linux-kernel - [PATCH tip/core/rcu 1/2] rcu: Fixes to NO_HZ

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Date:	Tue,  3 Mar 2015 09:50:42 -0800
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, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 1/2] rcu: Fixes to NO_HZ_FULL sysidle accounting

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

On second and subsequent passes through quiescent-state forcing, the
isidle variable was initialized to false, which would prevent full sysidle
state from being reached if a grace period needed more than one round
of quiescent-state forcing (which most should not).  However, the check
for offline CPUs in the quiescent-state forcing main loop had the wrong
sense, which could prevent CPUs from ever entering full sysidle state.

This commit fixes both of these bugs.  Given that sysidle is not yet
wired up, this has no effect in old kernels, but might have proven
frustrating had anyone attempted to wire it up.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 48d640ca1a05..735bd7ee749a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1798,7 +1798,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 		fqs_state = RCU_FORCE_QS;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
-		isidle = false;
+		isidle = true;
 		force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
 	}
 	/* Clear flag to prevent immediate re-entry. */
@@ -2596,8 +2596,8 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		bit = 1;
 		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
 			if ((rnp->qsmask & bit) != 0) {
-				if ((rnp->qsmaskinit & bit) != 0)
-					*isidle = false;
+				if ((rnp->qsmaskinit & bit) == 0)
+					*isidle = false; /* Pending hotplug. */
 				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
 					mask |= bit;
 			}
-- 
1.8.1.5

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