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Subject[PATCH v2 3/3] rcu/tree: Make FQS complaining about offline CPU more aggressive
Date
Make FQS loop consider it an immediate failure if the case of an offline CPU
reporting QS is detected, instead of a full second.

This is because rcu_report_dead() already reports quiescent states and
updates ->qsmaskinitnext under node lock.

Light testing with TREE03 and hotplug shows no warnings.

Convert the warning as well to WARN_ON_ONCE() to reduce log spam.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a621932cc385..39bdd744ba97 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1208,13 +1208,15 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
return 1;
}

- /* If waiting too long on an offline CPU, complain. */
- if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp)) &&
- time_after(jiffies, rcu_state.gp_start + HZ)) {
+ /*
+ * Complain if an offline CPU by RCU's books has not reported QS. Node
+ * lock is held ensuring offlining does not race here.
+ */
+ if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
bool onl;
struct rcu_node *rnp1;

- WARN_ON(1); /* Offline CPUs are supposed to report QS! */
+ WARN_ON_ONCE(1); /* Offline CPUs are supposed to report QS! */
pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
__func__, rnp->grplo, rnp->grphi, rnp->level,
(long)rnp->gp_seq, (long)rnp->completedqs);
--
2.28.0.163.g6104cc2f0b6-goog
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