rcu-tasks: Add an RCU-tasks rude variant
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. ]
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@ -37,6 +37,7 @@
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/* Exported common interfaces */
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void call_rcu(struct rcu_head *head, rcu_callback_t func);
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void rcu_barrier_tasks(void);
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void rcu_barrier_tasks_rude(void);
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void synchronize_rcu(void);
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#ifdef CONFIG_PREEMPT_RCU
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@ -138,6 +139,8 @@ static inline void rcu_init_nohz(void) { }
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#define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
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void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
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void synchronize_rcu_tasks(void);
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void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
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void synchronize_rcu_tasks_rude(void);
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void exit_tasks_rcu_start(void);
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void exit_tasks_rcu_finish(void);
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#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
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@ -71,7 +71,7 @@ config TREE_SRCU
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This option selects the full-fledged version of SRCU.
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config TASKS_RCU_GENERIC
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def_bool TASKS_RCU
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def_bool TASKS_RCU || TASKS_RUDE_RCU
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select SRCU
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help
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This option enables generic infrastructure code supporting
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@ -84,6 +84,15 @@ config TASKS_RCU
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only voluntary context switch (not preemption!), idle, and
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user-mode execution as quiescent states. Not for manual selection.
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config TASKS_RUDE_RCU
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def_bool 0
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help
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This option enables a task-based RCU implementation that uses
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only context switch (including preemption) and user-mode
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execution as quiescent states. It forces IPIs and context
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switches on all online CPUs, including idle ones, so use
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with caution.
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config RCU_STALL_COMMON
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def_bool TREE_RCU
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help
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@ -180,6 +180,9 @@ static void __init rcu_tasks_bootup_oddness(void)
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else
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pr_info("\tTasks RCU enabled.\n");
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#endif /* #ifdef CONFIG_TASKS_RCU */
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#ifdef CONFIG_TASKS_RUDE_RCU
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pr_info("\tRude variant of Tasks RCU enabled.\n");
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#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
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}
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#endif /* #ifndef CONFIG_TINY_RCU */
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@ -410,3 +413,98 @@ static int __init rcu_spawn_tasks_kthread(void)
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core_initcall(rcu_spawn_tasks_kthread);
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#endif /* #ifdef CONFIG_TASKS_RCU */
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#ifdef CONFIG_TASKS_RUDE_RCU
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////////////////////////////////////////////////////////////////////////
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//
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// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
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// passing an empty function to schedule_on_each_cpu(). This approach
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// provides an asynchronous call_rcu_tasks_rude() API and batching
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// of concurrent calls to the synchronous synchronize_rcu_rude() API.
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// This sends IPIs far and wide and induces otherwise unnecessary context
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// switches on all online CPUs, whether idle or not.
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// Empty function to allow workqueues to force a context switch.
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static void rcu_tasks_be_rude(struct work_struct *work)
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{
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}
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// Wait for one rude RCU-tasks grace period.
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static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
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{
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schedule_on_each_cpu(rcu_tasks_be_rude);
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}
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void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
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DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude);
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/**
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* call_rcu_tasks_rude() - Queue a callback rude task-based grace period
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* @rhp: structure to be used for queueing the RCU updates.
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* @func: actual callback function to be invoked after the grace period
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*
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* The callback function will be invoked some time after a full grace
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* period elapses, in other words after all currently executing RCU
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* read-side critical sections have completed. call_rcu_tasks_rude()
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* assumes that the read-side critical sections end at context switch,
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* cond_resched_rcu_qs(), or transition to usermode execution. As such,
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* there are no read-side primitives analogous to rcu_read_lock() and
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* rcu_read_unlock() because this primitive is intended to determine
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* that all tasks have passed through a safe state, not so much for
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* data-strcuture synchronization.
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*
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* See the description of call_rcu() for more detailed information on
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* memory ordering guarantees.
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*/
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void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
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{
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call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
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}
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EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
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/**
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* synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
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*
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* Control will return to the caller some time after a rude rcu-tasks
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* grace period has elapsed, in other words after all currently
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* executing rcu-tasks read-side critical sections have elapsed. These
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* read-side critical sections are delimited by calls to schedule(),
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* cond_resched_tasks_rcu_qs(), userspace execution, and (in theory,
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* anyway) cond_resched().
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*
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* This is a very specialized primitive, intended only for a few uses in
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* tracing and other situations requiring manipulation of function preambles
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* and profiling hooks. The synchronize_rcu_tasks_rude() function is not
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* (yet) intended for heavy use from multiple CPUs.
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*
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* See the description of synchronize_rcu() for more detailed information
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* on memory ordering guarantees.
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*/
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void synchronize_rcu_tasks_rude(void)
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{
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synchronize_rcu_tasks_generic(&rcu_tasks_rude);
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}
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EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
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/**
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* rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
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*
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* Although the current implementation is guaranteed to wait, it is not
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* obligated to, for example, if there are no pending callbacks.
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*/
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void rcu_barrier_tasks_rude(void)
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{
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/* There is only one callback queue, so this is easy. ;-) */
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synchronize_rcu_tasks_rude();
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}
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EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
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static int __init rcu_spawn_tasks_rude_kthread(void)
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{
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rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
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return 0;
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}
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core_initcall(rcu_spawn_tasks_rude_kthread);
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#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
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