linux-kernel - Re: [RFC][PATCH 0/7] sched: balance callbacks

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Message-ID: <20150601141637.GT19282@twins.programming.kicks-ass.net>
Date:	Mon, 1 Jun 2015 16:16:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	umgwanakikbuti@...il.com, mingo@...e.hu
Cc:	ktkhai@...allels.com, rostedt@...dmis.org, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/7] sched: balance callbacks

On Mon, Jun 01, 2015 at 03:58:18PM +0200, Peter Zijlstra wrote:
> Hi,
> 
> Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.
> 
> The exact scenario is __sched_setscheduler() changing a (runnable) task from
> FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity
> drop rq->lock. This gap allows regular cfs load-balancing to step in and
> migrate our.

s/\./ task&/

> However, check_class_changed() will happily continue with switched_to_fair()
> which assumes our task is still on the old rq and makes the kernel go boom.
> 
> Instead of trying to patch this up and make things complicated; simply disallow
> these methods to drop rq->lock and extend the current post_schedule stuff into
> a balancing callback list, and use that.
> 
> This survives Mike's testcase for well over an hour on my ivb-ep. I've not yet
> tested it on anything bigger.
> 
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