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SubjectRe: [PATCH tip/core/rcu 22/23] rcu: Reduce synchronize_rcu_expedited() latency
On Thu, Aug 30, 2012 at 11:18:37AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> 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@linux.vnet.ibm.com>

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

> 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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