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rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overload

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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>
This commit is contained in:
Paul E. McKenney 2019-07-16 02:17:00 -07:00
parent 296181d78d
commit f48fe4c586
1 changed files with 5 additions and 2 deletions
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7
kernel/rcu/tree_plugin.h
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@ -1909,8 +1909,10 @@ 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);
@ -1929,6 +1931,7 @@ 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);
}