Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Frederic Weisbecker
2014-07-31T23:57:58Z

On Thu, Jul 31, 2014 at 02:55:01PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> This commit adds a new RCU-tasks flavor of RCU, which provides
> call_rcu_tasks(). This RCU flavor's quiescent states are voluntary
> context switch (not preemption!), userspace execution, and the idle loop.
> Note that unlike other RCU flavors, these quiescent states occur in tasks,
> not necessarily CPUs. Includes fixes from Steven Rostedt.
>
> This RCU flavor is assumed to have very infrequent latency-tolerate
> updaters. This assumption permits significant simplifications, including
> a single global callback list protected by a single global lock, along
> with a single linked list containing all tasks that have not yet passed
> through a quiescent state. If experience shows this assumption to be
> incorrect, the required additional complexity will be added.
>
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> include/linux/init_task.h | 9 +++
> include/linux/rcupdate.h | 36 ++++++++++
> include/linux/sched.h | 23 ++++---
> init/Kconfig | 10 +++
> kernel/rcu/tiny.c | 2 +
> kernel/rcu/tree.c | 2 +
> kernel/rcu/update.c | 171 ++++++++++++++++++++++++++++++++++++++++++++++
> 7 files changed, 242 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index 6df7f9fe0d01..78715ea7c30c 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -124,6 +124,14 @@ extern struct group_info init_groups;
> #else
> #define INIT_TASK_RCU_PREEMPT(tsk)
> #endif
> +#ifdef CONFIG_TASKS_RCU
> +#define INIT_TASK_RCU_TASKS(tsk) \
> + .rcu_tasks_holdout = false, \
> + .rcu_tasks_holdout_list = \
> + LIST_HEAD_INIT(tsk.rcu_tasks_holdout_list),
> +#else
> +#define INIT_TASK_RCU_TASKS(tsk)
> +#endif
>
> extern struct cred init_cred;
>
> @@ -231,6 +239,7 @@ extern struct task_group root_task_group;
> INIT_FTRACE_GRAPH \
> INIT_TRACE_RECURSION \
> INIT_TASK_RCU_PREEMPT(tsk) \
> + INIT_TASK_RCU_TASKS(tsk) \
> INIT_CPUSET_SEQ(tsk) \
> INIT_RT_MUTEXES(tsk) \
> INIT_VTIME(tsk) \
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 6a94cc8b1ca0..829efc99df3e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -197,6 +197,26 @@ void call_rcu_sched(struct rcu_head *head,
>
> void synchronize_sched(void);
>
> +/**
> + * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
> + * @head: 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() assumes
> + * that the read-side critical sections end at a voluntary context
> + * switch (not a preemption!), entry into idle, 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(struct rcu_head *head, void (*func)(struct rcu_head *head));
> +
> #ifdef CONFIG_PREEMPT_RCU
>
> void __rcu_read_lock(void);
> @@ -294,6 +314,22 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
> rcu_irq_exit(); \
> } while (0)
>
> +/*
> + * Note a voluntary context switch for RCU-tasks benefit. This is a
> + * macro rather than an inline function to avoid #include hell.
> + */
> +#ifdef CONFIG_TASKS_RCU
> +#define rcu_note_voluntary_context_switch(t) \
> + do { \
> + preempt_disable(); /* Exclude synchronize_sched(); */ \
> + if (ACCESS_ONCE((t)->rcu_tasks_holdout)) \
> + ACCESS_ONCE((t)->rcu_tasks_holdout) = 0; \
> + preempt_enable(); \
> + } while (0)
> +#else /* #ifdef CONFIG_TASKS_RCU */
> +#define rcu_note_voluntary_context_switch(t) do { } while (0)
> +#endif /* #else #ifdef CONFIG_TASKS_RCU */
> +
> #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)
> bool __rcu_is_watching(void);
> #endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 306f4f0c987a..3cf124389ec7 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1273,6 +1273,11 @@ struct task_struct {
> #ifdef CONFIG_RCU_BOOST
> struct rt_mutex *rcu_boost_mutex;
> #endif /* #ifdef CONFIG_RCU_BOOST */
> +#ifdef CONFIG_TASKS_RCU
> + unsigned long rcu_tasks_nvcsw;
> + int rcu_tasks_holdout;
> + struct list_head rcu_tasks_holdout_list;
> +#endif /* #ifdef CONFIG_TASKS_RCU */
>
> #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> struct sched_info sched_info;
> @@ -1998,31 +2003,27 @@ extern void task_clear_jobctl_pending(struct task_struct *task,
> unsigned int mask);
>
> #ifdef CONFIG_PREEMPT_RCU
> -
> #define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
> #define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
> +#endif /* #ifdef CONFIG_PREEMPT_RCU */
>
> static inline void rcu_copy_process(struct task_struct *p)
> {
> +#ifdef CONFIG_PREEMPT_RCU
> p->rcu_read_lock_nesting = 0;
> p->rcu_read_unlock_special = 0;
> -#ifdef CONFIG_TREE_PREEMPT_RCU
> p->rcu_blocked_node = NULL;
> -#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
> #ifdef CONFIG_RCU_BOOST
> p->rcu_boost_mutex = NULL;
> #endif /* #ifdef CONFIG_RCU_BOOST */
> INIT_LIST_HEAD(&p->rcu_node_entry);
> +#endif /* #ifdef CONFIG_PREEMPT_RCU */
> +#ifdef CONFIG_TASKS_RCU
> + p->rcu_tasks_holdout = false;
> + INIT_LIST_HEAD(&p->rcu_tasks_holdout_list);
> +#endif /* #ifdef CONFIG_TASKS_RCU */
> }
>
> -#else
> -
> -static inline void rcu_copy_process(struct task_struct *p)
> -{
> -}
> -
> -#endif
> -
> static inline void tsk_restore_flags(struct task_struct *task,
> unsigned long orig_flags, unsigned long flags)
> {
> diff --git a/init/Kconfig b/init/Kconfig
> index 9d76b99af1b9..c56cb62a2df1 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -507,6 +507,16 @@ config PREEMPT_RCU
> This option enables preemptible-RCU code that is common between
> the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
>
> +config TASKS_RCU
> + bool "Task_based RCU implementation using voluntary context switch"
> + default n
> + help
> + This option enables a task-based RCU implementation that uses
> + only voluntary context switch (not preemption!), idle, and
> + user-mode execution as quiescent states.
> +
> + If unsure, say N.

I don't remember who said that, but indeed this is a pure internal feature
only. The user doesn't need to select that option ever.

> +
> config RCU_STALL_COMMON
> def_bool ( TREE_RCU || TREE_PREEMPT_RCU || RCU_TRACE )
> help
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index d9efcc13008c..717f00854fc0 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -254,6 +254,8 @@ void rcu_check_callbacks(int cpu, int user)
> rcu_sched_qs(cpu);
> else if (!in_softirq())
> rcu_bh_qs(cpu);
> + if (user)
> + rcu_note_voluntary_context_switch(current);
> }
>
> /*
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 625d0b0cd75a..f958c52f644d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2413,6 +2413,8 @@ void rcu_check_callbacks(int cpu, int user)
> rcu_preempt_check_callbacks(cpu);
> if (rcu_pending(cpu))
> invoke_rcu_core();
> + if (user)
> + rcu_note_voluntary_context_switch(current);
> trace_rcu_utilization(TPS("End scheduler-tick"));
> }
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index bc7883570530..50453589e3ca 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -47,6 +47,7 @@
> #include <linux/hardirq.h>
> #include <linux/delay.h>
> #include <linux/module.h>
> +#include <linux/kthread.h>
>
> #define CREATE_TRACE_POINTS
>
> @@ -350,3 +351,173 @@ static int __init check_cpu_stall_init(void)
> early_initcall(check_cpu_stall_init);
>
> #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
> +
> +#ifdef CONFIG_TASKS_RCU
> +
> +/*
> + * Simple variant of RCU whose quiescent states are voluntary context switch,
> + * user-space execution, and idle. As such, grace periods can take one good
> + * long time. There are no read-side primitives similar to rcu_read_lock()
> + * and rcu_read_unlock() because this implementation is intended to get
> + * the system into a safe state for some of the manipulations involved in
> + * tracing and the like. Finally, this implementation does not support
> + * high call_rcu_tasks() rates from multiple CPUs. If this is required,
> + * per-CPU callback lists will be needed.
> + */
> +
> +/* Lists of tasks that we are still waiting for during this grace period. */
> +static LIST_HEAD(rcu_tasks_holdouts);
> +
> +/* Global list of callbacks and associated lock. */
> +static struct rcu_head *rcu_tasks_cbs_head;
> +static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
> +static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
> +
> +/* Post an RCU-tasks callback. */
> +void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
> +{
> + unsigned long flags;
> +
> + rhp->next = NULL;
> + rhp->func = func;
> + raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> + *rcu_tasks_cbs_tail = rhp;
> + rcu_tasks_cbs_tail = &rhp->next;
> + raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(call_rcu_tasks);
> +
> +/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
> +static int __noreturn rcu_tasks_kthread(void *arg)
> +{
> + unsigned long flags;
> + struct task_struct *g, *t;
> + struct rcu_head *list;
> + struct rcu_head *next;
> +
> + /* FIXME: Add housekeeping affinity. */
> +
> + /*
> + * Each pass through the following loop makes one check for
> + * newly arrived callbacks, and, if there are some, waits for
> + * one RCU-tasks grace period and then invokes the callbacks.
> + * This loop is terminated by the system going down. ;-)
> + */
> + for (;;) {
> +
> + /* Pick up any new callbacks. */
> + raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> + smp_mb__after_unlock_lock(); /* Enforce GP memory ordering. */

I have no idea which against what this is exactly ordering. GP is a vast thing.
Especially for tricky barriers like __after_unlock_lock() which suggest very
counter-intuitive ordering, a detailed comment is very welcome :)

> + list = rcu_tasks_cbs_head;
> + rcu_tasks_cbs_head = NULL;
> + rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
> + raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> +
> + /* If there were none, wait a bit and start over. */
> + if (!list) {
> + schedule_timeout_interruptible(HZ);

So this thread is going to poll every second? I guess something prevents it
to run when system is idle somewhere? But I'm not familiar with the whole patchset
yet. But even without that it looks like a very annoying noise. why not use something
wait/wakeup based?

> + flush_signals(current);
> + continue;
> + }
> +
> + /*
> + * Wait for all pre-existing t->on_rq and t->nvcsw
> + * transitions to complete. Invoking synchronize_sched()
> + * suffices because all these transitions occur with
> + * interrupts disabled. Without this synchronize_sched(),
> + * a read-side critical section that started before the
> + * grace period might be incorrectly seen as having started
> + * after the grace period.
> + *
> + * This synchronize_sched() also dispenses with the
> + * need for a memory barrier on the first store to
> + * ->rcu_tasks_holdout, as it forces the store to happen
> + * after the beginning of the grace period.
> + */
> + synchronize_sched();
> +
> + /*
> + * There were callbacks, so we need to wait for an
> + * RCU-tasks grace period. Start off by scanning
> + * the task list for tasks that are not already
> + * voluntarily blocked. Mark these tasks and make
> + * a list of them in rcu_tasks_holdouts.
> + */
> + rcu_read_lock();
> + for_each_process_thread(g, t) {
> + if (t != current && ACCESS_ONCE(t->on_rq) &&
> + !is_idle_task(t)) {
> + get_task_struct(t);
> + t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> + ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
> + list_add(&t->rcu_tasks_holdout_list,
> + &rcu_tasks_holdouts);
> + }
> + }
> + rcu_read_unlock();
> +
> + /*
> + * Each pass through the following loop scans the list
> + * of holdout tasks, removing any that are no longer
> + * holdouts. When the list is empty, we are done.
> + */
> + while (!list_empty(&rcu_tasks_holdouts)) {
> + schedule_timeout_interruptible(HZ / 10);

OTOH here it is not annoying because it should only happen when rcu task
is used, which should be rare.

Thanks.

> + flush_signals(current);
> + rcu_read_lock();
> + list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> + rcu_tasks_holdout_list) {
> + if (ACCESS_ONCE(t->rcu_tasks_holdout)) {
> + if (t->rcu_tasks_nvcsw ==
> + ACCESS_ONCE(t->nvcsw) &&
> + ACCESS_ONCE(t->on_rq))
> + continue;
> + ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> + }
> + list_del_rcu(&t->rcu_tasks_holdout_list);
> + put_task_struct(t);
> + }
> + rcu_read_unlock();
> + }
> +
> + /*
> + * Because ->on_rq and ->nvcsw are not guaranteed
> + * to have a full memory barriers prior to them in the
> + * schedule() path, memory reordering on other CPUs could
> + * cause their RCU-tasks read-side critical sections to
> + * extend past the end of the grace period. However,
> + * because these ->nvcsw updates are carried out with
> + * interrupts disabled, we can use synchronize_sched()
> + * to force the needed ordering on all such CPUs.
> + *
> + * This synchronize_sched() also confines all
> + * ->rcu_tasks_holdout accesses to be within the grace
> + * period, avoiding the need for memory barriers for
> + * ->rcu_tasks_holdout accesses.
> + */
> + synchronize_sched();
> +
> + /* Invoke the callbacks. */
> + while (list) {
> + next = list->next;
> + local_bh_disable();
> + list->func(list);
> + local_bh_enable();
> + list = next;
> + cond_resched();
> + }
> + }
> +}
> +
> +/* Spawn rcu_tasks_kthread() at boot time. */
> +static int __init rcu_spawn_tasks_kthread(void)
> +{
> + struct task_struct __maybe_unused *t;
> +
> + t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
> + BUG_ON(IS_ERR(t));
> + return 0;
> +}
> +early_initcall(rcu_spawn_tasks_kthread);
> +
> +#endif /* #ifdef CONFIG_TASKS_RCU */
> --
> 1.8.1.5
>