linux-kernel - Re: [PATCH] sched: Fix bug in balance

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Message-ID: <20070807071128.GD19745@elte.hu>
Date:	Tue, 7 Aug 2007 09:11:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Williams <pwil3058@...pond.net.au>
Cc:	Nick Piggin <npiggin@...e.de>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Fix bug in balance_tasks()


* Peter Williams <pwil3058@...pond.net.au> wrote:

> There are two problems with balance_tasks() and how it used:
> 
> 1. The variables best_prio and best_prio_seen (inherited from the old 
> move_tasks()) were only required to handle problems caused by the 
> active/expired arrays, the order in which they were processed and the 
> possibility that the task with the highest priority could be on 
> either. These issues are no longer present and the extra overhead 
> associated with their use is unnecessary (and possibly wrong).

indeed.

> 2. In the absence of CONFIG_FAIR_GROUP_SCHED being set, the same 
> this_best_prio variable needs to be used by all scheduling classes or 
> there is a risk of moving too much load.  E.g. if the highest priority 
> task on this at the beginning is a fairly low priority task and the rt 
> class migrates a task (during its turn) then that moved task becomes the 
> new highest priority task on this_rq but when the sched_fair class 
> initializes its copy of this_best_prio it will get the priority of the 
> original highest priority task as, due to the run queue locks being 
> held, the reschedule triggered by pull_task() will not have taken place. 
>  This could result in inappropriate overriding of skip_for_load and 
> excessive load being moved.

looks good to me - i've applied your fixes to my tree. (I'll give it a 
good workout to see if there's any negative impact on the quality 
balancing.)

	Ingo
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