Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 02/23/2011 09:39 AM, Paul E. McKenney wrote:
> From: Paul E. McKenney <paul.mckenney@linaro.org>
> 
> If RCU priority boosting is to be meaningful, callback invocation must
> be boosted in addition to preempted RCU readers.  Otherwise, in presence
> of CPU real-time threads, the grace period ends, but the callbacks don't
> get invoked.  If the callbacks don't get invoked, the associated memory
> doesn't get freed, so the system is still subject to OOM.
> 
> But it is not reasonable to priority-boost RCU_SOFTIRQ, so this commit
> moves the callback invocations to a kthread, which can be boosted easily.
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/interrupt.h           |    1 -
>  include/trace/events/irq.h          |    3 +-
>  kernel/rcutree.c                    |  324 ++++++++++++++++++++++++++++++++++-
>  kernel/rcutree.h                    |    8 +
>  kernel/rcutree_plugin.h             |    4 +-
>  tools/perf/util/trace-event-parse.c |    1 -
>  6 files changed, 331 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 79d0c4f..ed47deb 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -385,7 +385,6 @@ enum
>  	TASKLET_SOFTIRQ,
>  	SCHED_SOFTIRQ,
>  	HRTIMER_SOFTIRQ,
> -	RCU_SOFTIRQ,	/* Preferable RCU should always be the last softirq */
>  
>  	NR_SOFTIRQS
>  };
> diff --git a/include/trace/events/irq.h b/include/trace/events/irq.h
> index 1c09820..ae045ca 100644
> --- a/include/trace/events/irq.h
> +++ b/include/trace/events/irq.h
> @@ -20,8 +20,7 @@ struct softirq_action;
>  			 softirq_name(BLOCK_IOPOLL),	\
>  			 softirq_name(TASKLET),		\
>  			 softirq_name(SCHED),		\
> -			 softirq_name(HRTIMER),		\
> -			 softirq_name(RCU))
> +			 softirq_name(HRTIMER))
>  
>  /**
>   * irq_handler_entry - called immediately before the irq action handler
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 0ac1cc0..2241f28 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -47,6 +47,8 @@
>  #include <linux/mutex.h>
>  #include <linux/time.h>
>  #include <linux/kernel_stat.h>
> +#include <linux/wait.h>
> +#include <linux/kthread.h>
>  
>  #include "rcutree.h"
>  
> @@ -82,6 +84,18 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
>  int rcu_scheduler_active __read_mostly;
>  EXPORT_SYMBOL_GPL(rcu_scheduler_active);
>  
> +/* Control variables for per-CPU and per-rcu_node kthreads. */

I think "per-leaf-rcu_node" is better. It seems that only the leaf rcu_node
of rcu_sched are used for rcu_node kthreads and they also serve for
other rcu domains(rcu_bh, rcu_preempt)? I think we need to add some
comments for it.

> +/*
> + * Timer handler to initiate the waking up of per-CPU kthreads that
> + * have yielded the CPU due to excess numbers of RCU callbacks.
> + */
> +static void rcu_cpu_kthread_timer(unsigned long arg)
> +{
> +	unsigned long flags;
> +	struct rcu_data *rdp = (struct rcu_data *)arg;
> +	struct rcu_node *rnp = rdp->mynode;
> +	struct task_struct *t;
> +
> +	raw_spin_lock_irqsave(&rnp->lock, flags);
> +	rnp->wakemask |= rdp->grpmask;

I think there is no reason that the rnp->lock also protects the
rnp->node_kthread_task. "raw_spin_unlock_irqrestore(&rnp->lock, flags);"
can be moved up here.

> +	t = rnp->node_kthread_task;
> +	if (t == NULL) {
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +		return;
> +	}
> +	wake_up_process(t);
> +	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +}
> +
> +/*
> + * Drop to non-real-time priority and yield, but only after posting a
> + * timer that will cause us to regain our real-time priority if we
> + * remain preempted.  Either way, we restore our real-time priority
> + * before returning.
> + */
> +static void rcu_yield(int cpu)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(rcu_sched_state.rda, cpu);
> +	struct sched_param sp;
> +	struct timer_list yield_timer;
> +
> +	setup_timer(&yield_timer, rcu_cpu_kthread_timer, (unsigned long)rdp);
> +	mod_timer(&yield_timer, jiffies + 2);
> +	sp.sched_priority = 0;
> +	sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
> +	schedule();
> +	sp.sched_priority = RCU_KTHREAD_PRIO;
> +	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
> +	del_timer(&yield_timer);
> +}
> +
> +/*
> + * Handle cases where the rcu_cpu_kthread() ends up on the wrong CPU.
> + * This can happen while the corresponding CPU is either coming online
> + * or going offline.  We cannot wait until the CPU is fully online
> + * before starting the kthread, because the various notifier functions
> + * can wait for RCU grace periods.  So we park rcu_cpu_kthread() until
> + * the corresponding CPU is online.
> + *
> + * Return 1 if the kthread needs to stop, 0 otherwise.
> + *
> + * Caller must disable bh.  This function can momentarily enable it.
> + */
> +static int rcu_cpu_kthread_should_stop(int cpu)
> +{
> +	while (cpu_is_offline(cpu) || smp_processor_id() != cpu) {
> +		if (kthread_should_stop())
> +			return 1;
> +		local_bh_enable();
> +		schedule_timeout_uninterruptible(1);
> +		if (smp_processor_id() != cpu)
> +			set_cpus_allowed_ptr(current, cpumask_of(cpu));

The current task is PF_THREAD_BOUND,
Why do "set_cpus_allowed_ptr(current, cpumask_of(cpu));" ?

> +		local_bh_disable();
> +	}
> +	return 0;
> +}
> +
> +/*
> + * Per-CPU kernel thread that invokes RCU callbacks.  This replaces the
> + * earlier RCU softirq.
> + */
> +static int rcu_cpu_kthread(void *arg)
> +{
> +	int cpu = (int)(long)arg;
> +	unsigned long flags;
> +	int spincnt = 0;
> +	wait_queue_head_t *wqp = &per_cpu(rcu_cpu_wq, cpu);
> +	char work;
> +	char *workp = &per_cpu(rcu_cpu_has_work, cpu);
> +
> +	for (;;) {
> +		wait_event_interruptible(*wqp,
> +					 *workp != 0 || kthread_should_stop());
> +		local_bh_disable();
> +		if (rcu_cpu_kthread_should_stop(cpu)) {
> +			local_bh_enable();
> +			break;
> +		}
> +		local_irq_save(flags);
> +		work = *workp;
> +		*workp = 0;
> +		local_irq_restore(flags);
> +		if (work)
> +			rcu_process_callbacks();
> +		local_bh_enable();
> +		if (*workp != 0)
> +			spincnt++;
> +		else
> +			spincnt = 0;
> +		if (spincnt > 10) {

"10" is a magic number here.

> +			rcu_yield(cpu);
> +			spincnt = 0;
> +		}
> +	}
> +	return 0;
> +}
> +


> +/*
> + * Per-rcu_node kthread, which is in charge of waking up the per-CPU
> + * kthreads when needed.
> + */
> +static int rcu_node_kthread(void *arg)
> +{
> +	int cpu;
> +	unsigned long flags;
> +	unsigned long mask;
> +	struct rcu_node *rnp = (struct rcu_node *)arg;
> +	struct sched_param sp;
> +	struct task_struct *t;
> +
> +	for (;;) {
> +		wait_event_interruptible(rnp->node_wq, rnp->wakemask != 0 ||
> +						       kthread_should_stop());
> +		if (kthread_should_stop())
> +			break;
> +		raw_spin_lock_irqsave(&rnp->lock, flags);
> +		mask = rnp->wakemask;
> +		rnp->wakemask = 0;
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
> +			if ((mask & 0x1) == 0)
> +				continue;
> +			preempt_disable();
> +			per_cpu(rcu_cpu_has_work, cpu) = 1;
> +			t = per_cpu(rcu_cpu_kthread_task, cpu);
> +			if (t == NULL) {
> +				preempt_enable();
> +				continue;
> +			}

Obviously preempt_disable() is not for protecting remote percpu data.
Is it for disabling cpu hotplug? I am afraid the @t may leave
and become invalid.