Commit 1943c89d authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Reduce synchronize_rcu_expedited() latency 



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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent bcfa57ce

kernel/rcutree_plugin.h

+22 −8
Original line number Diff line number Diff line
@@ -799,34 +799,48 @@ void synchronize_rcu_expedited(void) 
	snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;

	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);

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