linux-kernel - [PATCH tip/core/rcu 3/6] rcu: Throttle invoke_rcu_core() invocations due to non-lazy callbacks

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Message-Id: <1380073816-32381-3-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 24 Sep 2013 18:50:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 3/6] rcu: Throttle invoke_rcu_core() invocations due to non-lazy callbacks

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

If a non-lazy callback arrives on a CPU that has previously gone idle
with no non-lazy callbacks, invoke_rcu_core() forces the RCU core to
run.  However, it does not update the conditions, which could result
in several closely spaced invocations of the RCU core, which in turn
could result in an excessively high context-switch rate and resulting
high overhead.

This commit therefore updates the ->all_lazy and ->nonlazy_posted_snap
fields to prevent closely spaced invocations.

Reported-by: Tibor Billes <tbilles@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Tested-by: Tibor Billes <tbilles@....com>
---
 kernel/rcutree_plugin.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index d81e385..2c15d7c 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1745,6 +1745,8 @@ static void rcu_prepare_for_idle(int cpu)
 	 */
 	if (rdtp->all_lazy &&
 	    rdtp->nonlazy_posted != rdtp->nonlazy_posted_snap) {
+		rdtp->all_lazy = false;
+		rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
 		invoke_rcu_core();
 		return;
 	}
-- 
1.8.1.5

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