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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. ]
This commit is contained in:
Paul E. McKenney 2020-03-02 21:06:43 -08:00
parent 5873b8a94e
commit c84aad7654
3 changed files with 111 additions and 1 deletions
include/linux
kernel/rcu

View File

@ -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 */

View File

@ -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

View File

@ -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 */