Date:	Fri, 16 Mar 2012 12:28:50 -0500
From:	Dimitri Sivanich <sivanich@....com>
To:	Mike Galbraith <efault@....de>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu: Limit GP initialization to CPUs that have
 been online

On Fri, Mar 16, 2012 at 09:45:35AM +0100, Mike Galbraith wrote:
> On Fri, 2012-03-16 at 09:09 +0100, Mike Galbraith wrote: 
> > On Fri, 2012-03-16 at 08:27 +0100, Mike Galbraith wrote: 
> > > On Thu, 2012-03-15 at 12:59 -0500, Dimitri Sivanich wrote: 
> 
> > > > Could I try your 3.0 enterprise patch?
> > > 
> > > Sure, v3 below.
> > 
> > Erm, a bit of that went missing.  I'll put it back.
> 
> OK, box finally finished rebuild/boot.
> 

This patch also shows great improvement in the two
rcu_for_each_node_breadth_first() (nothing over 20 usec and most less than
10 in initial testing).

However, there are spinlock holdoffs at the following tracebacks (my nmi
handler does work on the 3.0 kernel):

[  584.157019]  [<ffffffff8144c5a0>] nmi+0x20/0x30
[  584.157023]  [<ffffffff8144bc8a>] _raw_spin_lock_irqsave+0x1a/0x30
[  584.157026]  [<ffffffff810c5f18>] force_qs_rnp+0x58/0x170
[  584.157030]  [<ffffffff810c6192>] force_quiescent_state+0x162/0x1d0
[  584.157033]  [<ffffffff810c6c95>] __rcu_process_callbacks+0x165/0x200
[  584.157037]  [<ffffffff810c6d4d>] rcu_process_callbacks+0x1d/0x80
[  584.157041]  [<ffffffff81061eaf>] __do_softirq+0xef/0x220
[  584.157044]  [<ffffffff81454cbc>] call_softirq+0x1c/0x30
[  584.157048]  [<ffffffff810043a5>] do_softirq+0x65/0xa0
[  584.157051]  [<ffffffff81061c85>] irq_exit+0xb5/0xe0
[  584.157054]  [<ffffffff810212c8>] smp_apic_timer_interrupt+0x68/0xa0
[  584.157057]  [<ffffffff81454473>] apic_timer_interrupt+0x13/0x20
[  584.157061]  [<ffffffff8102b352>] native_safe_halt+0x2/0x10
[  584.157064]  [<ffffffff8100adf5>] default_idle+0x145/0x150
[  584.157067]  [<ffffffff810020c6>] cpu_idle+0x66/0xc0

> 
> rcu: Limit GP initialization to CPUs that have been online
> 
> The current grace-period initialization initializes all leaf rcu_node
> structures, even those corresponding to CPUs that have never been online.
> This is harmless in many configurations, but results in 200-microsecond
> latency spikes for kernels built with NR_CPUS=4096.
> 
> This commit therefore keeps track of the largest-numbered CPU that has
> ever been online, and limits grace-period initialization to rcu_node
> structures corresponding to that CPU and to smaller-numbered CPUs.
> 
> Reported-by: Dimitri Sivanich <sivanich@....com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Acked-by: Mike Galbraith <mgalbraith@...e.de>
> 
> ---
>  kernel/rcutree.c |   36 ++++++++++++++++++++++++++++++++----
>  kernel/rcutree.h |   16 ++++++++++++++--
>  2 files changed, 46 insertions(+), 6 deletions(-)
> 
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -84,6 +84,8 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_d
>  
>  static struct rcu_state *rcu_state;
>  
> +int rcu_max_cpu __read_mostly;	/* Largest # CPU that has ever been online. */
> +
>  /*
>   * The rcu_scheduler_active variable transitions from zero to one just
>   * before the first task is spawned.  So when this variable is zero, RCU
> @@ -827,25 +829,31 @@ rcu_start_gp(struct rcu_state *rsp, unsi
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
>  	if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
> +		struct rcu_node *rnp_root = rnp;
> +
>  		if (cpu_needs_another_gp(rsp, rdp))
>  			rsp->fqs_need_gp = 1;
>  		if (rnp->completed == rsp->completed) {
> -			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +			raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
>  			return;
>  		}
> -		raw_spin_unlock(&rnp->lock);	 /* irqs remain disabled. */
>  
>  		/*
>  		 * Propagate new ->completed value to rcu_node structures
>  		 * so that other CPUs don't have to wait until the start
>  		 * of the next grace period to process their callbacks.
> +		 * We must hold the root rcu_node structure's ->lock
> +		 * across rcu_for_each_node_breadth_first() in order to
> +		 * synchronize with CPUs coming online for the first time.
>  		 */
>  		rcu_for_each_node_breadth_first(rsp, rnp) {
> +			raw_spin_unlock(&rnp_root->lock); /* remain disabled. */
>  			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
>  			rnp->completed = rsp->completed;
>  			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +			raw_spin_lock(&rnp_root->lock); /* already disabled. */
>  		}
> -		local_irq_restore(flags);
> +		raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
>  		return;
>  	}
>  
> @@ -935,7 +943,7 @@ static void rcu_report_qs_rsp(struct rcu
>  		rsp->gp_max = gp_duration;
>  	rsp->completed = rsp->gpnum;
>  	rsp->signaled = RCU_GP_IDLE;
> -	rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
> +	rcu_start_gp(rsp, flags);  /* releases root node's ->lock. */
>  }
>  
>  /*
> @@ -1862,6 +1870,7 @@ rcu_init_percpu_data(int cpu, struct rcu
>  	unsigned long mask;
>  	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
>  	struct rcu_node *rnp = rcu_get_root(rsp);
> +	struct rcu_node *rnp_init;
>  
>  	/* Set up local state, ensuring consistent view of global state. */
>  	raw_spin_lock_irqsave(&rnp->lock, flags);
> @@ -1882,6 +1891,20 @@ rcu_init_percpu_data(int cpu, struct rcu
>  	/* Exclude any attempts to start a new GP on large systems. */
>  	raw_spin_lock(&rsp->onofflock);		/* irqs already disabled. */
>  
> +	/*
> +	 * Initialize any rcu_node structures that will see their first use.
> +	 * Note that rcu_max_cpu cannot change out from under us because the
> +	 * hotplug locks are held.
> +	 */
> +	raw_spin_lock(&rnp->lock);		/* irqs already disabled. */
> +	for (rnp_init = per_cpu_ptr(rsp->rda, rcu_max_cpu)->mynode + 1;
> +	     rnp_init <= rdp->mynode;
> +	     rnp_init++) {
> +		rnp_init->gpnum = rsp->gpnum;
> +		rnp_init->completed = rsp->completed;
> +	}
> +	raw_spin_unlock(&rnp->lock);		/* irqs remain disabled. */
> +
>  	/* Add CPU to rcu_node bitmasks. */
>  	rnp = rdp->mynode;
>  	mask = rdp->grpmask;
> @@ -1907,6 +1930,11 @@ static void __cpuinit rcu_prepare_cpu(in
>  	rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
>  	rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
>  	rcu_preempt_init_percpu_data(cpu);
> +	if (cpu > rcu_max_cpu) {
> +		smp_mb(); /* Initialization before rcu_max_cpu assignment. */
> +		rcu_max_cpu = cpu;
> +		smp_mb(); /* rcu_max_cpu assignment before later uses. */
> +	}
>  }
>  
>  /*
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -191,11 +191,23 @@ struct rcu_node {
>  
>  /*
>   * Do a full breadth-first scan of the rcu_node structures for the
> - * specified rcu_state structure.
> + * specified rcu_state structure.  The caller must hold either the
> + * ->onofflock or the root rcu_node structure's ->lock.
>   */
> +extern int rcu_max_cpu;
> +static inline int rcu_get_max_cpu(void)
> +{
> +	int ret;
> +
> +	smp_mb();  /* Pairs with barriers in rcu_prepare_cpu(). */
> +	ret = rcu_max_cpu;
> +	smp_mb();  /* Pairs with barriers in rcu_prepare_cpu(). */
> +	return ret;
> +}
>  #define rcu_for_each_node_breadth_first(rsp, rnp) \
>  	for ((rnp) = &(rsp)->node[0]; \
> -	     (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
> +	     (rnp) <= per_cpu_ptr((rsp)->rda, rcu_get_max_cpu())->mynode; \
> +	     (rnp)++)
>  
>  /*
>   * Do a breadth-first scan of the non-leaf rcu_node structures for the
> 
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