linux-kernel - Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only one task

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Date:	Fri, 12 Sep 2008 09:52:57 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	ego@...ibm.com
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only
 one	task

Gautham R Shenoy wrote:
> 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.

Thank you!  I'd been looking suspiciously at this routine as well due to 
strange load-balancing behaviour that I saw while testing the fair group 
code, but I hadn't yet tracked down the exact problem.

Peter/Ingo, this appears to explain the issues described in the mail I 
sent on the 4th.  After applying this change the imbalance between tasks 
in the same group is substantially reduced.

Chris
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