Re: 3.0-git15 Atomic scheduling in pidmap_init

[Josh Boyer <jwboyer@redhat.com>]

On Thu, Aug 18, 2011 at 02:55:40PM -0700, Paul E. McKenney wrote:
> And here is this patch ported to v3.1-rc2, FYI.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> rcu: Avoid having just-onlined CPU resched itself when RCU is idle
> 
> CPUs set rdp->qs_pending when coming online to resolve races with
> grace-period start.  However, this means that if RCU is idle, the
> just-onlined CPU might needlessly send itself resched IPIs.  Adjust the
> online-CPU initialization to avoid this, and also to correctly cause
> the CPU to respond to the current grace period if needed.
> 
> This patch is believed to fix or otherwise suppress problems in
> https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the
> relationship is not apparent to this patch's author.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This is what has been in rawhide for a while now.

Tested-by: Josh Boyer <jwboyer@redhat.com>

> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index ba06207..6986d34 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
>  
>  	/* Set up local state, ensuring consistent view of global state. */
>  	raw_spin_lock_irqsave(&rnp->lock, flags);
> -	rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
> -	rdp->qs_pending = 1;	 /*  so set up to respond to current GP. */
>  	rdp->beenonline = 1;	 /* We have now been online. */
>  	rdp->preemptible = preemptible;
>  	rdp->qlen_last_fqs_check = 0;
> @@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
>  		rnp->qsmaskinit |= mask;
>  		mask = rnp->grpmask;
>  		if (rnp == rdp->mynode) {
> -			rdp->gpnum = rnp->completed; /* if GP in progress... */
> +			/*
> +			 * If there is a grace period in progress, we will
> +			 * set up to wait for it next time we run the
> +			 * RCU core code.
> +			 */
> +			rdp->gpnum = rnp->completed;
>  			rdp->completed = rnp->completed;
> +			rdp->passed_quiesc = 0;
> +			rdp->qs_pending = 1;
>  			rdp->passed_quiesc_completed = rnp->completed - 1;
>  		}
>  		raw_spin_unlock(&rnp->lock); /* irqs already disabled. */