Re: [PATCH tip/core/rcu 03/23] rcu: Move RCU grace-period cleanup into kthread

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 11:18:18AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> As a first step towards allowing grace-period cleanup to be preemptible,
> this commit moves the RCU grace-period cleanup into the same kthread
> that is now used to initialize grace periods.  This is needed to keep
> scheduling latency down to a dull roar.
> 
> 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 |  112 ++++++++++++++++++++++++++++++------------------------
>  1 files changed, 62 insertions(+), 50 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index ef56aa3..9fad21c 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1045,6 +1045,7 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
>  static int rcu_gp_kthread(void *arg)
>  {
>  	unsigned long flags;
> +	unsigned long gp_duration;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp;
>  	struct rcu_state *rsp = arg;
> @@ -1135,6 +1136,65 @@ static int rcu_gp_kthread(void *arg)
>  		rsp->fqs_state = RCU_SIGNAL_INIT;
>  		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  		put_online_cpus();
> +
> +		/* Handle grace-period end. */
> +		rnp = rcu_get_root(rsp);
> +		for (;;) {
> +			wait_event_interruptible(rsp->gp_wq,
> +						 !ACCESS_ONCE(rnp->qsmask) &&
> +						 !rcu_preempt_blocked_readers_cgp(rnp));
> +			if (!ACCESS_ONCE(rnp->qsmask) &&
> +			    !rcu_preempt_blocked_readers_cgp(rnp))
> +				break;
> +			flush_signals(current);
> +		}
> +
> +		raw_spin_lock_irqsave(&rnp->lock, flags);
> +		gp_duration = jiffies - rsp->gp_start;
> +		if (gp_duration > rsp->gp_max)
> +			rsp->gp_max = gp_duration;
> +
> +		/*
> +		 * We know the grace period is complete, but to everyone else
> +		 * it appears to still be ongoing.  But it is also the case
> +		 * that to everyone else it looks like there is nothing that
> +		 * they can do to advance the grace period.  It is therefore
> +		 * safe for us to drop the lock in order to mark the grace
> +		 * period as completed in all of the rcu_node structures.
> +		 *
> +		 * But if this CPU needs another grace period, it will take
> +		 * care of this while initializing the next grace period.
> +		 * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
> +		 * because the callbacks have not yet been advanced: Those
> +		 * callbacks are waiting on the grace period that just now
> +		 * completed.
> +		 */
> +		if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
> +			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.
> +			 */
> +			rcu_for_each_node_breadth_first(rsp, rnp) {
> +				/* irqs already disabled. */
> +				raw_spin_lock(&rnp->lock);
> +				rnp->completed = rsp->gpnum;
> +				/* irqs remain disabled. */
> +				raw_spin_unlock(&rnp->lock);
> +			}
> +			rnp = rcu_get_root(rsp);
> +			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +		}
> +
> +		rsp->completed = rsp->gpnum; /* Declare grace period done. */
> +		trace_rcu_grace_period(rsp->name, rsp->completed, "end");
> +		rsp->fqs_state = RCU_GP_IDLE;
> +		if (cpu_needs_another_gp(rsp, rdp))
> +			rsp->gp_flags = 1;
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  	}
>  	return 0;
>  }
> @@ -1182,57 +1242,9 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
>  static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
>  	__releases(rcu_get_root(rsp)->lock)
>  {
> -	unsigned long gp_duration;
> -	struct rcu_node *rnp = rcu_get_root(rsp);
> -	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
> -
>  	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
> -
> -	/*
> -	 * Ensure that all grace-period and pre-grace-period activity
> -	 * is seen before the assignment to rsp->completed.
> -	 */
> -	smp_mb(); /* See above block comment. */
> -	gp_duration = jiffies - rsp->gp_start;
> -	if (gp_duration > rsp->gp_max)
> -		rsp->gp_max = gp_duration;
> -
> -	/*
> -	 * We know the grace period is complete, but to everyone else
> -	 * it appears to still be ongoing.  But it is also the case
> -	 * that to everyone else it looks like there is nothing that
> -	 * they can do to advance the grace period.  It is therefore
> -	 * safe for us to drop the lock in order to mark the grace
> -	 * period as completed in all of the rcu_node structures.
> -	 *
> -	 * But if this CPU needs another grace period, it will take
> -	 * care of this while initializing the next grace period.
> -	 * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
> -	 * because the callbacks have not yet been advanced: Those
> -	 * callbacks are waiting on the grace period that just now
> -	 * completed.
> -	 */
> -	if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
> -		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.
> -		 */
> -		rcu_for_each_node_breadth_first(rsp, rnp) {
> -			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> -			rnp->completed = rsp->gpnum;
> -			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> -		}
> -		rnp = rcu_get_root(rsp);
> -		raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> -	}
> -
> -	rsp->completed = rsp->gpnum;  /* Declare the grace period complete. */
> -	trace_rcu_grace_period(rsp->name, rsp->completed, "end");
> -	rsp->fqs_state = RCU_GP_IDLE;
> -	rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
> +	raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> +	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
>  }
>  
>  /*
> -- 
> 1.7.8
>