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rcu/nocb: Reduce contention at no-CBs registry-time CB advancement 

Currently, __call_rcu_nocb_wake() conditionally acquires the leaf rcu_node
structure's ->lock, and only afterwards does rcu_advance_cbs_nowake()
check to see if it is possible to advance callbacks without potentially
needing to awaken the grace-period kthread.  Given that the no-awaken
check can be done locklessly, this commit reverses the order, so that
rcu_advance_cbs_nowake() is invoked without holding the leaf rcu_node
structure's ->lock and rcu_advance_cbs_nowake() checks the grace-period
state before conditionally acquiring that lock, thus reducing the number
of needless acquistions of the leaf rcu_node structure's ->lock.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-06-01 06:16:38 -07:00
parent 9fcb09bddd
commit 6608c3a027
2 changed files with 4 additions and 5 deletions
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5
kernel/rcu/tree.c
View File

@ -1341,10 +1341,11 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
struct rcu_data *rdp)
{
raw_lockdep_assert_held_rcu_node(rnp);
if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||
!raw_spin_trylock_rcu_node(rnp))
return;
WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
raw_spin_unlock_rcu_node(rnp);
}
/*

4
kernel/rcu/tree_plugin.h
View File

@ -1668,10 +1668,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
if (!rdp->nocb_cb_sleep &&
rcu_segcblist_ready_cbs(&rdp->cblist)) {
// Already going full tilt, so don't try to rewake.
} else if (rcu_segcblist_pend_cbs(&rdp->cblist) &&
raw_spin_trylock_rcu_node(rdp->mynode)) {
} else if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
rcu_advance_cbs_nowake(rdp->mynode, rdp);
raw_spin_unlock_rcu_node(rdp->mynode);
} else {
wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
TPS("WakeOvfIsDeferred"));