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Subject[PATCH RFC tip/core/rcu 09/14] rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overload
Date
When under overload conditions, __call_rcu_nocb_wake() will wake the
no-CBs GP kthread any time the no-CBs CB kthread is asleep or there
are no ready-to-invoke callbacks, but only after a timer delay. If the
no-CBs GP kthread has a ->nocb_bypass_timer pending, the deferred wakeup
from __call_rcu_nocb_wake() is redundant. This commit therefore makes
__call_rcu_nocb_wake() avoid posting the redundant deferred wakeup if
->nocb_bypass_timer is pending. This requires adding a bit of ordering
of timer actions.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
kernel/rcu/tree_plugin.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index efd7f6fa2790..379cb7e50a62 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1906,8 +1906,10 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
rcu_advance_cbs_nowake(rdp->mynode, rdp);
rdp->nocb_gp_adv_time = j;
}
- if (rdp->nocb_cb_sleep ||
- !rcu_segcblist_ready_cbs(&rdp->cblist))
+ smp_mb(); /* Enqueue before timer_pending(). */
+ if ((rdp->nocb_cb_sleep ||
+ !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
+ !timer_pending(&rdp->nocb_bypass_timer))
wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
TPS("WakeOvfIsDeferred"));
rcu_nocb_unlock_irqrestore(rdp, flags);
@@ -1926,6 +1928,7 @@ static void do_nocb_bypass_wakeup_timer(struct timer_list *t)

trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
rcu_nocb_lock_irqsave(rdp, flags);
+ smp_mb__after_spinlock(); /* Timer expire before wakeup. */
__call_rcu_nocb_wake(rdp, true, flags);
}

--
2.17.1
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