linux-kernel - [PATCH tip/core/rcu 2/4] rcu: Hold off RCU_FAST_NO

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Date:	Tue, 21 Feb 2012 09:58:27 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 2/4] rcu: Hold off RCU_FAST_NO_HZ after timer posted

From: "Paul E. McKenney" <paul.mckenney@...aro.org>

This commit handles workloads that transition quickly between idle and
non-idle, and where the CPU's callbacks cannot be invoked, but where
RCU does not have anything immediate for the CPU to do.  Without this
patch, the RCU_FAST_NO_HZ code can be invoked repeatedly on each entry
to idle.  The commit sets the per-CPU rcu_dyntick_holdoff variable to
hold off further attempts for a tick.

Reported-by: "Abou Gazala, Neven M" <neven.m.abou.gazala@...el.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree_plugin.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 392a651..c023464 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2138,7 +2138,7 @@ static void rcu_prepare_for_idle(int cpu)
 		/* Can we go dyntick-idle despite still having callbacks? */
 		trace_rcu_prep_idle("Dyntick with callbacks");
 		per_cpu(rcu_dyntick_drain, cpu) = 0;
-		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
+		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
 		if (rcu_cpu_has_nonlazy_callbacks(cpu))
 			hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
 				      rcu_idle_gp_wait, HRTIMER_MODE_REL);
-- 
1.7.8

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