linux-kernel - [PATCH tip/core/rcu 16/19] rcu: Make large and small sysidle systems use same state machine

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Date:	Mon, 28 Apr 2014 16:56:29 -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,
	oleg@...hat.com, sbw@....edu,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 16/19] rcu: Make large and small sysidle systems use same state machine

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

Currently, small systems move back into RCU_SYSIDLE_NOT from
RCU_SYSIDLE_SHORT and large systems do not.  This works because moving
aggressively to RCU_SYSIDLE_NOT affects only performance, not correctness,
and on small systems, the performance impact should be negligible.  That
said, this difference does make RCU a bit more complex, and RCU does not
seem to be suffering from any lack of complexity.  This commit therefore
adjusts small-system operation to match that of large systems, so that
the state never moves back to RCU_SYSIDLE_NOT from RCU_SYSIDLE_SHORT.

Reported-by: Frederic Weisbecker <fweisbec@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 045c6d04ed3f..f7593c2536b0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2659,7 +2659,8 @@ static void rcu_sysidle(unsigned long j)
 static void rcu_sysidle_cancel(void)
 {
 	smp_mb();
-	ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
+	if (full_sysidle_state > RCU_SYSIDLE_SHORT)
+		ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
 }
 
 /*
-- 
1.8.1.5

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