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rcu/nocb: Reduce __call_rcu_nocb_wake() leaf rcu_node ->lock contention

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Currently, __call_rcu_nocb_wake() advances callbacks each time that it
detects excessive numbers of callbacks, though only if it succeeds in
conditionally acquiring its leaf rcu_node structure's ->lock.  Despite
the conditional acquisition of ->lock, this does increase contention.
This commit therefore avoids advancing callbacks unless there are
callbacks in ->cblist whose grace period has completed and advancing
has not yet been done during this jiffy.

Note that this decision does not take the presence of new callbacks
into account.  That is because on this code path, there will always be
at least one new callback, namely the one we just enqueued.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-07-15 06:06:40 -07:00
parent 1d5a81c18d
commit 296181d78d
1 changed files with 10 additions and 3 deletions
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13
kernel/rcu/tree_plugin.h
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@ -1872,6 +1872,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
unsigned long flags)
__releases(rdp->nocb_lock)
{
unsigned long cur_gp_seq;
unsigned long j;
long len;
struct task_struct *t;
@ -1900,12 +1902,17 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
} else if (len > rdp->qlen_last_fqs_check + qhimark) {
/* ... or if many callbacks queued. */
rdp->qlen_last_fqs_check = len;
if (rdp->nocb_cb_sleep ||
!rcu_segcblist_ready_cbs(&rdp->cblist)) {
j = jiffies;
if (j != rdp->nocb_gp_adv_time &&
rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
rcu_advance_cbs_nowake(rdp->mynode, rdp);
rdp->nocb_gp_adv_time = j;
}
if (rdp->nocb_cb_sleep ||
!rcu_segcblist_ready_cbs(&rdp->cblist))
wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
TPS("WakeOvfIsDeferred"));
}
rcu_nocb_unlock_irqrestore(rdp, flags);
} else {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));