Re: [PATCH tip/core/rcu 22/23] rcu: Reducesynchronize_rcu_expedited() latency

[Josh Triplett]

On Thu, Aug 30, 2012 at 11:18:37AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> The synchronize_rcu_expedited() function disables interrupts across a
> scan of all leaf rcu_node structures, which is not good for real-time
> scheduling latency on large systems (hundreds or especially thousands
> of CPUs).  This commit therefore holds off CPU-hotplug operations using
> get_online_cpus(), and removes the prior acquisiion of the ->onofflock
> (which required disabling interrupts).
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

>  kernel/rcutree_plugin.h |   30 ++++++++++++++++++++++--------
>  1 files changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 7ed45c9..f1e06f6 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -800,33 +800,47 @@ void synchronize_rcu_expedited(void)
>  	smp_mb(); /* Above access cannot bleed into critical section. */
>  
>  	/*
> +	 * Block CPU-hotplug operations.  This means that any CPU-hotplug
> +	 * operation that finds an rcu_node structure with tasks in the
> +	 * process of being boosted will know that all tasks blocking
> +	 * this expedited grace period will already be in the process of
> +	 * being boosted.  This simplifies the process of moving tasks
> +	 * from leaf to root rcu_node structures.
> +	 */
> +	get_online_cpus();
> +
> +	/*
>  	 * Acquire lock, falling back to synchronize_rcu() if too many
>  	 * lock-acquisition failures.  Of course, if someone does the
>  	 * expedited grace period for us, just leave.
>  	 */
>  	while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
> +		if (ULONG_CMP_LT(snap,
> +		    ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
> +			put_online_cpus();
> +			goto mb_ret; /* Others did our work for us. */
> +		}
>  		if (trycount++ < 10) {
>  			udelay(trycount * num_online_cpus());
>  		} else {
> +			put_online_cpus();
>  			synchronize_rcu();
>  			return;
>  		}
> -		if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
> -			goto mb_ret; /* Others did our work for us. */
>  	}
> -	if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
> +	if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
> +		put_online_cpus();
>  		goto unlock_mb_ret; /* Others did our work for us. */
> +	}
>  
>  	/* force all RCU readers onto ->blkd_tasks lists. */
>  	synchronize_sched_expedited();
>  
> -	raw_spin_lock_irqsave(&rsp->onofflock, flags);
> -
>  	/* Initialize ->expmask for all non-leaf rcu_node structures. */
>  	rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
> -		raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +		raw_spin_lock_irqsave(&rnp->lock, flags);
>  		rnp->expmask = rnp->qsmaskinit;
> -		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  	}
>  
>  	/* Snapshot current state of ->blkd_tasks lists. */
> @@ -835,7 +849,7 @@ void synchronize_rcu_expedited(void)
>  	if (NUM_RCU_NODES > 1)
>  		sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
>  
> -	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> +	put_online_cpus();
>  
>  	/* Wait for snapshotted ->blkd_tasks lists to drain. */
>  	rnp = rcu_get_root(rsp);
> -- 
> 1.7.8
> 
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