linux-kernel - [PATCH RT] Fix RT balancing tasks pulling

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Date:	Tue, 24 Jul 2007 15:42:02 +0200
From:	Sébastien Dugué <sebastien.dugue@...l.net>
To:	Linux RT Users <linux-rt-users@...r.kernel.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>,
	Darren Hart <dvhltc@...ibm.com>,
	john stultz <johnstul@...ibm.com>,
	Josh Triplett <josht@...ux.vnet.ibm.com>
Subject: [PATCH RT] Fix RT balancing tasks pulling

  Hi Ingo, Thomas,

  this one-liner fixes a bug in balance_rt_tasks() which sometimes manifests by
having a lower prio task being scheduled while a higher prio task is sitting
waiting on another runqueue.

  This is pretty hard to reproduce on low cpu count machines, for example, I
had to have sched_football run in a loop for ~38h before it failed on a dual HT
Xeon box.

  Sébastien.

---

  In the CPU loop in balance_rt_tasks(), the 'next' task pointer is only ever
updated if this_rq->lock has been dropped in double_lock_balance().

  This sometimes lead to 'this_cpu' pulling tasks that are only garanteed to
have a higher priority than the 'next' picked before the loop.

  Fix this to update 'next' to the last picked RT task.

Signed-off-by: Sébastien Dugué <sebastien.dugue@...l.net>

---
 kernel/sched.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Index: linux-2.6.22.1-rt6/kernel/sched.c
===================================================================
--- linux-2.6.22.1-rt6.orig/kernel/sched.c	2007-07-24 11:34:18.000000000 +0200
+++ linux-2.6.22.1-rt6/kernel/sched.c	2007-07-24 11:34:35.000000000 +0200
@@ -1499,6 +1499,14 @@ static void balance_rt_tasks(struct rq *
 			 * in another runqueue. (low likelyhood
 			 * but possible)
 			 */
+
+			/*
+			 * Update next so that we won't pick a task
+			 * on another cpu with a priority lower (or equal)
+			 * than the one we just picked.
+			 */
+			next = p;
+
 		}
 		spin_unlock(&src_rq->lock);
 	}
-
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