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Message-ID: <CAJhHMCDXm7Zu0cehHzO2OKoZRm0oFqAgA5ZOM=_OJ3=uc3+79w@mail.gmail.com>
Date:	Thu, 14 Aug 2014 16:46:34 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v5 tip/core/rcu 01/16] rcu: Add call_rcu_tasks()

On Mon, Aug 11, 2014 at 6:48 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paulmck@...ux.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-tolerant
> 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@...dmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Please find comments below. I did not read all the ~100 emails in this
series, so please forgive if I ask something repetitive and just point
that out. I will go digging :)

> ---
>  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

rcu_tasks_holdout is defined as an int. So use 0 may be?

I see that there are other locations which set it to 'false'. So may
just change the definition to bool, as it seems more appropriate.

Also why is rcu_tasks_nvcsw not being initialized? I see that it can
be read before initialized, no?

>
>  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

-ENOPARSE :(

> + * @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.

s/strcuture/structure

> + *
> + * 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 */
>  }

I think rcu_tasks_nvcsw needs to be set here too.

>
> -#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.
> +
>  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..f6f164119a14 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.
> + */
> +
> +/* 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);
> +
> +/* See if tasks are still holding out, complain if so. */
> +static void check_holdout_task(struct task_struct *t)
> +{
> +       if (!ACCESS_ONCE(t->rcu_tasks_holdout) ||
> +           t->rcu_tasks_nvcsw != ACCESS_ONCE(t->nvcsw) ||
> +           !ACCESS_ONCE(t->on_rq)) {
> +               ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> +               list_del_rcu(&t->rcu_tasks_holdout_list);
> +               put_task_struct(t);
> +       }
> +}
> +

I don't see a WARN() for the "complain if so" part. :)


> +/* 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;
> +       LIST_HEAD(rcu_tasks_holdouts);
> +
> +       /* 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);
> +               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);
> +                       WARN_ON(signal_pending(current));
> +                       continue;
> +               }

Why not use a wait queue here? Since this is called very infrequently,
it should be a win when compared to periodically waking up and
checking, no?

> +
> +               /*
> +                * 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();

I don't see why this is a read side critical section. What am I missing?

> +
> +               /*
> +                * 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);
> +                       WARN_ON(signal_pending(current));
> +                       rcu_read_lock();
> +                       list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> +                                               rcu_tasks_holdout_list)
> +                               check_holdout_task(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;

I think adding a prefetch(next) here should be helpful.

> +                       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
>



-- 
Pranith
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