Re: [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 11:18:16AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> As the first step towards allowing grace-period initialization to be
> preemptible, this commit moves the RCU grace-period initialization
> into its own kthread.  This is needed to keep large-system scheduling
> latency at reasonable levels.
> 
> Reported-by: Mike Galbraith <mgalbraith@suse.de>
> Reported-by: Dimitri Sivanich <sivanich@sgi.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcutree.c |  191 ++++++++++++++++++++++++++++++++++++------------------
>  kernel/rcutree.h |    3 +
>  2 files changed, 130 insertions(+), 64 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index f280e54..e1c5868 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1040,6 +1040,103 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
>  }
>  
>  /*
> + * Body of kthread that handles grace periods.
> + */
> +static int rcu_gp_kthread(void *arg)
> +{
> +	unsigned long flags;
> +	struct rcu_data *rdp;
> +	struct rcu_node *rnp;
> +	struct rcu_state *rsp = arg;
> +
> +	for (;;) {
> +
> +		/* Handle grace-period start. */
> +		rnp = rcu_get_root(rsp);
> +		for (;;) {
> +			wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
> +			if (rsp->gp_flags)
> +				break;
> +			flush_signals(current);
> +		}
> +		raw_spin_lock_irqsave(&rnp->lock, flags);
> +		rsp->gp_flags = 0;
> +		rdp = this_cpu_ptr(rsp->rda);
> +
> +		if (rcu_gp_in_progress(rsp)) {
> +			/*
> +			 * A grace period is already in progress, so
> +			 * don't start another one.
> +			 */
> +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +			continue;
> +		}
> +
> +		if (rsp->fqs_active) {
> +			/*
> +			 * We need a grace period, but force_quiescent_state()
> +			 * is running.  Tell it to start one on our behalf.
> +			 */
> +			rsp->fqs_need_gp = 1;
> +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +			continue;
> +		}
> +
> +		/* Advance to a new grace period and initialize state. */
> +		rsp->gpnum++;
> +		trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
> +		WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
> +		rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */
> +		rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
> +		record_gp_stall_check_time(rsp);
> +		raw_spin_unlock(&rnp->lock);  /* leave irqs disabled. */
> +
> +		/* Exclude any concurrent CPU-hotplug operations. */
> +		raw_spin_lock(&rsp->onofflock);  /* irqs already disabled. */
> +
> +		/*
> +		 * Set the quiescent-state-needed bits in all the rcu_node
> +		 * structures for all currently online CPUs in breadth-first
> +		 * order, starting from the root rcu_node structure.
> +		 * This operation relies on the layout of the hierarchy
> +		 * within the rsp->node[] array.  Note that other CPUs will
> +		 * access only the leaves of the hierarchy, which still
> +		 * indicate that no grace period is in progress, at least
> +		 * until the corresponding leaf node has been initialized.
> +		 * In addition, we have excluded CPU-hotplug operations.
> +		 *
> +		 * Note that the grace period cannot complete until
> +		 * we finish the initialization process, as there will
> +		 * be at least one qsmask bit set in the root node until
> +		 * that time, namely the one corresponding to this CPU,
> +		 * due to the fact that we have irqs disabled.
> +		 */
> +		rcu_for_each_node_breadth_first(rsp, rnp) {
> +			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +			rcu_preempt_check_blocked_tasks(rnp);
> +			rnp->qsmask = rnp->qsmaskinit;
> +			rnp->gpnum = rsp->gpnum;
> +			rnp->completed = rsp->completed;
> +			if (rnp == rdp->mynode)
> +				rcu_start_gp_per_cpu(rsp, rnp, rdp);
> +			rcu_preempt_boost_start_gp(rnp);
> +			trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
> +						    rnp->level, rnp->grplo,
> +						    rnp->grphi, rnp->qsmask);
> +			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +		}
> +
> +		rnp = rcu_get_root(rsp);
> +		raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +		/* force_quiescent_state() now OK. */
> +		rsp->fqs_state = RCU_SIGNAL_INIT;
> +		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +		raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> +	}
> +	return 0;
> +}
> +
> +/*
>   * Start a new RCU grace period if warranted, re-initializing the hierarchy
>   * in preparation for detecting the next grace period.  The caller must hold
>   * the root node's ->lock, which is released before return.  Hard irqs must
> @@ -1056,77 +1153,20 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
>  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
> -	if (!rcu_scheduler_fully_active ||
> +	if (!rsp->gp_kthread ||
>  	    !cpu_needs_another_gp(rsp, rdp)) {
>  		/*
> -		 * Either the scheduler hasn't yet spawned the first
> -		 * non-idle task or this CPU does not need another
> -		 * grace period.  Either way, don't start a new grace
> -		 * period.
> -		 */
> -		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> -		return;
> -	}
> -
> -	if (rsp->fqs_active) {
> -		/*
> -		 * This CPU needs a grace period, but force_quiescent_state()
> -		 * is running.  Tell it to start one on this CPU's behalf.
> +		 * Either we have not yet spawned the grace-period
> +		 * task or this CPU does not need another grace period.
> +		 * Either way, don't start a new grace period.
>  		 */
> -		rsp->fqs_need_gp = 1;
>  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  		return;
>  	}
>  
> -	/* Advance to a new grace period and initialize state. */
> -	rsp->gpnum++;
> -	trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
> -	WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
> -	rsp->fqs_state = RCU_GP_INIT; /* Hold off force_quiescent_state. */
> -	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
> -	record_gp_stall_check_time(rsp);
> -	raw_spin_unlock(&rnp->lock);  /* leave irqs disabled. */
> -
> -	/* Exclude any concurrent CPU-hotplug operations. */
> -	raw_spin_lock(&rsp->onofflock);  /* irqs already disabled. */
> -
> -	/*
> -	 * Set the quiescent-state-needed bits in all the rcu_node
> -	 * structures for all currently online CPUs in breadth-first
> -	 * order, starting from the root rcu_node structure.  This
> -	 * operation relies on the layout of the hierarchy within the
> -	 * rsp->node[] array.  Note that other CPUs will access only
> -	 * the leaves of the hierarchy, which still indicate that no
> -	 * grace period is in progress, at least until the corresponding
> -	 * leaf node has been initialized.  In addition, we have excluded
> -	 * CPU-hotplug operations.
> -	 *
> -	 * Note that the grace period cannot complete until we finish
> -	 * the initialization process, as there will be at least one
> -	 * qsmask bit set in the root node until that time, namely the
> -	 * one corresponding to this CPU, due to the fact that we have
> -	 * irqs disabled.
> -	 */
> -	rcu_for_each_node_breadth_first(rsp, rnp) {
> -		raw_spin_lock(&rnp->lock);	/* irqs already disabled. */
> -		rcu_preempt_check_blocked_tasks(rnp);
> -		rnp->qsmask = rnp->qsmaskinit;
> -		rnp->gpnum = rsp->gpnum;
> -		rnp->completed = rsp->completed;
> -		if (rnp == rdp->mynode)
> -			rcu_start_gp_per_cpu(rsp, rnp, rdp);
> -		rcu_preempt_boost_start_gp(rnp);
> -		trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
> -					    rnp->level, rnp->grplo,
> -					    rnp->grphi, rnp->qsmask);
> -		raw_spin_unlock(&rnp->lock);	/* irqs remain disabled. */
> -	}
> -
> -	rnp = rcu_get_root(rsp);
> -	raw_spin_lock(&rnp->lock);		/* irqs already disabled. */
> -	rsp->fqs_state = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
> -	raw_spin_unlock(&rnp->lock);		/* irqs remain disabled. */
> -	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> +	rsp->gp_flags = 1;
> +	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +	wake_up(&rsp->gp_wq);
>  }
>  
>  /*
> @@ -2627,6 +2667,28 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
>  }
>  
>  /*
> + * Spawn the kthread that handles this RCU flavor's grace periods.
> + */
> +static int __init rcu_spawn_gp_kthread(void)
> +{
> +	unsigned long flags;
> +	struct rcu_node *rnp;
> +	struct rcu_state *rsp;
> +	struct task_struct *t;
> +
> +	for_each_rcu_flavor(rsp) {
> +		t = kthread_run(rcu_gp_kthread, rsp, rsp->name);
> +		BUG_ON(IS_ERR(t));
> +		rnp = rcu_get_root(rsp);
> +		raw_spin_lock_irqsave(&rnp->lock, flags);
> +		rsp->gp_kthread = t;
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +	}
> +	return 0;
> +}
> +early_initcall(rcu_spawn_gp_kthread);
> +
> +/*
>   * This function is invoked towards the end of the scheduler's initialization
>   * process.  Before this is called, the idle task might contain
>   * RCU read-side critical sections (during which time, this idle
> @@ -2727,6 +2789,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
>  	}
>  
>  	rsp->rda = rda;
> +	init_waitqueue_head(&rsp->gp_wq);
>  	rnp = rsp->level[rcu_num_lvls - 1];
>  	for_each_possible_cpu(i) {
>  		while (i > rnp->grphi)
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 4d29169..117a150 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -385,6 +385,9 @@ struct rcu_state {
>  	u8	boost;				/* Subject to priority boost. */
>  	unsigned long gpnum;			/* Current gp number. */
>  	unsigned long completed;		/* # of last completed gp. */
> +	struct task_struct *gp_kthread;		/* Task for grace periods. */
> +	wait_queue_head_t gp_wq;		/* Where GP task waits. */
> +	int gp_flags;				/* Commands for GP task. */
>  
>  	/* End of fields guarded by root rcu_node's lock. */
>  
> -- 
> 1.7.8
>