linux-kernel - [RFC PATCH v2 2/7] sched: Fix __load_balance

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Message-ID: <20080908131740.3221.99223.stgit@drishya.in.ibm.com>
Date:	Mon, 08 Sep 2008 18:47:40 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	David Collier-Brown <davecb@....com>,
	Tim Connors <tconnors@...ro.swin.edu.au>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: [RFC PATCH v2 2/7] sched: Fix __load_balance_iterator() for cfq with
	only one task.

From: Gautham R Shenoy <ego@...ibm.com>

The __load_balance_iterator() returns a NULL when there's only one
sched_entity which is a task. It is caused by the following code-path.


	/* Skip over entities that are not tasks */
	do {
		se = list_entry(next, struct sched_entity, group_node);
		next = next->next;
	} while (next != &cfs_rq->tasks && !entity_is_task(se));

	if (next == &cfs_rq->tasks)
		return NULL;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      This will return NULL even when se is a task.

As a side-effect, there was a regression in sched_mc behavior since 2.6.25,
since iter_move_one_task() when it calls load_balance_start_fair(),
would not get any tasks to move!

Fix this by checking if the last entity was a task or not.

Reference:
http://lkml.org/lkml/2008/9/5/135

Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc: 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 fb8994c..f1c96e3 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1451,7 +1451,7 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
 		next = next->next;
 	} while (next != &cfs_rq->tasks && !entity_is_task(se));
 
-	if (next == &cfs_rq->tasks)
+	if (next == &cfs_rq->tasks && !entity_is_task(se))
 		return NULL;
 
 	cfs_rq->balance_iterator = next;

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