linux-kernel - [PATCH 21/39] autonuma: fix selecting task runqueue

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Date:	Mon, 26 Mar 2012 19:46:08 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: [PATCH 21/39] autonuma: fix selecting task runqueue

From: Hillf Danton <dhillf@...il.com>

Without coments, the following three hunks, I guess,
======
@@ -2788,6 +2801,7 @@ select_task_rq_fair(struct task_struct *p,
 		goto unlock;
 	}

+	prev_cpu = new_cpu;
 	while (sd) {
 		int load_idx = sd->forkexec_idx;
 		struct sched_group *group;
@@ -2811,6 +2825,7 @@ select_task_rq_fair(struct task_struct *p,
 		if (new_cpu == -1 || new_cpu == cpu) {
 			/* Now try balancing at a lower domain level of cpu */
 			sd = sd->child;
+			new_cpu = prev_cpu;
 			continue;
 		}

@@ -2826,6 +2841,7 @@ select_task_rq_fair(struct task_struct *p,
 		}
 		/* while loop will break here if sd == NULL */
 	}
+	BUG_ON(new_cpu < 0);
 unlock:
 	rcu_read_unlock();

======
were added for certain that selected cpu is valid, based on BUG_ON.

But question raised, why prev_cpu is changed?

Andrea's answer: yes the BUG_ON was introduced to verify the function
wouldn't return -1. This patch fixes the problem too.

Signed-off-by: Hillf Danton <dhillf@...il.com>
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 kernel/sched/fair.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 25e9e5b..a8498e0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2769,7 +2769,6 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 		goto unlock;
 	}
 
-	prev_cpu = new_cpu;
 	while (sd) {
 		int load_idx = sd->forkexec_idx;
 		struct sched_group *group;
@@ -2793,7 +2792,10 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 		if (new_cpu == -1 || new_cpu == cpu) {
 			/* Now try balancing at a lower domain level of cpu */
 			sd = sd->child;
-			new_cpu = prev_cpu;
+			if (new_cpu == -1) {
+				/* Only for certain that new cpu is valid */
+				new_cpu = prev_cpu;
+			}
 			continue;
 		}
 
--
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