linux-kernel - [PATCH 2/3] sched: fix SCHED

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Date:	Tue, 27 May 2008 18:59:39 -0600
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: [PATCH 2/3] sched: fix SCHED_FAIR wake-idle logic error

We currently use an optimization to skip the overhead of wake-idle

processing if more than one task is assigned to a run-queue.  The
assumption is that the system must already be load-balanced or we
wouldnt be overloaded to begin with.

The problem is that we are looking at rq->nr_running, which may include
RT tasks in addition to CFS tasks.  Since the presence of RT tasks
really has no bearing on the balance status of CFS tasks, this throws
the calculation off.

This patch changes the logic to only consider the number of CFS tasks
when making the decision to optimze the wake-idle.

Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
CC: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---

 kernel/sched_fair.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 24c0830..0ade6f8 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -951,7 +951,7 @@ static int wake_idle(int cpu, struct task_struct *p)
 	 * sibling runqueue info. This will avoid the checks and cache miss
 	 * penalities associated with that.
 	 */
-	if (idle_cpu(cpu) || cpu_rq(cpu)->nr_running > 1)
+	if (idle_cpu(cpu) || cpu_rq(cpu)->cfs.nr_running > 1)
 		return cpu;
 
 	for_each_domain(cpu, sd) {

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