On Sun, Mar 22, 2009 at 06:27:37PM -0700, Paul E. McKenney wrote:
> On Sun, Mar 22, 2009 at 04:16:37PM -0700, Arjan van de Ven wrote:
> > On Sun, 22 Mar 2009 16:03:31 -0700
> > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
[ . . . ]
> > > But in any case, I will see what I can do about speeding up
> > > synchronize_rcu(). I will likely start with TREE_RCU, and I may need
> > > some sort of indication that boot is in progress.
> >
> > system_state == SYSTEM_BOOTING is not a very elegant test but it works
>
> Fair enough... This would be until init, correct?
Here is a first attempt. The idea is to accelerate grace-period
detection when all but one of the CPUs is in nohz state by checking
for nohz CPUs immediately upon starting a grace period, instead of
waiting for three jiffies in that case. Lightly tested.
Not for inclusion.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 97ce315..77ce937 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -373,7 +373,26 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
return rcu_implicit_offline_qs(rdp);
}
-#endif /* #ifdef CONFIG_SMP */
+/*
+ * If almost all the other CPUs are in dyntick-idle mode, invoke
+ * force_quiescent_state() in order to make the RCU grace period
+ * happen more quickly. The main purpose is to speed up boot
+ * on multiprocessor machines with CONFIG_NO_HZ.
+ */
+static void accelerate_almost_idle(struct rcu_state *rsp)
+{
+ if (cpumask_weight(cpu_online_mask) -
+ cpumask_weight(nohz_cpu_mask) <= 1)
+ force_quiescent_state(rsp, 0);
+}
+
+#else /* #ifdef CONFIG_SMP */
+
+static void accelerate_almost_idle(struct rcu_state *rsp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_SMP */
#else /* #ifdef CONFIG_NO_HZ */
@@ -407,6 +426,10 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
#endif /* #ifdef CONFIG_SMP */
+static void accelerate_almost_idle(struct rcu_state *rsp)
+{
+}
+
#endif /* #else #ifdef CONFIG_NO_HZ */
#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
@@ -577,6 +600,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
rnp->qsmask = rnp->qsmaskinit;
rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
spin_unlock_irqrestore(&rnp->lock, flags);
+ accelerate_almost_idle(rsp);
return;
}
@@ -627,6 +651,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
spin_unlock_irqrestore(&rsp->onofflock, flags);
+ accelerate_almost_idle(rsp);
}
/*
--
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