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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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