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Commit ab5e869c authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Make rcu_nocb_wait_gp() check if GP already requested 



This commit makes rcu_nocb_wait_gp() check rdp->gp_seq_needed to see
if the current CPU already knows about the needed grace period having
already been requested.  If so, it avoids acquiring the corresponding
leaf rcu_node structure's ->lock, thus decreasing contention.  This
optimization is intended for cases where either multiple leader rcuo
kthreads are running on the same CPU or these kthreads are running on
a non-offloaded (e.g., housekeeping) CPU.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Move lock release past "if" as suggested by Joel Fernandes. ]
[ paulmck: Fix caching of furthest-future requested grace period. ]
parent 7a1d0f23

kernel/rcu/tree.c

+5 −0
Original line number Diff line number Diff line
@@ -1618,6 +1618,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, 
	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum), TPS("newreq"));

	ret = true;  /* Caller must wake GP kthread. */

unlock_out:

	/* Push furthest requested GP to leaf node and rcu_data structure. */

	if (ULONG_CMP_LT(c, rnp_root->gp_seq_needed)) {

		rnp->gp_seq_needed = rnp_root->gp_seq_needed;

		rdp->gp_seq_needed = rnp_root->gp_seq_needed;

	}

	if (rnp != rnp_root)

		raw_spin_unlock_rcu_node(rnp_root);

	return ret;

kernel/rcu/tree_plugin.h

+10 −5
Original line number Diff line number Diff line
@@ -2104,12 +2104,17 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp) 
	bool needwake;

	struct rcu_node *rnp = rdp->mynode;



	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	local_irq_save(flags);

	c = rcu_seq_snap(&rdp->rsp->gp_seq);

	if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {

		local_irq_restore(flags);

	} else {

		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */

		needwake = rcu_start_this_gp(rnp, rdp, c);

		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

		if (needwake)

			rcu_gp_kthread_wake(rdp->rsp);

	}



	/*

	 * Wait for the grace period.  Do so interruptibly to avoid messing