 0;
+}
+core_initcall(rcu_spawn_tasks_rude_kthread);
+
+#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
--
2.9.5