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rcu: Make rcu_nocb_wait_gp() check if GP already requested

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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: Paul 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. ]
This commit is contained in:
Paul E. McKenney 2018-05-01 11:07:23 -07:00
parent 7a1d0f23ad
commit ab5e869c1f
2 changed files with 15 additions and 5 deletions
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5
kernel/rcu/tree.c
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@ -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;

15
kernel/rcu/tree_plugin.h
View file

@ -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);
needwake = rcu_start_this_gp(rnp, rdp, c);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
if (needwake)
rcu_gp_kthread_wake(rdp->rsp);
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