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Date: Fri, 20 May 2011 15:46:53 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
yong.zhang0@...il.com, oleg@...hat.com,
linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
mingo@...e.hu, lizf@...fujitsu.com, miaox@...fujitsu.com
Subject: Re: [PATCH v2 1/2] rcu: don't bind offline cpu
On Thu, May 19, 2011 at 11:41:53AM +0200, Peter Zijlstra wrote:
> On Thu, 2011-05-19 at 17:50 +0900, KOSAKI Motohiro wrote:
> > I'm sorry. Please give me a bit time.
>
> N/P, as I've already found out CPU_STARTING isn't a suitable location
> either.
>
> How about something like the below...
>
>
> ---
> Subject: rcu: Remove waitqueue usage for cpu,node and boost kthreads
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Thu May 19 11:27:39 CEST 2011
>
> Since wait-queue usage is inconsistent (node) and generally not needed
> since we know exactly which thread to wake, rip it out.
>
> The problem is that wake_up() only issues an actual wake-up when there's
> a thread waiting on the queue which means we need an extra, explicit,
> wakeup to kick-start the kthread itself (they're born sleeping).
>
> Now, since we already know which exact thread we want to wake up,
> there's not point really in enqueing it on a waitqueue. So provide Paul
> with a nice rcu_wait() macro to replace wait_event_interruptible() with
> and rip out all the waitqueue stuff.
>
> This moves us to an explicit, unconditional, wake_up_process() which can
> be used to wake us from both conditions.
Hello, Peter,
Thank you! I have queued this and will test it.
I am having some difficulty understanding the synchronization in the
rcu_node_kthread() case, where I want to hold the rcu_node structure's
->lock over the "if (cond) break;" in rcu_wait(), but cannot prove that
it is really necessary.
Will look it over again after the jetlag has worn off a bit.
Thanx, Paul
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
> Index: linux-2.6/kernel/rcutree.c
> ===================================================================
> --- linux-2.6.orig/kernel/rcutree.c
> +++ linux-2.6/kernel/rcutree.c
> @@ -94,7 +94,6 @@ static DEFINE_PER_CPU(struct task_struct
> DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
> DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu);
> DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
> -static DEFINE_PER_CPU(wait_queue_head_t, rcu_cpu_wq);
> DEFINE_PER_CPU(char, rcu_cpu_has_work);
> static char rcu_kthreads_spawnable;
>
> @@ -1475,7 +1474,7 @@ static void invoke_rcu_cpu_kthread(void)
> local_irq_restore(flags);
> return;
> }
> - wake_up(&__get_cpu_var(rcu_cpu_wq));
> + wake_up_process(__this_cpu_read(rcu_cpu_kthread_task));
> local_irq_restore(flags);
> }
>
> @@ -1598,14 +1597,12 @@ static int rcu_cpu_kthread(void *arg)
> unsigned long flags;
> int spincnt = 0;
> unsigned int *statusp = &per_cpu(rcu_cpu_kthread_status, cpu);
> - wait_queue_head_t *wqp = &per_cpu(rcu_cpu_wq, cpu);
> char work;
> char *workp = &per_cpu(rcu_cpu_has_work, cpu);
>
> for (;;) {
> *statusp = RCU_KTHREAD_WAITING;
> - wait_event_interruptible(*wqp,
> - *workp != 0 || kthread_should_stop());
> + rcu_wait(*workp != 0 || kthread_should_stop());
> local_bh_disable();
> if (rcu_cpu_kthread_should_stop(cpu)) {
> local_bh_enable();
> @@ -1656,7 +1653,6 @@ static int __cpuinit rcu_spawn_one_cpu_k
> per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
> WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
> per_cpu(rcu_cpu_kthread_task, cpu) = t;
> - wake_up_process(t);
> sp.sched_priority = RCU_KTHREAD_PRIO;
> sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> return 0;
> @@ -1679,7 +1675,7 @@ static int rcu_node_kthread(void *arg)
>
> for (;;) {
> rnp->node_kthread_status = RCU_KTHREAD_WAITING;
> - wait_event_interruptible(rnp->node_wq, rnp->wakemask != 0);
> + rcu_wait(rnp->wakemask != 0);
> rnp->node_kthread_status = RCU_KTHREAD_RUNNING;
> raw_spin_lock_irqsave(&rnp->lock, flags);
> mask = rnp->wakemask;
> @@ -1764,7 +1760,6 @@ static int __cpuinit rcu_spawn_one_node_
> raw_spin_lock_irqsave(&rnp->lock, flags);
> rnp->node_kthread_task = t;
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> - wake_up_process(t);
> sp.sched_priority = 99;
> sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> }
> @@ -1781,21 +1776,16 @@ static int __init rcu_spawn_kthreads(voi
>
> rcu_kthreads_spawnable = 1;
> for_each_possible_cpu(cpu) {
> - init_waitqueue_head(&per_cpu(rcu_cpu_wq, cpu));
> per_cpu(rcu_cpu_has_work, cpu) = 0;
> if (cpu_online(cpu))
> (void)rcu_spawn_one_cpu_kthread(cpu);
> }
> rnp = rcu_get_root(rcu_state);
> - init_waitqueue_head(&rnp->node_wq);
> - rcu_init_boost_waitqueue(rnp);
> (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
> - if (NUM_RCU_NODES > 1)
> - rcu_for_each_leaf_node(rcu_state, rnp) {
> - init_waitqueue_head(&rnp->node_wq);
> - rcu_init_boost_waitqueue(rnp);
> + if (NUM_RCU_NODES > 1) {
> + rcu_for_each_leaf_node(rcu_state, rnp)
> (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
> - }
> + }
> return 0;
> }
> early_initcall(rcu_spawn_kthreads);
> Index: linux-2.6/kernel/rcutree.h
> ===================================================================
> --- linux-2.6.orig/kernel/rcutree.h
> +++ linux-2.6/kernel/rcutree.h
> @@ -28,6 +28,17 @@
> #include <linux/cpumask.h>
> #include <linux/seqlock.h>
>
> +#define rcu_wait(cond) \
> +do { \
> + for (;;) { \
> + set_current_state(TASK_INTERRUPTIBLE); \
> + if (cond) \
> + break; \
> + schedule(); \
> + } \
> + __set_current_state(TASK_RUNNING); \
> +} while (0)
> +
> /*
> * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
> * In theory, it should be possible to add more levels straightforwardly.
> @@ -157,9 +168,6 @@ struct rcu_node {
> struct task_struct *boost_kthread_task;
> /* kthread that takes care of priority */
> /* boosting for this rcu_node structure. */
> - wait_queue_head_t boost_wq;
> - /* Wait queue on which to park the boost */
> - /* kthread. */
> unsigned int boost_kthread_status;
> /* State of boost_kthread_task for tracing. */
> unsigned long n_tasks_boosted;
> @@ -186,9 +194,6 @@ struct rcu_node {
> /* kthread that takes care of this rcu_node */
> /* structure, for example, awakening the */
> /* per-CPU kthreads as needed. */
> - wait_queue_head_t node_wq;
> - /* Wait queue on which to park the per-node */
> - /* kthread. */
> unsigned int node_kthread_status;
> /* State of node_kthread_task for tracing. */
> } ____cacheline_internodealigned_in_smp;
> @@ -443,7 +448,6 @@ static void __cpuinit rcu_preempt_init_p
> static void rcu_preempt_send_cbs_to_online(void);
> static void __init __rcu_init_preempt(void);
> static void rcu_needs_cpu_flush(void);
> -static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp);
> static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
> static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
> cpumask_var_t cm);
> Index: linux-2.6/kernel/rcutree_plugin.h
> ===================================================================
> --- linux-2.6.orig/kernel/rcutree_plugin.h
> +++ linux-2.6/kernel/rcutree_plugin.h
> @@ -1196,8 +1196,7 @@ static int rcu_boost_kthread(void *arg)
>
> for (;;) {
> rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
> - wait_event_interruptible(rnp->boost_wq, rnp->boost_tasks ||
> - rnp->exp_tasks);
> + rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
> rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
> more2boost = rcu_boost(rnp);
> if (more2boost)
> @@ -1275,14 +1274,6 @@ static void rcu_preempt_boost_start_gp(s
> }
>
> /*
> - * Initialize the RCU-boost waitqueue.
> - */
> -static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp)
> -{
> - init_waitqueue_head(&rnp->boost_wq);
> -}
> -
> -/*
> * Create an RCU-boost kthread for the specified node if one does not
> * already exist. We only create this kthread for preemptible RCU.
> * Returns zero if all is well, a negated errno otherwise.
> @@ -1306,7 +1297,6 @@ static int __cpuinit rcu_spawn_one_boost
> raw_spin_lock_irqsave(&rnp->lock, flags);
> rnp->boost_kthread_task = t;
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> - wake_up_process(t);
> sp.sched_priority = RCU_KTHREAD_PRIO;
> sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> return 0;
> @@ -1328,10 +1318,6 @@ static void rcu_preempt_boost_start_gp(s
> {
> }
>
> -static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp)
> -{
> -}
> -
> static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
> struct rcu_node *rnp,
> int rnp_index)
>
--
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